Trimix Best Mix
Set the depth, your pO2 ceiling and the narcosis you will accept — and get the trimix that satisfies all three, with hypoxic warnings, travel-gas depth and gas density at the bottom.
Documented figures — verify depths and conditions locally on the day.
The maths, explained
Two ceilings shape a trimix. Oxygen first, exactly as for nitrox: FO2 = pO2 ceiling ÷ P. Then narcosis: pick the equivalent narcotic depth you are willing to accept and solve for helium. Treating oxygen as narcotic — the conservative convention we teach — the helium fraction is:
FHe = 1 − (END + 10) ÷ (depth + 10)
Whatever remains after oxygen and helium is nitrogen. Below about 18% oxygen the mix is hypoxic at the surface — it cannot safely be breathed shallow, so you descend on a travel gas and switch at depth. The tool flags the switch depth for you.
Helium also solves the quieter problem: gas density. Above 5.2 g/L the work of breathing and CO2 retention climb steeply; 6.2 g/L is the hard ceiling (Anthony & Mitchell, 2016). At depth, helium is not a luxury. It is the plan.
This Is Exactly What We Teach On Trimix and Advanced Trimix
The calculator picks the mix; the courses teach you to dive it — narcosis and density management, multi-gas switching, and the long disciplined ascents that bring you back from 60 m and beyond.
Checked, not trusted: the formulas on this page are pinned by 130 automated reference checks that run on every change — cross-validated against Subsurface’s published planner tests, the independent DecoTengu implementation, printed MOD tables and the team’s own Splash apps.
For training and planning purposes only. Analyse every mix before you breathe it and always dive within your certification.