Education

Best Nitrox Mix for Your Dive

How to calculate the optimal nitrox mix for a given depth — maximizing bottom time while staying within safe oxygen limits.

Nitrox is not one gas — it is a spectrum. EAN28, EAN32, EAN36, or any other blend can be optimized for your specific dive. The best mix formula tells you the highest safe oxygen percentage for your planned depth.

What it is

The “best mix” calculation works backwards from your maximum depth and ppO2 limit to find the richest nitrox blend you can safely use. A richer mix (more oxygen, less nitrogen) means less decompression obligation and more bottom time — but you are limited by how much oxygen is safe at your planned depth.

The formula

O2% = (ppO2max / P) × 100
VariableMeaning
O2%Optimal oxygen percentage for the mix
ppO2maxMaximum allowable oxygen partial pressure (typically 1.4 bar)
PAbsolute pressure at your maximum planned depth in bar

Worked example

What is the best nitrox mix for a dive to 28 meters with a ppO2 limit of 1.4 bar?

Step by step

P = (28 / 10) + 1 = 3.8 bar
O2% = (1.4 / 3.8) × 100
O2% = 0.368 × 100
O2% = 36.8% → round down to 36%

For a 28-meter dive with a 1.4 bar ppO2 limit, the best nitrox mix is EAN36 (36% oxygen). This gives you the maximum reduction in nitrogen loading while staying within safe oxygen limits at your planned depth.

Why it matters

Using the optimal mix for your depth has real practical benefits:

  • More bottom time: Compared to air, EAN36 at 28 meters gives you significantly longer no-decompression limits
  • Reduced nitrogen loading: Less nitrogen absorption means shorter surface intervals and reduced DCS risk
  • Safety margin: If you stay shallower than planned, your ppO2 is even further from the limit

Practical considerations

In practice, you will not always get exactly the mix the formula suggests:

  • Available mixes: Many dive shops offer only standard mixes like EAN32 or EAN36. The formula tells you the theoretical optimum; you may need to use the next lower standard blend
  • Always round down: If the formula gives you 36.8%, use 36%. Rounding up increases your ppO2 beyond your chosen limit at the planned depth
  • Multiple depths: If your dive profile includes a range of depths, calculate for the deepest point. The mix must be safe at your maximum depth
  • Deco mixes: For decompression stops at fixed shallow depths, a different (richer) best mix applies. Some technical divers carry a separate deco bottle with EAN50 or even pure oxygen for shallow stops

Relationship to MOD

Best mix and MOD are inverse calculations:

  • MOD: “Given this mix, what is the deepest I can go?”
  • Best mix: “Given this depth, what is the richest mix I can use?”

If you calculate the MOD of your best mix result, you will get back your original target depth (or very close to it, depending on rounding).

Safety considerations

  • Never round up: Always round the oxygen percentage down to the nearest whole number
  • Analyze your gas: Always verify the actual oxygen content of your filled tank with an oxygen analyzer. Do not rely on what was requested or what the label says
  • Plan for maximum depth: Calculate best mix for the deepest point of your planned profile, not the average depth
  • Consider a ppO2 buffer: Some divers use 1.3 bar instead of 1.4 bar for additional margin

Sources