Calculator
Best mix calculator — Optimal gas for the dive
Best mix is the richest gas you can breathe at a target depth without exceeding your ppO₂ ceiling, while keeping the equivalent narcotic depth (END) inside an acceptable window. Set depth, ppO₂, and your END limit — the calculator returns the highest-O₂, lowest-helium mix that satisfies both, plus a few alternatives.
How best mix is selected
Two constraints bracket the answer. The ppO₂ cap fixes a maximum oxygen fraction (FO₂ = ppO₂ / P_ambient). The END limit caps how narcotic the breathable inert can be — when nitrogen alone is too narcotic, helium replaces some of it.
FO₂ = ppO₂ ÷ P_ambient END(m) ≤ END_limit → P_amb × (FO₂ + FN₂) ≤ P(END_limit) FO₂ oxygen fraction selected FHe helium fraction (raises N2 enough to satisfy END)
Worked example: 50 m, 1.4 bar ppO₂, 30 m END limit (salt water). The maximum FO₂ is about 0.23 — close to air. To bring END below 30 m at 6 ATA you need roughly 35% helium, leaving ~21/35 Trimix as the standard choice.
Why discrete mixes
Most blenders and standard fills cluster around a small set of mixes (21/35, 18/45, 15/55, 10/70) because partial-pressure blending makes them easy to mix and the spacing covers a wide depth range. The calculator presents the standard mix that fits, with one or two adjacent alternatives so you can match what your team or fill station actually carries.
Frequently asked questions
- What's the best mix for 50 m / 1.4 ppO₂ / 30 m END?
- Trimix 21/35 is the textbook answer. Some teams run 18/45 for slightly more conservative END at the cost of a little more helium.
- Why does END matter?
- Above ~30 m of nitrogen-equivalent depth, narcosis impairs judgment, motor skills, and CO₂ tolerance. Helium addresses this by replacing nitrogen — a 21/35 trimix has the same density as air but markedly less narcotic load.
- What if no standard mix satisfies both?
- Then either the depth is impractical for the chosen ppO₂ (lower it, or accept hypoxic mix at depth) or the END limit is unrealistic. Sport / tech standards typically settle around 1.4 bar working ppO₂ and 30 m END.
- Does the calculator handle hypoxic mixes?
- Yes. For dives below ~60 m the recommendation will drop O₂ below 21% — a hypoxic trimix needs a travel gas to descend safely past the surface.