Nitrox MOD and ppO2 Calculator
Calculate maximum operating depth and oxygen partial pressure for any nitrox mix.
Find the MOD for your chosen ppO2 limit, 1.4 or 1.6 bar.
Why Oxygen Becomes Dangerous at Depth
At the surface, breathing air (21% oxygen) produces an oxygen partial pressure of about 0.21 bar. As you descend, pressure increases by 1 bar per 10 meters of depth, and the partial pressure of each gas rises proportionally. Pure oxygen hits the 1.6 bar toxicity limit at just 6 meters, where ppO2 = 1.0 × (6/10 + 1) = 1.6 bar. Air, being only 21% oxygen, does not reach that same 1.6 bar until about 66 meters. The richer the oxygen mix, the shallower its safe limit.
Oxygen toxicity can cause convulsions underwater with no warning signs. The working limit for most dive agencies is 1.4 bar ppO2, and the absolute (contingency) limit is 1.6 bar ppO2. Most sport divers stay at or below 1.4 bar.
The Formula
Oxygen partial pressure at depth: ppO2 = fraction_O2 x (depth/10 + 1)
Maximum operating depth (meters): MOD = (ppO2_limit / fraction_O2 - 1) x 10
For EAN32 (32% oxygen) with a 1.4 bar limit: MOD = (1.4 / 0.32 - 1) x 10 = (4.375 - 1) x 10 = 33.75 meters.
EAN32 vs EAN36
EAN32 (32% O2) is the most common nitrox blend. It extends no-decompression limits compared to air but limits maximum depth to about 34m at the working limit. EAN36 further extends NDL at shallow depths but limits depth to about 29m. No significant benefit using nitrox below its MOD versus just using air.
Oxygen Analyser
Always analyze your own tank before diving. Never dive based on what the label says. Blending errors happen. A 1% error in blend changes your MOD by roughly 1 to 2 meters.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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