Scuba diving calculators
Nitrox, gas planning and dive physics: the calculations taught in diver training, with every formula shown. Free, no sign-up, everything is computed in your browser.
Educational tool. It does not replace your dive computer, official dive tables or training with an instructor. Always double-check your calculations.
Maximum operating depth (MOD) Nitrox
Depth not to exceed with a gas mix, for a given PPO2.
Best mix Nitrox
The richest Nitrox you can use at a given depth.
PPO2, PPN2 and EAD Dalton
Partial pressures (Dalton's law) and equivalent air depth of a Nitrox mix.
Oxygen toxicity CNS %, OTU
Share of the oxygen dose tolerated by the central nervous system (NOAA limits) and pulmonary dose, for a Nitrox dive.
Air autonomy Air
Time at a given depth before reaching your reserve, excluding descent, ascent and stops.
My SAC rate SAC
Work out your surface air consumption from a dive you have already done.
MN90 tables (FFESSM) MN90
Decompression stops, total ascent time and repetitive group for an air or Nitrox dive, then a repetitive or consecutive dive, using the French federation (FFESSM) tables based on the French Navy MN90 tables. Sea water, sea level, metres only.
First dive
Source: FFESSM air diving tables based on the French Navy MN90 tables, user guide by J.-L. Blanchard and F. Imbert (July 2005 edition). Training tool: when diving, use your official tables or your dive computer.
US Navy dive tables (Revision 7) US Navy
No-decompression limits (Table 9-7), repetitive dives (Table 9-8) and in-water air decompression stops (Table 9-9) from the US Navy Diving Manual, Revision 7. The tables are in feet of sea water (fsw): a depth in metres is converted, then rounded up to the next table depth.
First dive
Source: U.S. Navy Diving Manual, Revision 7, chapter 9 (Tables 9-7, 9-8 and 9-9), a US government work in the public domain. Otoolia is not affiliated with or endorsed by the US Navy.
Bühlmann ZH-L16C calculator GF
Stops computed with Bühlmann's algorithm (ZH-L16C, 16 compartments) and gradient factors. This is a calculator, not an official table: its results do not replace tables or a dive computer.
100/100: original Bühlmann
Algorithm published by A. A. Bühlmann (ZH-L16C coefficients), gradient factors by E. Baker. Results checked against the open-source DecoTengu library. Descent 20 m/min, ascent 10 m/min, stops every 3 m.
Dive physics
The calculations of diver training exams: pressure, Boyle's law and Archimedes.
Absolute pressure
Boyle's law (Mariotte)
Archimedes' principle
Formulas used
P is the absolute pressure in bar (ata) at the given depth: P = 1 + depth / k, with k = 10 m in salt water
(convention of dive tables and training), 10.3 m in fresh water, 33 ft and 34 ft in imperial units.
Surface pressure is taken as 1 bar: at altitude (mountain lakes) it is lower and specific procedures apply.
| Calculation | Formula | Example |
|---|---|---|
| MOD | (max PPO2 / FO2 − 1) × k, rounded down | Nitrox 32%, 1.4 bar: 4.375 bar → 33.7 m (111 ft) |
| Best mix | FO2 = max PPO2 / P, rounded down | 30 m, 1.4 bar: 1.4 / 4 = 35% |
| Dalton's law | partial pressure = fraction × P | Nitrox 32% at 30 m: PPO2 = 0.32 × 4 = 1.28 bar |
| EAD | (P × FN2 / 0.79 − 1) × k, rounded up | Nitrox 32% at 30 m: 24.43 → 24.5 m |
| CNS % | time / NOAA limit(PPO2) × 100, limit interpolated in the NOAA table (1.4 bar: 150 min; 1.6 bar: 45 min) | Nitrox 32% at 30 m, 40 min: PPO2 1.28 bar → limit 186 min → 21.5% |
| OTU (pulmonary dose) | time × ((PPO2 − 0.5) / 0.5)^0.83 | 1.28 bar, 40 min: ≈ 58 OTU |
| Autonomy | cylinder volume × (start − reserve) / (SAC × P) | 15 L, 200 → 50 bar, 20 m, 20 L/min: 2,250 / 60 = 37 min |
| SAC rate | cylinder volume × (start − end) / (duration × average P) | 15 L, 200 → 70 bar, 45 min at 15 m: 17.3 L/min |
| Boyle | P1 × V1 = P2 × V2 | 10 L lift bag at 30 m brought to the surface: 40 L |
| Archimedes | apparent weight = mass − volume × density | 80 kg, 85 L in salt water (1.025): −7.1 kg, it floats |
In imperial units, a cylinder is described by its capacity (cu ft of gas) at its rated pressure (psi): usable gas = capacity × (start − reserve) / rated pressure. Depth limits and the suggested mix are rounded on the safe side; autonomy is rounded down to the minute. Gas calculations assume an ideal gas, as in diver training; above 200 bar (2,900 psi) the cylinder holds slightly less gas than calculated.
