MOD (maximum operating depth) is the deepest a breathing gas can go before its oxygen partial pressure passes a limit: 1.4 bar while swimming and working, 1.6 bar at rest on a deco stop. EAD (equivalent air depth) is the air depth with the same nitrogen partial pressure, so this also works as an EAD calculator: enter an EANx mix (or trimix) and a depth to get both MODs, plus the PPO₂, EAD, END and gas density there.
The formula
Worked example
A diver plans a dive on EANx32 (32 % oxygen) to 30 m in EN 13319 water and wants the deepest point the mix allows at the 1.4 bar working limit.
- 1The ambient pressure at which 32 % O₂ reaches 1.4 bar is 1.4 ÷ 0.32 = 4.375 bar.
- 2Take away the surface pressure and convert to depth: (4.375 − 1.01325) × 10.00 = 33.6 m.
- 3Floored to the whole metre, the working MOD is 33 m; at the 1.6 bar deco limit it is 39 m.
- 4At the planned 30 m the mix reads PPO₂ 1.28 bar, gas density 5.0 g/L, and the answer bar reads “Breathable from the surface to 33 m at PPO₂ 1.4”.
- 5EAD at 30 m: the ambient pressure is 4.013 bar, so (0.68 ÷ 0.79 × 4.013 − 1.01325) × 10.00 = 24.4 m: the depth to read in air tables for this dive's nitrogen loading.
Because Dive Kit starts from 1.01325 bar at the surface rather than a round 1 bar, every MOD in EN 13319 water sits (1.01325 − 1) × 10.00 = 0.13 m shallower than the 10 m per bar rule of thumb gives, before flooring.
In imperial: EANx36 has a working MOD of 94 ft and a deco MOD of 112 ft. At 90 ft it reads PPO₂ 1.35 bar and an EAD of 67 ft: the depth to read in air tables for its nitrogen loading.
MOD table by nitrox mix
Maximum operating depth for common mixes at three PPO₂ limits: 1.2 bar (the low end of the 1.2 to 1.3 bar working range GUE's standard gases are designed for), 1.4 bar (working) and 1.6 bar (deco stops, at rest).
| Mix | PPO₂ 1.2 (m) | PPO₂ 1.2 (ft) | PPO₂ 1.4 (m) | PPO₂ 1.4 (ft) | PPO₂ 1.6 (m) | PPO₂ 1.6 (ft) |
|---|---|---|---|---|---|---|
| Air | 47 | 154 | 56 | 185 | 66 | 216 |
| EANx25 | 37 | 124 | 45 | 150 | 53 | 176 |
| EANx28 | 32 | 107 | 39 | 130 | 47 | 154 |
| EANx30 | 29 | 97 | 36 | 119 | 43 | 141 |
| EANx32 | 27 | 89 | 33 | 110 | 39 | 130 |
| EANx34 | 25 | 82 | 31 | 101 | 36 | 121 |
| EANx36 | 23 | 76 | 28 | 94 | 34 | 112 |
| EANx40 | 19 | 65 | 24 | 81 | 29 | 97 |
| EANx50 | 13 | 45 | 17 | 58 | 21 | 71 |
| EANx80 | 4 | 15 | 7 | 24 | 9 | 32 |
| O₂ | 1 | 6 | 3 | 12 | 5 | 19 |
EN 13319 water (10.00 m per bar), from a surface pressure of 1.01325 bar, each figure floored to the whole metre or foot. That is why oxygen at 1.6 bar reads 5 m rather than the 6 m rule of thumb: the 6 m stop reads 1.61 bar, which Dive Kit's gas switch allows within its 0.03 bar tolerance.
EAD by mix and depth
Equivalent air depth for three common nitrox mixes, in metres and feet: the air depth with the same nitrogen partial pressure. A dash marks a depth past the mix's 1.4 bar working MOD.
| Depth (m) | Depth (ft) | EANx28 EAD (m / ft) | EANx32 EAD (m / ft) | EANx36 EAD (m / ft) |
|---|---|---|---|---|
| 18 | 59 | 15.5 / 51 | 14.1 / 46 | 12.7 / 42 |
| 21 | 69 | 18.2 / 60 | 16.7 / 55 | 15.1 / 50 |
| 24 | 79 | 21.0 / 69 | 19.2 / 63 | 17.5 / 57 |
| 27 | 89 | 23.7 / 78 | 21.8 / 72 | 19.9 / 65 |
| 30 | 98 | 26.4 / 87 | 24.4 / 80 | – |
| 33 | 108 | 29.2 / 96 | 27.0 / 89 | – |
| 36 | 118 | 31.9 / 105 | – | – |
| 40 | 131 | – | – | – |
EN 13319 water, nitrogen only, from a surface pressure of 1.01325 bar.
How Dive Kit computes this
Depth turns into pressure from the real surface pressure of 1.01325 bar rather than a round 1 bar, with the water type you pick: EN 13319 (10.00 m per bar, the app's default), salt, fresh or Red Sea. That puts every MOD shallower than the 10 m per bar rule of thumb by one fixed amount, the extra 0.01325 bar of surface pressure turned into depth (the worked example works it out); the conventions page explains why.
A MOD is converted to feet from the unrounded metres, then floored to the whole metre or foot, so the figure shown never lies past the true MOD. Flooring in metres first would read a feet MOD a couple of feet short.
PPO₂ reads within 1.4, deco only past 1.4 bar and too high past 1.6 bar, each with the 0.03 bar tolerance the deco planner switches gases by. Gas density is judged at one decimal against 5.2 g/L (elevated) and 6.2 g/L (too dense), the limits the planner warns on. A mix with too little oxygen to breathe at the surface (PPO₂ under 0.16 bar) shows the shallowest depth it can be breathed.
The app's Gas Specs screen adds a no-stop time (NDL) for the mix at that depth, which needs the decompression engine and is left off this page. O₂ counted as narcotic mirrors the app's Settings › Diving toggle and changes END only.
The app screen for this tool is documented in the user guide: Gas Specs in the Dive Kit guide.
How do you calculate the MOD of a nitrox mix?
Should I plan with a PPO₂ of 1.4 or 1.6?
How do I calculate EAD?
What is the difference between EAD and END?
Why is this MOD shallower than my nitrox table?
Does helium change the MOD?
Sources
- GUE General Training Standards, Policies, and Procedures, Version 10.4 (section 2.2) : A planned PPO₂ of at most 1.4 bar while working and 1.6 bar while resting, and an END limit of 30 m (100 ft) with helium the only non-narcotic gas.
- InDepth: Standard Gases, the Simplicity of Everyone Singing the Same Song : GUE's standard gases are designed for a working PPO₂ of 1.2 to 1.3 bar; about 1.4 bar is the usual working maximum elsewhere.
- Anthony and Mitchell (2016), Respiratory Physiology of Rebreather Diving, in Rebreathers and Scientific Diving Proceedings : The 5.2 g/L ideal maximum and 6.2 g/L absolute maximum gas density.
- EN 13319:2000, Diving accessories: depth gauges and combined depth and time measuring devices (BSI) : The depth-gauge standard the default EN 13319 water type is named after. The standard's text is paywalled; this page lists it.
Last updated 30 October 2026
