Nitrox MOD & Best Mix Calculator

How it works

MOD (m) = (ppO₂ limit ÷ FO₂ − 1) × 10; best FO₂ = ppO₂ limit ÷ (depth/10 + 1); EAD = ((1 − FO₂) × (depth/10 + 1) ÷ 0.79 − 1) × 10; CNS % = time ÷ NOAA limit at that ppO₂

Breathing oxygen-enriched air reduces the nitrogen you absorb — so no-decompression limits lengthen — at the price of oxygen toxicity if the partial pressure climbs too high. Every mix therefore has a maximum operating depth where ppO₂ reaches the agreed limit, normally 1.4 bar for the working part of a dive and 1.6 as an emergency ceiling. Turning the formula round gives the “best mix” for a planned depth: the richest blend that stays under the limit, which yields the longest bottom time. The equivalent air depth converts a nitrox dive into the air depth with the same nitrogen loading so air tables can be used, and the NOAA oxygen-exposure table tracks how much of the day’s CNS “oxygen clock” a dive uses. Tank analysis and labelling are non-negotiable.

Worked example

EAN32 at a 1.4 bar limit: MOD 33.8 m (40 m at 1.6). At 30 m the ppO₂ is 1.28 bar, the equivalent air depth is 24.4 m, and 40 minutes uses about 22% of the CNS clock.

Frequently asked questions