Scuba Gas Planning, Rock Bottom & Turn Pressure Calculator
Estimate dive gas requirements with SAC rates, rock bottom reserves, and Nitrox calculations.
Last reviewed: June 2026Diver Profile
Tank
Dive Plan
Gas Mix
Gas Plan Summary
Dive Phase Breakdown
| Phase | Depth | Time (min) | Gas Used | Gas Remaining |
|---|
How the Scuba Gas Planning Calculator Works
This page estimates open-circuit gas use from SAC rate, tank size, starting pressure, maximum depth, bottom time, ascent rate, reserve rule, and gas mix. It is an educational model for checking assumptions. It is not a decompression planner, a substitute for training, or permission to conduct a dive.
The calculator converts each phase of the dive into gas volume. Ambient pressure in salt water is estimated as ATA = 1 + depth / 33. Phase gas is then SAC x ATA x minutes. With the default 0.5 cuft/min SAC rate, 60 fsw depth, 30 minute bottom time, and a 3 minute stop at 15 fsw, the model estimates about 47.3 cuft of total gas used: about 1.0 cuft during descent, 42.3 cuft on the bottom, 1.9 cuft during ascent, and 2.2 cuft during the safety stop.
Rock Bottom and Turn Pressure
The rock-bottom estimate uses a stressed SAC rate, one minute of problem solving at maximum depth, a 30 ft/min ascent, and a 3 minute stop at 15 fsw, then doubles the result for two divers sharing gas. With the default AL80 and stress factor, that reserve is about 16.5 cuft, or about 640 psi. When the reserve rule is set to rule of thirds, this calculator treats rock bottom as the protected reserve and turns after one third of the gas above that reserve has been used.
That rule is still only a model. Actual reserve planning depends on training agency procedures, buddy configuration, cylinder sizes, current, visibility, temperature, exertion, local rules, and whether the dive has a direct ascent. Overhead, decompression, solo, technical, and professional dives require planning methods outside this page.
Nitrox and NDL Notes
Nitrox does not make a cylinder last longer for the same SAC rate, depth, and time. It changes oxygen exposure and equivalent air depth assumptions, which can affect MOD and nitrogen loading. This calculator shows MOD, EAD, and a simple no-decompression-limit lookup, but those outputs are not a replacement for a dive computer, tables, oxygen-analysis procedures, or training.
Safety References
The 2025 NOAA Diving Standards and Safety Manual discusses 15 to 20 foot safety stops for 3 to 5 minutes and reserve gas sufficient to reach the surface while maintaining a 30 ft/min ascent rate. Divers Alert Network's ascent-rate discussion explains why ascent-rate choices vary and why divers should monitor their computer and training limits.
Common Mistakes
Do not use a best-case SAC rate for the entire plan. Cold, current, task loading, stress, buoyancy problems, and a teammate breathing from your gas supply can change the numbers quickly. Do not assume a fixed 500 psi reserve is adequate at every depth or for every cylinder. Do not treat the simple NDL lookup as a full repetitive-dive or decompression model. The safest use of this page is to expose assumptions before the dive, not to make real-time emergency decisions underwater.