Diving guide
Best Nitrox Mix for Any Depth
Calculate the richest theoretical Nitrox mix for a planned depth and PPO2 limit, then learn how to apply the result safely.
Maximum Operating Depth starts with a gas mix and finds its depth limit. Best-mix planning reverses the question: for the deepest point of a planned dive, what is the highest oxygen fraction that stays within the chosen PPO₂ limit?
How the best-mix calculation works
Divide the selected PPO₂ limit by absolute ambient pressure at the target depth. The result is the maximum fraction of oxygen. At 30 meters, ambient pressure is approximately 4 ATA, so 1.4 ÷ 4 = 0.35, or 35% oxygen. At 100 ft, pressure is about 4.03 ATA; 1.4 ÷ 4.03 = 0.347, so the result rounds down to EAN34.
Try it: Best Nitrox Mix
Find the richest theoretical mix that stays at or below your chosen PPO₂ at the planned maximum depth.
Allowed range: 6 to 60 m.
Analyze and label every cylinder. Set your computer to the measured mix—not the planned mix—and follow your Nitrox training.
Why you round down
If the calculation produces 32.7%, calling it EAN33 would slightly exceed the selected ceiling at the target depth. Rounding down to EAN32 preserves the margin. After filling, always plan from the measured oxygen percentage and confirm its MOD with the MOD calculator.
Why the safer mix is rounded down
MOD reference: common Nitrox mixes
Rounded down to the whole unit, because the convention for an oxygen limit is to round toward the shallower depth. Many agencies teach a further round-down (EAN32 as 110 ft rather than 111) — follow the figure your training specifies. Always plan from the oxygen percentage you personally analyzed, not the label on the cylinder.
| Mix | MOD at 1.4 ATA | MOD at 1.6 ATA |
|---|---|---|
| EAN32 | 33 m111 ft | 40 m132 ft |
| EAN36 | 28 m95 ft | 34 m113 ft |
| EAN40 | 25 m82 ft | 30 m99 ft |
| Air (21%) | 56 m187 ft | 66 m218 ft |
The air row is the oxygen limit only. Training limits, narcosis, gas density, gas supply, and decompression obligations all impose shallower ceilings.
The richest mix is not always the operational best
- Contingency depth: a modest depth excursion must not immediately cross MOD.
- Standard gases: a familiar, readily available mix can simplify team planning. Many shops bank EAN32 or EAN36 rather than blending every theoretical result. If the math says EAN35 and the shop hands you EAN32, that leaner standard mix is often the practical answer; recalculate its NDL and confirm its MOD.
- Whole-profile exposure: PPO₂ and cumulative oxygen exposure matter beyond the single deepest point.
- Computer settings: every diver must set the analyzed mix and agreed PPO₂ limits correctly.
A repeatable pre-dive workflow
- Choose the maximum planned depth, including an appropriate margin.
- Select the PPO₂ value required by your training and conditions.
- Calculate and round the maximum oxygen percentage down.
- Obtain, personally analyze, and label the cylinder.
- Calculate MOD again from the measured mix and configure the dive computer.
New to enriched air? Start with What is Nitrox?, then review PPO₂ safety limits before using best-mix calculations.
Sources
References and further reading
These official and specialist sources support the concepts in this guide. Always use current training materials and operational procedures for dive planning.
- NOAANOAA Diving Manual: Diving for Science and Technology (6th ed.)
Reference text for diving physics, breathing gases, decompression, and oxygen exposure.
- Divers Alert NetworkOxygen Toxicity
- PADIWhat Is Nitrox?