Use this scuba tank calculator to estimate how much breathing gas your tank holds and how long it may last during a dive. Enter your tank size, pressure, SAC rate, and average depth to estimate available gas, gas consumption, remaining air, and approximate dive time.
Scuba tank size and pressure determine the amount of compressed breathing gas available. Your breathing rate, depth, and activity level then affect how quickly that gas is consumed.
Calculate your estimated tank air volume, available gas, remaining gas, pressure used, and dive time in seconds. Use the results for recreational dive planning and gas-management estimates.
What Is a Scuba Tank Calculator?
A scuba tank calculator is a tool that estimates the amount of breathing gas available in a scuba tank. It uses factors such as tank volume, starting pressure, remaining pressure, SAC rate, and dive depth to provide useful gas-management estimates.
A scuba tank has a physical capacity, which is the internal space inside the tank. However, the amount of breathing gas it contains depends on how much the gas is compressed. A larger tank does not automatically mean more usable gas unless its pressure is also considered.
Pressure matters because higher pressure means more compressed gas inside the same tank. For example, a 12-liter tank filled to 200 bar contains more gas than the same tank filled to 100 bar.
Tank volume alone, therefore, cannot tell you how much breathing gas is available. You need both tank volume and pressure to estimate the total gas volume.
Your actual dive time also depends on how quickly you consume that gas. Depth, breathing rate, physical effort, and other conditions can change your gas consumption during a dive.
What Can You Calculate With This Tool?
This calculator can help you estimate:
Tank capacity — Determine the physical volume of your scuba tank.
Air volume — Estimate the total gas volume based on tank size and pressure.
Gas consumed — Estimate how much gas is used during a dive.
Remaining gas — Calculate the approximate gas left in the tank.
Estimated tank time — Estimate how long your available gas may last.
Final tank pressure — Estimate the pressure remaining after gas consumption.
These calculations are useful for understanding scuba gas usage and planning. Always follow your training, planned gas reserves, and dive computer or dive tables when making actual dive decisions.
Dive planning tool
Scuba Tank Calculator
Estimate tank capacity, available gas, consumption at depth, remaining pressure, and dive time in seconds.
Live results
Your planning estimate
Based on your tank, depth, reserve, and gas-consumption input.
Tank gauge
1,000 psi remaining
Estimated gas timeline
How we calculated your result
How the scuba tank calculator works
This calculator uses a simplified recreational gas-planning model. It converts your starting and reserve pressures into usable gas, adjusts your consumption for ambient pressure at depth, and estimates how long that usable gas could last.
capacity × starting pressure ÷ rated pressureATA = depth ÷ 33 + 1surface rate × ATAusable gas ÷ depth consumptionExample: Aluminum 80
With an Aluminum 80 starting at 3,000 psi, a 1,000 psi reserve, 60 ft average depth, and a 20 psi/min SAC rate, the calculator estimates the usable gas and time remaining after accounting for depth pressure.
The result is an estimate—not guaranteed dive time. A diver's workload, breathing pattern, stress, equipment, temperature, current, and actual depth all affect gas use.
Common scuba tank sizes
| Tank | Approx. capacity |
|---|---|
| Aluminum 63 | 63 cu ft |
| Aluminum 80 | 80 cu ft |
| Aluminum 100 | 100 cu ft |
| Steel 80 | 80 cu ft |
| Steel 100 | 100 cu ft |
| Steel 120 | 120 cu ft |
| Metric cylinders | 10–15 L internal volume |
This calculator is for educational and recreational planning estimates. Actual gas consumption varies with depth, workload, breathing rate, equipment, water conditions, temperature, and other factors. Always follow your training agency's gas-management procedures and use an appropriate dive computer or dive tables.
Scuba tank calculator FAQs
How much air is in an Aluminum 80 tank?
An Aluminum 80 is commonly referenced as approximately 80 cubic feet of gas at its rated pressure. The exact specification depends on the cylinder and its rated pressure, so check the markings on your tank.
What is the difference between SAC and RMV?
SAC is commonly entered as pressure used per minute, while RMV is the volume of gas breathed per minute at the surface. This calculator supports both methods and adjusts the rate for ambient pressure at depth.
Does this calculate safe dive time?
No. It calculates an estimated dive time based on the values entered. It does not account for every safety limit, ascent requirement, decompression obligation, or change in breathing rate. Use trained dive-planning procedures and keep the reserve you planned.
