Nitrox MOD Calculator
Maximum operating depth and equivalent air depth for any nitrox mix.
What it does
Enriched air buys bottom time by trimming nitrogen, but it charges oxygen in return. The maximum operating depth is the line where your chosen oxygen partial pressure limit runs out, and the equivalent air depth tells table users what air depth the mix behaves like.
The tool above takes your mix's oxygen fraction, a ppO2 ceiling of 1.2 to 1.6, and your planned depth. It returns the MOD, the equivalent air depth at plan, the actual oxygen partial pressure at plan, and whether the plan fits inside the limit.
The defaults plan EAN32 to 100 feet under the standard 1.4 working limit: MOD 111.4 feet, EAD 81.5 feet, actual ppO2 1.29, all clear. The metric mirror at 30 meters gives 33.8 meters, 24.4 meters, and 1.28.
The rules follow the working consensus cited in OSHA's 2003 enriched air rulemaking, CMAS guidance of 1.4 bar for the working phase and 1.6 bar in decompression, and the NOAA Diving Manual formula book's equation forms.
How to use
- Pick imperial feet or metric meters.
- Enter the oxygen fraction of your mix, like 32 for EAN32.
- Choose the ppO2 limit; keep 1.4 for working dives.
- Enter your planned depth to check it against the limit.
- Read the MOD first, then the EAD for table use.
Formula
- FO2
- Oxygen fraction of the mix, 0.32 for EAN32.
- ppO2
- Oxygen partial pressure in atmospheres absolute; 1.4 ATA is the customary working ceiling.
- MOD
- The depth where ppO2 reaches your limit on this mix.
- EAD
- The air depth with the same nitrogen loading as your nitrox at the real depth.
Every 33 ft of seawater, or 10 m, adds one atmosphere, so ppO2 grows linearly with depth. The EAD formula works off nitrogen only and the 0.79 fraction of nitrogen in air; shallower than about 20 ft or 6 m the equation can run negative, which just means the mix carries less nitrogen than air there.
Worked example
Example: EAN32 to 100 ft, and the metric mirror
Defaults: imperial, 32 percent oxygen, 1.4 ppO2 limit, 100 ft.
- MOD: (1.4 / 0.32 - 1) x 33 = 3.375 x 33 = 111.4 ft.
- ppO2 at 100 ft: 0.32 x (100 / 33 + 1) = 1.29 ATA, inside the limit.
- EAD: (0.68 x 133 / 0.79) - 33 = 81.5 ft.
- So at 100 ft, EAN32 breathes like air at about 81 ft.
- Metric: 30 m gives MOD 33.8 m, EAD 24.4 m, ppO2 1.28.
Reference table
| Mix | Oxygen | MOD (ft) | MOD (m) |
|---|---|---|---|
| EAN32 | 32% | 111 | 33.8 |
| EAN36 | 36% | 95 | 28.9 |
| EAN40 | 40% | 83 | 25.0 |
| EAN50 | 50% | 59 | 18.0 |
| Air | 21% | 187 | 56.7 |
Limits
- Oxygen math only; nitrogen plans need tables or a computer.
- Does not model CNS or OTU accumulation.
- Assumes sea level surface pressure.
- Altitude diving shifts the depth arithmetic.
Common mistakes
- Diving the MOD as a target instead of a ceiling.
- Using 1.6 at depth all dive long.
- Confusing EAD with an actual depth limit.
- Forgetting to analyze the tank's real mix.
- Planning at altitude with sea level formulas.
Try it yourself
Quick checks. Answers sit tucked under each question.
EAN36 at 1.4 ATA?
MOD = (1.4 / 0.36 - 1) x 33 = 95.3 ft, about 28.9 m.
EAN32 at 80 ft?
ppO2 = 0.32 x (80 / 33 + 1) = 1.10 ATA, well inside 1.4.
EAN36 at 110 ft?
ppO2 = 0.36 x (110 / 33 + 1) = 1.56 ATA, over the 1.4 working limit.
Questions
What is MOD?
Maximum operating depth: the deepest point where your mix keeps oxygen partial pressure at or under the limit you chose.
Why 1.4 ATA?
It is the widely adopted working limit that keeps oxygen exposure low for the working phase of a dive; 1.6 is reserved for restful decompression.
What is EAD used for?
Tables built for air: entering the EAD instead of your real depth gives the nitrogen loading of your nitrox dive.
Is nitrox safer than air?
It lowers nitrogen and extends no-decompression limits at moderate depths, but it adds oxygen limits and requires analyzing your own tank.
Can I dive deeper on EAN36 than EAN32?
The opposite: richer mixes run out of oxygen allowance shallower, though they stretch time at moderate depths.
Does this replace a dive computer?
No; it is planning arithmetic for a single gas, not a computer's multi-level model.