Calculate Maximum Operating Depth and PO₂ at any depth for nitrox diving planning.
| Mix (O₂%) | MOD at PO₂ 1.4 | MOD at PO₂ 1.6 |
|---|---|---|
| 21% (Air) | 56.7 m / 186 ft | 66.3 m / 218 ft |
| 28% (EAN28) | 40.0 m / 132 ft | 47.1 m / 155 ft |
| 32% (EAN32) | 33.8 m / 111 ft | 40.0 m / 132 ft |
| 36% (EAN36) | 28.9 m / 95 ft | 34.4 m / 113 ft |
| 40% (EAN40) | 25.0 m / 82 ft | 30.0 m / 99 ft |
Nitrox — formally called Enriched Air Nitrox (EANx) — is a breathing gas mixture that contains a higher percentage of oxygen than normal air. Standard air is approximately 21% oxygen and 79% nitrogen. Common recreational nitrox mixes are EAN32 (32% O₂) and EAN36 (36% O₂), though mixes from 22% up to 40% are widely used.
The main benefit of nitrox is the reduced nitrogen fraction: because you are breathing less nitrogen at any given depth, your body absorbs nitrogen more slowly, which extends your no-decompression limit (NDL) compared to diving the same profile on air. For recreational divers, this translates to longer bottom times on repetitive dives and reduced post-dive fatigue.
The trade-off is oxygen toxicity. More oxygen means a higher partial pressure of oxygen (PO₂) at depth, and PO₂ above 1.4–1.6 ATA can trigger central nervous system (CNS) oxygen toxicity, potentially causing convulsions with little or no warning. This is why nitrox divers must understand and respect the Maximum Operating Depth of their specific mix.
Most recreational nitrox courses are offered by PADI, SSI, NAUI, and other agencies and can be completed in a day. A nitrox certification qualifies you to plan and execute dives using enriched air up to 40% oxygen.
This nitrox calculator has two modes, accessible via the tabs above. Select your unit system (metric or imperial) with the toggle, then choose the appropriate mode:
The Maximum Operating Depth is the deepest point at which a specific nitrox mix can be safely breathed without the partial pressure of oxygen exceeding a predetermined limit. The formulas used are:
For example, EAN32 (FO₂ = 0.32) with a PO₂ limit of 1.4 gives a MOD of ((1.4 ÷ 0.32) − 1) × 10 = 33.75 m, rounded to 33.8 m. This means you must ascend above 33.8 m if breathing EAN32 with a 1.4 ATA limit.
The 1.4 ATA limit is the standard recreational working limit recommended by virtually all training agencies. The 1.6 ATA limit may be used only during decompression stops, when the diver is stationary and closely monitored — it should never be used as a bottom gas limit.
According to Dalton's Law of Partial Pressures, the total pressure of a gas mixture equals the sum of the partial pressures of its component gases. As you descend, the absolute pressure of the water column increases, and so does the partial pressure of each gas in your breathing mix.
At the surface, ATA = 1.0. At 10 m (33 ft) ATA = 2.0; at 20 m (66 ft) ATA = 3.0. Breathing EAN32 at 20 m gives PO₂ = 0.32 × 3.0 = 0.96 ATA — well within safe limits. At 34 m (112 ft), ATA = 4.4, so PO₂ = 0.32 × 4.4 = 1.408 — right at the recreational limit.
CNS oxygen toxicity risk rises steeply above 1.4 ATA and becomes severe above 1.6 ATA. Symptoms include visual disturbances, ringing in the ears, nausea, and convulsions. Because convulsions underwater are almost always fatal (the regulator is expelled), divers must never exceed their MOD.