Why does depth reduce estimated tank time?
Pressure increases with depth, so each breath uses more gas than it would at the surface. The calculator represents this with an ambient-pressure factor in ATA.
How to Use the Scuba Tank Calculator
Using the scuba tank calculator is simple. Enter your tank details, gas consumption rate, and dive depth to estimate available gas and approximate dive time.
1. Enter Your Scuba Tank Capacity
Start by entering your tank’s internal volume. Scuba tanks are commonly measured in liters (L) or cubic feet (cu ft).
For example, a tank may have a 10 L, 12 L, or 15 L internal volume. In the USA, scuba tanks are often described as 80 cu ft or 100 cu ft.
Make sure you enter the unit that matches the calculator field.
2. Enter the Tank Pressure
Enter the rated or starting pressure of your scuba tank. Common pressure ratings include 200 bar, 232 bar, and 300 bar.
A higher pressure means more compressed breathing gas can be stored in the same physical tank volume. Always use the actual pressure shown on your tank gauge or specifications when making an estimate.
3. Enter Starting and Final Pressure
Enter the pressure at the beginning and the final pressure you expect to have remaining.
The difference between these values represents the pressure used:
Pressure Used = Starting Pressure − Final Pressure
For example, dropping from 200 bar to 100 bar means approximately 100 bar of pressure has been used.
4. Enter Your SAC Rate
Enter your Surface Air Consumption (SAC) rate if the calculator uses it to estimate dive time.
SAC describes how much gas you would consume per minute at the surface. Your SAC rate can vary with breathing pattern, fitness, workload, experience, and diving conditions.
If you need to calculate your SAC rate first, use our Scuba Air Consumption Calculator (SAC).
5. Enter Your Average Dive Depth
Enter the average depth of your planned dive. Depth matters because ambient pressure increases as you descend.
At greater depths, each breath requires more gas than it would at the surface. This means your tank can be consumed faster even when your breathing rate feels unchanged.
Using average depth provides an estimate for planning. Actual gas consumption can vary throughout a dive.
6. Review Your Estimated Dive Time
After entering the required values, review the calculator’s estimated tank time and gas results.
The result represents an estimate, not a guaranteed dive duration. Real-world gas consumption can change because of depth, breathing rate, current, water temperature, physical effort, buoyancy, and other conditions.
Always maintain an appropriate gas reserve and follow your dive training, dive computer, and established gas-management procedures when making actual dive decisions.
How Does a Scuba Tank Calculator Work?
A scuba tank calculator works by combining tank volume and gas pressure to estimate how much breathing gas is available. It can then use your gas consumption rate and dive depth to estimate how quickly that gas may be used.
The key idea is simple: tank size tells you how much space the tank has, while pressure tells you how much gas is compressed into that space.
Scuba Tank Capacity vs Gas Volume
Tank capacity is the physical internal volume of the scuba tank. It describes the amount of space inside the tank and is commonly measured in liters or cubic feet.
For example, a scuba tank may have a 12-liter internal capacity. That 12 L does not mean the tank contains only 12 liters of breathing gas.
Gas volume refers to the amount of breathing gas available when the tank’s pressure is taken into account. Compressed gas expands when released to atmospheric pressure, so a small physical tank can contain a much larger volume of gas.
This distinction is important when you calculate the volume of air in a scuba tank. You need both the tank’s internal volume and its pressure to estimate the amount of gas available.
Basic Scuba Tank Air Volume Formula
For a simple estimate, use:
Gas Volume ≈ Tank Internal Volume × Pressure
For example, consider a 12-liter scuba tank filled to 200 bar:
12 L × 200 bar = 2,400 L
So, the tank contains approximately 2,400 liters of gas at approximately atmospheric pressure, based on this simplified calculation.
This is an ideal-gas estimate rather than an exact measurement of usable breathing gas. Actual gas volume can vary because of temperature, pressure behavior, tank characteristics, regulator performance, and equipment limitations.
The formula is therefore useful for understanding tank capacity and available gas. For actual dive planning, use your measured tank pressure, planned gas reserves, and appropriate dive-planning procedures.
How to Calculate the Volume of Air in a Scuba Tank
If you want to know how to calculate the volume of air in a scuba tank, you need two basic values: the tank’s internal volume and its pressure.
