The deepest safe depth for a gas is 10 × (1.4 ÷ oxygen fraction − 1) metres: about 57 m on air and about 34 m on nitrox 32.
Oxygen turns toxic above a partial pressure of about 1.4 bar when working (1.6 bar is the contingency ceiling), and the partial pressure is the oxygen fraction times the absolute pressure, which is 1 bar plus 1 bar for every 10 m. Air is 21% oxygen, so 1.4 bar arrives at a pressure of 6.67 bar, 56.7 m. Nitrox, with more oxygen, reaches the same limit shallower: 32% gives 33.8 m, 36% gives 28.9 m, and pure oxygen only 4 m.
The limit is one of several, and not the first to matter: nitrogen narcosis impairs judgement well above 57 m on air, and most training agencies set recreational limits at 40 m. It uses the diver’s rounding of 10 m of seawater to a bar. Gas planning belongs to your training and a qualified instructor, not to a formula on a web page.
Estimate with the rule, then check it against the calculator that models it properly.
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