Work out the pure O₂ pressure for a partial-pressure fill and the MOD of your mix, in bar and metres.
How to calculate a Nitrox mix
Partial-pressure blending, step by step
Partial-pressure blending is the most common way to make Nitrox in a dive centre: you first inject pure oxygen into the cylinder, then top it off with air from the compressor. The amount of oxygen to inject depends only on the final pressure you want and the oxygen fraction you are aiming for.
The calculation follows Dalton's law. In the finished cylinder, the oxygen already present in the residual gas, plus the pure oxygen you inject, plus the 21 % oxygen carried by the air top-up, must add up to the oxygen content of the target mix.
P_O₂ = [ P_final × (FO₂ − 0.21) − P_res × (FO₂_res − 0.21) ] ÷ 0.79
Starting from an empty cylinder the residual term drops out and the formula reduces to P_O₂ = P_final × (FO₂ − 0.21) ÷ 0.79. For EAN32 at 200 bar that gives 28 bar of pure oxygen before the air top-up.
Topping off a cylinder that is not empty
A cylinder coming back from a dive still holds gas, and that gas already contains oxygen. Ignoring it produces a mix that is too rich. Switch the calculator to "With residual", enter the pressure left in the tank and its oxygen fraction, and the residual term above is taken into account.
Some combinations have no physical solution. If the residual is richer than the target, even an air-only top-up would overshoot — you have to vent the cylinder first. If the target is very rich and the residual leaves too little room, there is no space for the required oxygen. The calculator detects both cases and tells you which one you hit instead of showing a misleading number.
MOD — the depth limit of your mix
The Maximum Operating Depth is the depth at which the oxygen partial pressure of your mix reaches the limit you set. Beyond it, the risk of central nervous system oxygen toxicity — which can cause a convulsion underwater — rises sharply.
Ambient pressure is 1 bar at the surface and increases by 1 bar every 10 metres, so the oxygen partial pressure at depth d is (1 + d/10) × FO₂. Solving for the depth at which that reaches your ppO₂ limit gives the MOD.
MOD = (ppO₂max ÷ FO₂ − 1) × 10
Recreational diving normally uses a 1.4 bar limit for the working part of the dive, and 1.6 bar is reserved for decompression stops under supervision. EAN32 at 1.4 bar gives a MOD of 33.8 m.
Nitrox MOD reference table
Maximum operating depth for the most common recreational mixes, at both usual oxygen partial pressure limits.
| Mix | ppO₂ 1.4 bar | ppO₂ 1.6 bar |
| EAN28 | 40.0 m | 47.1 m |
| EAN32 | 33.8 m | 40.0 m |
| EAN36 | 28.9 m | 34.4 m |
| EAN40 | 25.0 m | 30.0 m |
| EAN50 | 18.0 m | 22.0 m |
Depths in metres of sea water, computed with the formula above and rounded to one decimal.
Frequently asked questions
How much pure oxygen do I need for EAN32 at 200 bar?
Starting from an empty cylinder, 28 bar of pure oxygen, then top off with air to 200 bar. From a cylinder still holding 50 bar of EAN32, you only need 21 bar.
Why is the oxygen partial pressure limited to 1.4 bar?
Above roughly 1.4 bar of oxygen partial pressure, the risk of central nervous system oxygen toxicity becomes significant, and a convulsion underwater is usually fatal. Recreational agencies set 1.4 bar for the active part of the dive and allow 1.6 bar only for decompression stops.
What is the MOD of Nitrox 32?
At a 1.4 bar oxygen partial pressure limit, the maximum operating depth of EAN32 is 33.8 metres. At 1.6 bar it is 40 metres.
Can I use this calculator with a cylinder that still has gas in it?
Yes. Switch to "With residual" and enter the remaining pressure and its oxygen fraction. The calculator solves the full oxygen balance and warns you when the combination is physically impossible.
Does the calculator account for temperature and gas compressibility?
No. It uses the ideal gas approximation and Dalton’s law of partial pressures, which is the standard method taught in Nitrox blending courses. Real cylinders heat up while filling, so let the cylinder settle and always analyse the final mix.
Do I still need to analyse my mix with an oxygen analyser?
Always. A calculation tells you what should be in the cylinder, not what is. Analysing the mix with a calibrated oxygen analyser before every dive is mandatory practice, and this tool does not replace Nitrox training.