The tank volume tells you how much physical space is available. The pressure tells you how much breathing gas is compressed inside that space.
Formula
For a simple estimate, use:
Air Volume = Tank Volume × Tank Pressure
Keep the units consistent when entering your values. This calculation estimates the amount of gas the tank would provide if released to approximately atmospheric pressure.
Example Calculation
Suppose you have:
Tank volume: 12 L
Tank pressure: 200 bar
Apply the formula:
12 L × 200 bar = 2,400 L
This means a 12-liter tank filled to 200 bar contains approximately 2,400 liters of gas at atmospheric pressure, under simplified conditions.
In simple terms, the tank physically holds only 12 liters of space. The high pressure compresses much more gas into that space. When released to normal atmospheric pressure, that gas would occupy roughly 2,400 liters.
This is a theoretical estimate. Actual gas availability can vary with temperature, tank characteristics, pressure behavior, and equipment.
What Happens When Tank Pressure Drops?
As pressure decreases, the amount of gas remaining also decreases. Assuming the same tank and simplified conditions:
| Tank Pressure | Approximate Gas Remaining |
|---|---|
| 200 bar | 100% |
| 100 bar | 50% |
| 50 bar | 25% |
For example, if a 12-liter tank starts at 200 bar, its estimated gas volume is about 2,400 liters. At 100 bar, the estimate drops to about 1,200 liters. At 50 bar, it drops to about 600 liters.
These figures are simplified estimates and should not be treated as a recommended reserve or minimum tank pressure for a dive.
Scuba Tank Capacity vs Scuba Tank Volume
The terms scuba tank capacity and scuba tank volume are often used interchangeably. However, understanding the difference helps you interpret tank specifications correctly.
What Does Scuba Tank Capacity Mean?
Scuba tank capacity usually refers to the tank’s internal water volume, or the physical space inside the cylinder.
For example, a tank labeled as 12 L has an internal volume of approximately 12 liters. This does not mean it contains only 12 liters of breathable gas.
The amount of gas available depends on both the tank’s physical capacity and its filling pressure.
What Does Scuba Tank Volume Mean?
Scuba tank volume describes the amount of physical space inside the tank. It may be listed in liters (L) or cubic feet (cu ft), depending on the market and tank specification.
A larger-volume tank can generally store more gas at the same pressure. However, pressure must also be considered when estimating total gas volume.
For example, two tanks may have different internal volumes but the same pressure rating. The larger tank will generally contain more gas because it has more internal space.
Liters vs Cubic Feet
Scuba tanks can be described using either liters or cubic feet.
A useful conversion is:
1 cubic foot ≈ 28.32 liters
For example:
80 cu ft × 28.32 ≈ 2,266 liters
The USA commonly uses cubic feet when describing scuba tank sizes, such as 80 cu ft or 100 cu ft. Many international markets use liters, such as 10 L, 12 L, or 15 L.
When using a scuba tank calculator, always check whether the tank specification is given in liters or cubic feet and select the correct unit.
How to Calculate Scuba Tank Time
A scuba tank time calculator estimates how long your available breathing gas may last during a dive. The result depends on more than tank size. Your pressure, breathing rate, depth, and activity level all affect gas consumption.
The main factors that determine estimated tank time include:
Tank capacity: A larger tank can hold more gas at the same pressure.
Starting pressure: A higher starting pressure generally provides more available gas.
Reserve pressure: Gas kept in reserve reduces the amount available for planned use.
SAC rate: Your Surface Air Consumption rate indicates how quickly you use gas at the surface.
Average depth: Greater depth increases surrounding pressure and gas consumption.
Breathing gas: Different breathing gases can require different planning considerations.
Diver workload: Swimming against current, poor buoyancy, and physical exertion can increase gas consumption.
Because these factors vary between divers and dives, tank time should always be treated as an estimate, not a guaranteed dive duration.
Basic Tank Time Formula
The basic concept is:
Estimated Time = Available Gas ÷ Gas Consumption Rate
First, determine how much gas is available after accounting for your planned reserve. Then compare that amount with your estimated gas consumption rate.
Your consumption rate changes with depth because ambient pressure increases as you descend. Therefore, the same diver can use considerably more gas per minute at depth than at the surface.
For accurate gas-consumption estimates, your SAC rate is especially important. You can calculate or review your surface air consumption using our Scuba Air Consumption Calculator (SAC).
