Nitrox MOD & Best Mix Calculator
Maximum operating depth, best mix and equivalent air depth — the three nitrox numbers, with a depth diagram.
Nitrox requires certification, and this page is not it. These are the standard formulas so you can check a plan and understand where the numbers come from. Analyse every cylinder yourself before every dive and set your computer to what you measured, not what you ordered.
Planning the other way round
MOD by mix
| Mix | At 1.4 ata | At 1.6 ata |
|---|---|---|
| Air (21%) | 56.7 m / 187 ft | 66.2 m / 218 ft |
| EAN28 | 40.0 m / 132 ft | 47.1 m / 156 ft |
| EAN32 | 33.8 m / 111 ft | 40.0 m / 132 ft |
| EAN36 | 28.9 m / 95 ft | 34.4 m / 114 ft |
| EAN40 | 25.0 m / 82 ft | 30.0 m / 99 ft |
| EAN50 | 18.0 m / 59 ft | 22.0 m / 73 ft |
What the three numbers mean
Maximum Operating Depth is where the oxygen partial pressure reaches your chosen limit. Past it the risk is an oxygen toxicity seizure, which underwater is almost always fatal. Unlike nitrogen narcosis it gives no warning and no chance to feel your way toward it, which is why it is a hard limit rather than a guideline.
Best mix is the same formula run backwards: the richest blend still safe at a planned depth. Richer means more oxygen, which means shallower — the two move in opposite directions, and that is the trade nitrox is built on.
Equivalent Air Depth is the air depth with the same nitrogen partial pressure. It is what the decompression tables were built around, and it is where the actual benefit shows up: on EAN32 at 30 metres your body is loading nitrogen as though you were at 24, which is why your no-decompression time is longer.
Before every nitrox dive
- Analyse every cylinder yourself, immediately before the dive, and log the reading. The fill you were promised and the fill you received are different claims, and only one of them is yours to verify.
- Set your computer to the mix you actually analysed, not the mix you ordered. A computer set to air on a nitrox dive is merely conservative; a computer set to nitrox on an air fill is dangerous.
- Nitrox extends your no-decompression time. It does not extend your depth limit, reduce your gas consumption, or make you less tired — the last of those is widely believed and not supported.
- Exceeding the maximum operating depth risks an oxygen toxicity seizure, which underwater is almost always fatal. This is the one limit with no warning signs and no margin to feel your way toward.
How to use
- Set the oxygen fraction and your partial-pressure limit for a maximum operating depth.
- Or set a planned depth to get the richest mix that is still safe there.
- Read the equivalent air depth to see where the longer bottom time actually comes from.
Frequently asked questions
What is the MOD of EAN32?
About 33 metres or 111 feet at the 1.4 ata working limit, and exactly 40 metres at the 1.6 contingency limit. Richer mixes have shallower limits — that is the trade nitrox is built on, and it moves in the opposite direction to what beginners expect.
Why is the depth limit so absolute?
Because exceeding it risks an oxygen toxicity seizure, which underwater is almost always fatal. Unlike nitrogen narcosis it gives no warning and no chance to feel your way toward it, so there is nothing to notice before it happens.
What is equivalent air depth?
The air depth with the same nitrogen partial pressure — what the decompression tables were built around. On EAN32 at 30 metres your body loads nitrogen as though it were at 24, and that gap is exactly where the longer no-decompression time comes from.
Does nitrox reduce my air consumption or tiredness?
No to both. It extends no-decompression time and nothing else. The claim that it makes you less tired is widely believed and not supported, and it certainly does not extend your depth limit — it shortens it.
Do I need to analyse the cylinder myself?
Yes, every time, immediately before the dive, and log the reading. The fill you were promised and the fill you received are different claims. Then set your computer to what you measured — a computer set to nitrox on an air fill is genuinely dangerous.
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