Free scuba calculators and dive guides

No. The calculators are planning and training aids. They do not replace dive training, current printed tables or a dive computer. Use them before a dive to check your plan, and always dive within the limits of your certification.

Every calculator shows its formula, rounding and limits on the page, so you can check a result by hand. Where rounding affects safety, it goes to the safer side: MOD and best mix are rounded down, EAD and gas pressures are rounded up. Oxygen exposure uses the NOAA limits, and the dive planner uses Dr. Max Hahn's DECO 2000 table.

They are free and there is no sign-up. The calculations run in your browser, so the numbers you type stay on your device and are not sent to a server.

MOD (maximum operating depth) is the deepest point where a gas stays within your ppO₂ limit. MOD = (ppO₂ limit ÷ FO₂ − 1) × 10. For 32% nitrox at 1.4 bar that gives 33.7 m. The nitrox calculator also works out EAD and the best mix for a planned depth.

Common practice is 1.2-1.4 bar for the working part of the dive and up to 1.6 bar only for decompression stops, where you are resting. The difference in exposure time is large: the NOAA single-exposure limit is 150 minutes at 1.4 bar but 45 minutes at 1.6 bar. The nitrox calculator starts at 1.4 bar. If your training agency sets a lower limit, use that one.

SAC is the gas you breathe per minute at the surface. Take the gas used in litres (pressure drop × tank volume), divide it by the dive time, then divide by the average absolute pressure (average depth ÷ 10 + 1). There is no single good number, because body size, fitness, workload and water temperature all change it. Measure your own rate over several dives with the SAC calculator and plan with that.

Some sources give SAC in bar per minute, which only holds for one tank size, and RMV in litres per minute, which works with any tank. Others use SAC for both. Our calculator reports litres per minute, so you can use the result with any tank.

Gas use rises with depth. At 20 m the absolute pressure is 3 bar, so each breath uses three times as much gas as at the surface. Enter your depth, time, SAC rate and tank size in the gas planner and it shows the gas the dive needs in litres and bar.

The rule of thirds keeps one third of your gas for the way back and one third as a reserve. It comes from cave and wreck diving, where you cannot swim straight up. Rock bottom (minimum gas) is the gas you and your buddy need to reach the surface together if something goes wrong at the deepest point of the dive. The gas planner calculates both.

The CNS percentage shows how much of the NOAA time limit for your ppO₂ you have used, and 100% means you have reached it. The CNS and OTU calculator marks 80% and above as caution and anything over 100% as danger. The value falls again at the surface: Shearwater computers, for example, use a 90-minute half-time, so 80% drops to 40% after 90 minutes. Follow your training agency's limit and your dive computer's warnings.

Yes. Dive tables are still taught in training and give you a backup plan if your dive computer fails. DECO 2000 is Dr. Max Hahn's table, the one taught in CMAS courses. It is designed for 0-700 m altitude with an ascent rate of 10 m/min. You can read the full DECO 2000 table or plan single and repetitive dives with the dive planner.

END (equivalent narcotic depth) is the depth at which air would feel as narcotic as your trimix does at the planned depth. Helium is not narcotic, so more helium gives a shallower END. Training agencies differ on whether oxygen counts as narcotic, so the trimix calculator shows END both ways.

The calculators work in metric units: metres, bar and litres. To switch units, use the Dive Unit Converter. It converts bar and psi, metres and feet, kilograms and pounds, °C and °F, and metric cylinder sizes to cubic feet (a 12 L at 232 bar is about 97 cu ft, an AL80 about 80). Quick figures: 1 bar is about 14.5 psi and 1 m is about 3.28 ft.