Why Dive Time Changes With Depth
Depth directly affects how much gas you use with each breath. As you descend, the surrounding water pressure increases.
For example, a breath taken at greater depth contains more gas molecules than a breath of the same physical volume taken at the surface. Your tank therefore supplies gas more quickly at depth.
This means a tank that lasts longer during a shallow dive may be consumed faster during a deeper dive, even if your breathing pattern remains similar.
Your actual consumption can also change because of swimming effort, current, stress, temperature, buoyancy, and other diving conditions.
How to Calculate Final Pressure Inside a Scuba Tank
To calculate the final pressure inside a scuba tank, you need the starting pressure and the amount of pressure used.
A simple calculation can estimate the remaining pressure, but real dive planning should also consider your gas consumption, planned reserve, depth, and dive conditions.
Basic Final Pressure Formula
For a simplified calculation:
Final Pressure = Starting Pressure − Pressure Used
This formula works when the amount of pressure consumed is already known.
Example
Suppose your scuba tank starts at 200 bar and you consume 80 bar.
200 bar − 80 bar = 120 bar
The estimated final pressure is therefore 120 bar.
However, this simple calculation does not determine whether 120 bar is an appropriate pressure to end a dive. Actual dive planning should account for your expected gas consumption, depth, ascent requirements, and planned reserve.
Always follow established gas-management procedures and your dive training rather than relying on a calculator result alone.
Scuba Tank Pressure and Available Air
Scuba tank pressure determines how much compressed breathing gas is stored inside a tank. When tank volume stays the same, a higher pressure generally means more gas is available.
For example, the same 12-liter tank filled to 200 bar contains less gas than when filled to 232 bar. However, pressure should never be considered by itself when estimating dive time.
200-Bar vs 232-Bar Tanks
A 200-bar tank and a 232-bar tank can have the same physical capacity but different amounts of compressed gas.
For example, a 12-liter tank gives a simplified estimate of:
200 bar: 12 × 200 = 2,400 L
232 bar: 12 × 232 = 2,784 L
The 232-bar example therefore contains more theoretical gas in the same 12-liter internal volume.
Actual usable gas can differ from these simplified calculations. Always use the tank’s specifications and actual pressure when planning gas use.
300-Bar Scuba Tanks
300-bar scuba tanks store gas at a higher pressure than 200-bar and 232-bar tanks. This can provide more gas within a similar physical tank volume.
However, a higher-pressure tank is not automatically better for every diver. Tank weight, size, equipment compatibility, filling availability, and gas-management practices also matter.
Never exceed the pressure rating marked on the tank. Use only equipment that is appropriate and rated for the intended pressure.
Why Tank Pressure Alone Doesn’t Determine Dive Time
Tank pressure tells you how much gas is compressed inside the tank, but it does not tell you how quickly you will consume that gas.
Two divers using identical tanks can have very different dive times because of:
SAC rate: Higher gas consumption reduces available dive time.
Depth: Greater depth increases ambient pressure and gas consumption.
Current: Swimming against a current can increase breathing effort.
Workload: Physical activity can significantly increase gas use.
Temperature: Cold conditions can affect workload and breathing.
Experience: New divers may use more gas while developing breathing and buoyancy skills.
For this reason, tank pressure should always be considered together with tank volume, SAC rate, depth, and planned gas reserves.
Common Scuba Tank Sizes and Their Air Capacity
The table below shows simplified theoretical gas volumes for several common tank sizes and pressures.
| Tank Size | Pressure | Approx. Gas Volume |
|---|---|---|
| 10 L | 200 bar | 2,000 L |
| 12 L | 200 bar | 2,400 L |
| 15 L | 200 bar | 3,000 L |
| 12 L | 232 bar | 2,784 L |
| 15 L | 232 bar | 3,480 L |
These values use the simplified formula Gas Volume ≈ Tank Volume × Pressure. They represent theoretical gas volume at approximately atmospheric pressure, not guaranteed usable gas during a dive.
What Do 80 cu ft and 100 cu ft Mean?
In the United States, scuba tanks are commonly described by their approximate cubic-foot gas capacity, such as 80 cu ft or 100 cu ft.
An 80 cu ft tank is commonly used as a standard recreational scuba cylinder, while a 100 cu ft tank provides more gas under its rated conditions.
These labels can be confusing because they describe the tank’s approximate gas capacity rather than simply its physical internal volume.