Ascent, stops and flying
- Ascent rate: follow your procedure. Most dive computers and US agencies use about 9–10 m/min (30 ft/min); the French MN90 tables use 15–17 m/min to the first stop, then 6 m/min between stops and to the surface.
- Safety stop: recommended (not mandatory) at the end of every dive, even a no-decompression dive: 3 minutes at 3–5 m (10–15 ft).
- Flying after diving (DAN guidelines): at least 12 hours after a single no-decompression dive, 18 hours after multiple dives per day or multiple days of diving, and substantially longer after decompression dives.
- Mandatory decompression stops come from official tables or a dive computer; the “MN90 tables” card above gives them from the FFESSM tables.
Reading the MN90 tables
- Reading: maximum depth reached and duration from leaving the surface to leaving the bottom, in whole minutes. If a value is not in the table, use the next higher value; interpolation is forbidden.
- Ascent: 15–17 m/min to the first stop, then 6 m/min (30 s between stops and from the last stop to the surface). The DTR (total ascent time) includes these ascent times (Table IV).
- At most two dives per 24 hours. Diving beyond 60 m is forbidden; the 62 and 65 m tables are emergency tables.
- Repetitive dive (surface interval of 15 min to 12 h): the first dive's group and the interval (next lower value) give the residual nitrogen (Table I); that value and the second dive's depth give the penalty time (Table II); read the table with the fictitious duration = penalty + duration. Consecutive (under 15 min): a single dive, durations added, deepest depth of the two.
- Nitrox: enter the table with the equivalent depth
PE = (P + 10) × FN2 / 0.79 − 10; maximum PPO2 1.6 bar, maximum duration 2 h; stops are made at the depths shown. - Incidents (fast ascent, missed stop, slow ascent), altitude (fictitious depth
P × 1013 / H) and oxygen stops: follow the procedures of the FFESSM user guide; they are not calculated here.
US Navy tables
- Reading: depth and bottom time (from leaving the surface to leaving the bottom) rounded up to the next table value, in feet of sea water (1 m ≈ 3.28 fsw).
- Ascent at 30 ft/min (9 m/min); stops are at 20 fsw (6 m), 30 fsw (9 m)… and their times include travel, except for the first stop.
- Repetitive dive: the end-of-dive group and the surface interval give a new group (Table 9-8), then a residual nitrogen time added to the second dive's bottom time. Surface interval under 10 min: a single dive.
- The “oxygen decompression recommended” and “exceptional exposure” zones of Table 9-9 are flagged: they are beyond recreational diving.
Bühlmann calculator
- 16-compartment ZH-L16C model (nitrogen), published by A. A. Bühlmann, with gradient factors: GF low sets the depth of the first stop, GF high the margin when surfacing. 100/100 is the original model; lower values add margin.
- This is a calculator: it does not replace official tables or a dive computer, and its results differ from printed “Bühlmann” tables.
Nitrox guidelines
- A PPO2 of 1.4 bar is the recommended limit for recreational diving; 1.6 bar is the absolute maximum (decompression, contingency).
- The MOD depends on the mix actually in your cylinder: always analyse your cylinder before diving.
- The EAD lets you use air tables with Nitrox; it does not replace Nitrox training.
Glossary
- MOD: Maximum Operating Depth of a gas mix.
- PPO2 / PPN2: partial pressure of oxygen / nitrogen.
- FO2 / FN2: fraction of oxygen / nitrogen in the mix.
- EAD: Equivalent Air Depth.
- SAC / RMV: surface air consumption, the volume of gas breathed per minute at surface pressure.
- DCS: decompression sickness.