Gas Planning Calculator
Plan your gas supply using SAC rate, depth, and time. Calculates consumption at depth (Boyle's Law), maximum bottom time, turn pressure, and reserve adequacy.
About this calculator
This calculator converts a diver's Surface Air Consumption (SAC) rate into real gas math at depth, using Boyle's Law: pressure at depth (in atmospheres absolute, ATA) equals depth in metres divided by 10, plus 1. Multiplying SAC by ATA gives your actual breathing rate at depth, since compressed gas is consumed faster the deeper you go — a 15 L/min SAC rate at 25 m (3.5 ATA) becomes a real consumption of 52.5 L/min. Total gas in the tank is tank volume times fill pressure (an AL80's 11.1 L cylinder filled to 207 bar holds about 2,298 L of breathable gas at surface pressure), and usable gas subtracts out whatever you've reserved for the ascent. Dividing usable gas by consumption at depth gives the maximum bottom time the tank can support.
Turn pressure here follows the rule of thirds — splitting the fill into thirds for the swim out, the swim back, and an untouched reserve — though real thirds-planning normally divides remaining usable gas rather than starting pressure, so treat this as a simplified approximation, not a technical-diving turn-pressure calculation. The tool also estimates ascent gas as one minute at half depth plus a three-minute safety stop, then flags whether your plan is sufficient with that ascent included. Remember this only tracks gas volume, not decompression obligations, gas density limits, or your buddy's consumption — always dive the more conservative of your own and your buddy's numbers, and build in a real safety margin beyond the bare minimum this calculator shows.
Inputs
Results
Consumption at depth (L/min)
52.5
Max bottom time (min)
33.2
How to Use This Calculator
- Enter your Surface Air Consumption (SAC) rate in L/min — if unknown, use 15–20 L/min as a starting estimate.
- Input your planned maximum depth in meters and planned bottom time in minutes.
- Set your tank volume in litres and fill pressure in bar (standard fills are 200–207 bar).
- Enter your minimum reserve pressure in bar — typically 50 bar to reach the surface safely.
- Read Consumption at Depth (L/min) and Max Bottom Time to verify your tank has enough gas.
- Check Gas Sufficient? — if 0, either reduce planned depth/time or use a larger tank.
How the result changes with SAC rate (L/min)
| SAC rate (L/min) | Consumption at depth (L/min) | Max bottom time (min) |
|---|---|---|
| 7.5 | 26.3 | 66.4 |
| 11 | 38.5 | 45.3 |
| 23 | 80.5 | 21.6 |
| 38 | 133 | 13.1 |
What each input means
- SAC rate (L/min)
- Surface Air Consumption in litres per minute at 1 ATA. Typical range: 12-20 L/min.
- Depth (m)
- Planned maximum depth in metres.
- Planned time (min)
- Planned bottom time in minutes.
- Tank volume (L)
- Internal tank volume in litres. AL80 = 11.1 L, Steel 80 = 11.1 L, AL63 = 9.0 L.
- Fill pressure (bar)
- Starting tank pressure in bar. Standard fill is 200-207 bar.
- Reserve pressure (bar)
- Minimum reserve pressure to surface with. Typical: 50 bar.
What each result means
- Consumption at depth (L/min)
- Your air consumption rate at the planned depth (SAC x ATA).
- Gas needed (litres)
- Total gas required for the planned bottom time at depth.
- Max bottom time (min)
- Maximum time you can stay at depth with usable gas.
- Turn pressure (bar)
- Pressure at which to start your ascent (rule of thirds).
- Remaining pressure (bar)
- Tank pressure remaining after the planned dive (before ascent).
- Gas sufficient?
- 1 = yes, gas supply covers planned bottom time plus reserve. 0 = no.
- Sufficient with ascent?
- 1 = yes, including ascent + 3 min safety stop. 0 = no.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersSAC rate (L/min) = 15, Depth (m) = 25, Planned time (min) = 40, Tank volume (L) = 11.1 = 6 input(s) provided
- Calculate Consumption at depthConsumption at depth = sacLpm * ata52.5 = 52.5
- Calculate Max bottom timeMax bottom time = usableGasL / consumptionAtDepth33.2 = 33.2
- Calculate Gas neededGas needed = consumptionAtDepth * plannedTimeMin2100 = 2100
- Calculate Turn pressureTurn pressure = fillBar - (fillBar - reserveBar) / 2129 = 129
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why is my "consumption at depth" so much higher than my SAC rate?
SAC rate is measured at the surface, at 1 ATA. Compressed gas gets denser as pressure increases, so you consume more molecules per breath to fill the same lung volume at depth. This calculator multiplies your SAC rate by ATA (depth in metres ÷ 10, plus 1) to get real consumption — at 25 m (3.5 ATA), a 15 L/min SAC rate becomes 52.5 L/min of actual gas use.
How does the turn pressure calculation relate to the rule of thirds?
It sets turn pressure halfway between your fill pressure and reserve pressure, so the gas above reserve is split evenly into an outbound and return portion. Real thirds-planning normally divides the remaining usable gas at the start of the dive rather than starting pressure directly, so treat this turn pressure as a simplified approximation rather than a technical-diving calculation.
What's the difference between "gas sufficient" and "sufficient with ascent"?
"Gas sufficient" only checks whether your usable gas covers the planned bottom time at depth. "Sufficient with ascent" adds an estimated ascent cost — one minute of breathing at roughly half your planned depth's pressure, plus a three-minute safety stop — and checks whether the combined total still fits within your usable gas.
Why are SAC rate and depth capped at specific ranges in this calculator?
SAC rate is bounded between 5 and 40 L/min and depth between 0 and 100 m to keep results within physiologically realistic and recreationally relevant territory. Values outside those ranges would either represent an implausible breathing rate or a dive well beyond recreational limits that this simplified gas model isn't built to handle.
Related Calculators
The questions that sit next to this one — chosen by subject, including calculators filed under a different category.
Scuba Air Consumption Calculator
Calculate your surface air consumption (SAC) rate and estimate remaining bottom time at depth based on tank size and pressure readings.
DivingDive Log Statistics Calculator
Analyze your diving history to estimate SAC rate, air consumption efficiency, experience level, and depth-to-experience safety ratio.
DivingDive Profile Planner Calculator
Plan recreational dives using no-decompression limits (NDL). Calculates NDL at depth, safety stop duration, pressure group, and repetitive dive guidance.
DivingInstructor-to-Student Ratio Calculator
Determine the maximum and recommended instructor-to-student ratios for scuba courses based on certification level, water conditions, and assistant availability.
More in Sports, Outdoors & Recreation.