For comparison:
80 cu ft ≈ 2,266 L
100 cu ft ≈ 2,832 L
The exact gas capacity depends on the tank’s specifications, rated pressure, and measurement conditions. When using a calculator, always enter the tank information according to the unit and specifications provided by the manufacturer.
What Affects Scuba Tank Air Consumption?
Your scuba tank air consumption can change significantly from one dive to another. Tank size and pressure determine how much gas you start with, but your diving conditions determine how quickly you use it.
Understanding these factors can help you make better gas-consumption estimates and understand why your actual tank time may differ from a calculator result.
Depth
Greater depth means greater ambient pressure, which increases gas consumption per breath.
As you descend, the surrounding water pressure increases. Your regulator delivers breathing gas at the pressure needed for that depth, so each breath uses more gas than the same breath would at the surface.
This is why deeper dives generally reduce the amount of time a given tank supply can last.
Breathing Rate
Your breathing rate directly affects how quickly you consume gas.
A faster or deeper breathing pattern uses more gas per minute. Stress, excitement, poor breathing habits, and increased physical effort can all affect your breathing rate.
Developing relaxed and controlled breathing can help improve gas efficiency over time.
Physical Effort
Physical activity can increase your scuba tank air consumption.
Swimming against a strong current, carrying equipment, or performing underwater tasks requires more energy. Your body may respond by increasing breathing rate and gas consumption.
Planning should therefore consider the expected workload rather than assuming your normal consumption rate will remain constant.
Water Temperature
Cold water can affect both your breathing and physical workload.
Exposure to cold may increase your body’s energy demands. A thicker exposure suit can also require additional weighting and effort to maintain comfortable buoyancy.
These factors can contribute to higher gas consumption compared with an easy dive in warmer water.
Buoyancy and Weighting
Poor buoyancy control can increase your workload underwater.
If you are constantly adjusting your position, adding or releasing gas, or kicking to maintain depth, you may use more energy and breathe more heavily.
Proper weighting and good buoyancy skills can help you move efficiently and maintain a steady position in the water.
Experience
Beginners often consume more gas while developing their breathing, buoyancy, and diving skills.
New divers may experience increased breathing rates because of unfamiliar equipment, task loading, or nervousness. With training and experience, many divers become more relaxed and efficient underwater.
Your personal SAC rate should therefore be based on your own measured diving performance rather than someone else’s average.
Scuba Tank Calculator vs SAC Calculator
A scuba tank calculator and a SAC calculator serve different purposes, although they work well together.
Scuba Tank Calculator
A scuba tank calculator focuses on the tank itself and the gas available. It can estimate:
Tank capacity
Available gas
Starting and remaining pressure
Gas consumed
Estimated tank time
It answers questions such as, “How much gas do I have and how long might it last?”
SAC Calculator
A SAC calculator focuses on your personal gas-consumption rate. It calculates how efficiently you consume breathing gas using a surface-equivalent rate.
Your SAC rate can then be used with tank volume, pressure, and depth to make more useful gas-consumption estimates.
If you want to calculate your personal surface air consumption, use our Scuba Air Consumption Calculator (SAC).
Keeping these tools separate makes their purpose clearer: the scuba tank calculator estimates available gas and tank time, while the SAC calculator measures your gas-consumption rate. Using both together can provide a more complete picture of your expected gas use.
Common Scuba Tank Calculator Mistakes to Avoid
A scuba tank calculator can provide useful estimates, but incorrect inputs can produce misleading results. Avoid these common mistakes when calculating tank capacity, available gas, or estimated dive time.
Using Tank Volume Without Pressure
Tank volume alone does not tell you how much breathing gas is available. A 12-liter tank can contain different amounts of gas depending on its pressure.
Always consider tank volume and pressure together when estimating gas volume.
Confusing Tank Capacity With Gas Volume
A tank’s physical capacity is not the same as the amount of gas it contains at atmospheric pressure.
For example, a 12-liter tank filled to 200 bar can represent approximately 2,400 liters of gas at atmospheric pressure under simplified conditions.
Ignoring Dive Depth
Using your surface gas consumption rate without accounting for depth can overestimate your available dive time.
Ambient pressure increases as you descend, causing you to consume more gas per breath. Always include depth when the calculator uses it for dive-time estimates.
Forgetting Your Gas Reserve
Do not treat all the gas in your tank as available for planned bottom time.
