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Nitrox MOD Calculator

Enter your oxygen percentage and maximum ppO₂ to find the safe maximum depth.

How it is calculated

The maximum operating depth is the depth at which the oxygen partial pressure reaches your chosen limit. The tool computes MOD (m) = (ppO₂ / FO₂ − 1) × 10, where ppO₂ is the limit in ata (roughly bar) and FO₂ is the oxygen share as a fraction, i.e. the percentage you type divided by 100. The factor 10 assumes that every 10 m of seawater adds one atmosphere on top of the 1 ata at the surface. The depth in feet is the metre value times 3.28084. Both are shown to one decimal. The oxygen percentage must be between 21 (air) and 100, and the ppO₂ limit above 0 and at most 2.0 ata; values outside those ranges show no result. Type the percentage (32), not a fraction (0.32).

Worked example

For EAN32 (32% oxygen) with a working limit of 1.4 ata: FO₂ = 32 / 100 = 0.32. Then 1.4 / 0.32 = 4.375, and subtracting the 1 ata of surface air pressure leaves 3.375 ata of water pressure, which at 10 m per atmosphere is 33.75 m. The tool rounds this to 33.8 m (110.7 ft). A diver using this mix and this limit should stay shallower than that depth.

Try this example →

Common mistakes

  • Typing the oxygen fraction as 0.32 instead of 32. The field takes a percentage and accepts only 21 to 100, so 0.32 shows no result.
  • Entering the limit as a percentage or as a depth. The ppO₂ field is a pressure in ata, for example 1.4 or 1.6, and anything above 2.0 is rejected.
  • Rounding the result up. 33.75 m shown as 33.8 m is not a reason to plan a dive to 34 m; round down when you plan.
  • Using the result for a gas that is not plain nitrox. Only the oxygen share matters here, and nothing about helium, decompression or oxygen exposure time is included.
  • Choosing a limit that is too low for the mix. With 100% oxygen and a limit of 1.0 ata the depth is 0.0 m, meaning the limit is already reached at the surface.

About this calculator

The maximum operating depth (MOD) of a nitrox blend is the deepest a diver can descend on that gas mix without the oxygen partial pressure (ppO₂) climbing past a chosen safety limit. It is calculated as MOD = ((ppO₂ limit ÷ FO₂) − 1) × 10, where FO₂ is the fraction of oxygen in the mix (you enter it as a percentage, e.g. 32 for EAN32, and the tool divides by 100) and depth comes out in meters of seawater using the 10 m-per-atmosphere rule.

Recreational nitrox courses from PADI, NAUI, and TDI teach divers to plan dives to a working ppO₂ limit of 1.4 ATA, with 1.6 ATA reserved as an absolute contingency maximum for the bottom portion of a dive — beyond that, oxygen toxicity risk (CNS oxtox) rises sharply. Because a richer mix like EAN36 lets a diver breathe more oxygen at any given depth than air, its MOD is shallower than air's; divers and dive planners use this calculator before every enriched-air dive to confirm the planned depth stays within the mix's safe limit.

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