Nitrox safety: what is MOD?
What is a MOD?
Your MOD — Maximum Operating Depth — is the deepest you should breathe a given tank. Nitrox (enriched air) has more oxygen than normal air, and under the pressure of depth too much oxygen becomes toxic — so every mix has a depth past which it is no longer safe to breathe. The rule is simple: plan to stay shallower than your MOD, and never go past it. This app works out that depth from your oxygen percentage so you can plan your dive and label your cylinder. It's a planning aid, not a dive computer — always analyze your own gas and dive your training.
How MOD is calculated
MOD (m) = dpa × ( PPO₂ / FO₂ − 1 )
dpa = 10 m / 33 ft · fresh 10.3 m / 34 ft
dpa = 10 m / 33 ft · fresh 10.3 m / 34 ft
FO₂Oxygen fraction — 32% is written as 0.32.
PPO₂The oxygen partial pressure you plan to — set by your planning method below.
WaterSalt water uses 10 m / 33 ft per atmosphere; fresh water is ~3% more (10.3 m / 34 ft).
Two things move the number: a lower PPO₂ method gives a shallower (more conservative) limit; fresh water gives a slightly deeper one (~1 m on a 33 m MOD).
Before you dive
Analyze your gas and confirm the oxygen reading yourself.
Label the cylinder with the mix and its MOD.
Set your dive computer to the analyzed mix.
Dive within your certification, training and local rules.
Learn more
Trusted references to deepen your understanding of nitrox and MOD. These open in your browser.
This app provides simple nitrox gas calculations. It is not a dive computer, decompression planner, or substitute for certified scuba training. Always analyze your gas, label your cylinder, set your dive computer, and follow your training, local rules, and dive plan.