A proper dive plan includes an appropriate reserve for the ascent, safety requirements, and unexpected conditions. Your required reserve depends on your training, dive plan, environment, and applicable procedures.
Using Someone Else’s SAC Rate
Every diver consumes gas differently. Fitness, breathing pattern, experience, workload, and diving conditions can all affect SAC.
Use your own measured SAC rate when possible rather than copying another diver’s value.
Assuming Calculated Dive Time Is Guaranteed
A calculator provides an estimate based on the information you enter. Actual gas consumption can change during the dive because of current, temperature, stress, workload, buoyancy, and depth.
Use the result as a planning estimate, not as a guaranteed number of minutes underwater.
Entering the Wrong Units
Mixing liters with cubic feet or bar with another pressure unit can produce incorrect results.
Before calculating, check that your tank volume and pressure units match the calculator’s required inputs.
Ignoring Actual Tank Pressure
A tank’s rated pressure and its current pressure are not necessarily the same. A tank rated for 232 bar may currently contain much less gas if its pressure has dropped.
For gas estimates, use the appropriate actual pressure reading and follow the tank manufacturer’s specifications.
Treating the Calculator as a Replacement for Dive Planning
A calculator cannot account for every condition of a real dive. It should support your planning rather than replace proper training, gas-management procedures, dive tables, or a dive computer.
Always maintain an appropriate gas reserve and follow the procedures taught by your certified diving agency.
A scuba tank calculator makes it easier to estimate your available breathing gas, tank capacity, pressure, gas consumption, and approximate dive time. Tank size and pressure determine your available gas, while depth and SAC rate affect how quickly you use it.
Use the calculator to understand your gas supply and support your dive planning. Remember that real-world consumption can vary with workload, current, temperature, buoyancy, and experience.
For actual diving decisions, always follow your training, gas-management procedures, planned reserves, and dive computer.
Frequently Asked Questions
The amount of air in a scuba tank depends on its internal volume and pressure. For a simplified estimate, multiply tank volume by pressure. For example, a 12-liter tank filled to 200 bar contains approximately 2,400 liters of gas at atmospheric pressure. Actual usable gas can vary with temperature, tank characteristics, equipment, and the reserve required for safe dive planning.
You calculate scuba tank capacity by identifying the tank's internal volume, usually listed in liters or cubic feet. For example, a tank with a 12-liter internal volume has a physical capacity of approximately 12 liters. Its total compressed gas depends on the filling pressure. Therefore, capacity and available gas are related but should not be treated as the same measurement.
You can estimate scuba tank air volume by multiplying the tank's internal volume by its pressure. The simplified formula is Air Volume = Tank Volume × Tank Pressure. For example, 12 L × 200 bar gives approximately 2,400 liters of gas at atmospheric pressure. This is a theoretical estimate, so actual available gas may differ under real diving conditions.
A scuba tank can last anywhere from a short period to well over an hour, depending on the diver and dive conditions. Tank volume, starting pressure, SAC rate, depth, workload, and reserve pressure all affect estimated duration. A larger tank does not automatically guarantee longer dive time because gas consumption generally increases with depth and physical effort.
A 12-liter scuba tank filled to 200 bar contains approximately 2,400 liters of gas at atmospheric pressure under simplified conditions. At 232 bar, the same 12-liter tank represents approximately 2,784 liters. These values use tank volume × pressure. Actual usable gas depends on temperature, equipment, tank characteristics, and the reserve required for the dive.
A 300-bar scuba tank stores gas at a higher pressure than a 200-bar tank, so it can hold more gas in a similar physical volume. However, higher pressure does not automatically mean longer dive time. Tank size, SAC rate, depth, workload, equipment compatibility, and planned reserves also affect how long your breathing gas may last.
You can calculate the final pressure using Final Pressure = Starting Pressure − Pressure Used when the pressure consumed is already known. For example, if a tank starts at 200 bar and 80 bar is consumed, the final pressure is 120 bar. For actual dive planning, final pressure should also account for expected gas consumption, depth, ascent needs, and the planned gas reserve.
Yes, scuba tank size can affect dive time because a larger tank can generally store more gas at the same pressure. However, tank size is only one factor. Your SAC rate, depth, workload, breathing pattern, current, temperature, and planned reserve also affect duration. Two divers using identical tanks can therefore have very different dive times under the same general conditions.