Air planning is one division. A diver uses a known number of PSI per minute at the surface (their SAC rate); at depth they use more, by a factor of the depth in feet over 33 plus 1. Usable pressure — the tank minus the reserve you never touch — divided by that consumption gives the minutes. Round down, because 44.35 minutes means you are on the reserve at 45.
A 3,000 PSI tank with a 500 PSI reserve leaves 2,500. A 20 PSI/min SAC at 60 ft (factor 2.82) is 56.4 PSI/min, so 44 minutes.
The example
Three planned dives. The reserve is a shop policy and a bigger one for the deep dive; the SAC rate is the diver's own, from a logged dive.
| A | B | C | D | E | F | G | |
|---|---|---|---|---|---|---|---|
| 1 | Dive | Tank PSI | Reserve | SAC PSI/min | Depth (ft) | Factor | Minutes |
| 2 | Reef, 60 ft | 3000 | 500 | 20 | 60 | 2.82 | 44 |
| 3 | Wreck, 90 ft | 3000 | 700 | 25 | 90 | 3.73 | 24 |
| 4 | Training, 30 ft | 3000 | 500 | 15 | 30 | 1.91 | 87 |
The formula
The depth factor deserves its own column, because it is the part divers get wrong:
How it works
Gas planning in four steps:
B2-C2is usable PSI. 3,000 in the tank less a 500 PSI reserve is 2,500 you are allowed to breathe.E2/33+1is the absolute pressure in atmospheres. Every 33 feet of seawater adds one atmosphere to the one you started with; at 60 ft that is 2.82.D2*F2is consumption at depth. A diver who breathes 20 PSI a minute at the surface breathes 56.4 at 60 ft.ROUNDDOWN(2500/56.4, 0)is 44 minutes. Not 44.35 — the reserve is the reserve.
This is the gas limit, not the dive limit. The no-decompression limit at 60 ft is around 55 minutes on most tables, so this diver is gas-limited; at 90 ft the NDL is about 25 minutes and the gas gives 24, so it is close either way. Plan to whichever is shorter.
Try it: interactive demo
Enter the tank and reserve pressure, the diver's SAC rate, and the planned depth.
Variations
Metric: bar and meters
Same idea with 10 meters per atmosphere and SAC in bar per minute.
Shorter of gas and NDL
Look up the no-decompression limit for the depth and take the smaller of the two numbers.
Pitfalls & errors
SAC rate in PSI per minute is tank-specific. A diver's 20 PSI/min on an aluminum 80 is a different number on a steel 100. Either keep one rate per tank size or convert to cubic feet per minute (RMV) and back.
Use a real measured SAC from a logged dive, not the 20 in the example. New divers commonly run 30 or more; a calm experienced diver may be 12. The number is the whole plan.
Do not forget the +1. Depth over 33 alone gives the added pressure, not the total, and at 60 ft that under-counts consumption by more than a third — the diver runs out with 15 minutes still on the plan.
Practice workbook
Frequently asked questions
Is this a substitute for a dive computer?
What reserve should I use?
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