Trimix Calculator
See where a helium mix can be breathed, how narcotic it feels at depth, which mix fits your dive, and how to blend it.
Results update as you type.
Quick examples
Load a full example, then change any value.
From your tank's analysis label.
Helium percentage in the tank. The rest is nitrogen.
The depth you want to check.
The conservative model counts oxygen as narcotic as nitrogen.
Equivalent narcotic depth (END)
Within limitsTx 21/35 can be breathed from the surface down to 56.6 m. At 50 m it feels as narcotic as air at 29 m.
- Nitrogen (FN₂)
- 44%
- ppO₂ at this depth
- 1.26 bar
- Maximum operating depth (ppO₂ 1.4)
- 56.6 m
- Maximum operating depth (ppO₂ 1.6)
- 66.1 m
- Shallowest breathable depth
- Surface
Was this calculator useful?
About trimix, END and blending
Trimix replaces part of the nitrogen with helium. Helium is not narcotic, so a deep dive feels shallower. It does not change the oxygen partial pressure, so the MOD depends on the oxygen fraction alone, the same as with nitrox.
END (equivalent narcotic depth) is the depth on air that would feel as narcotic as your mix. Best Mix works back from a depth and an END target, and Blending turns the mix into pressures of helium, oxygen and top-up gas to add.
With your numbers
Enter valid values to see the calculation worked through with your numbers.
Good to know
- The two narcosis models agree only on air. The less nitrogen a mix has, the further apart they are, so check which model your course uses before comparing plans.
- A mix with less than 18% oxygen can't be breathed at the surface. Switch to it only once you are deeper than its shallowest breathable depth, and switch back before you pass that depth on the way up.
- Helium is the expensive part of a fill. Best Mix adds only as much helium as your END target needs, so a deeper END target means a cheaper fill.
- Blending uses ideal gas maths. Above about 200 bar the real oxygen fraction ends up lower than calculated, so always analyse the finished tank.
Formula
Maximum operating depth (MOD)
MOD = ((ppO2) ÷ (FO2) − 1) × 10
Shallowest breathable depth
dmin = ((0.18) ÷ (FO2) − 1) × 10
END, oxygen narcotic
END = ((1 − FHe) × P − 1) × 10
END, oxygen not narcotic
END = ((FN2 × P) ÷ (0.79) − 1) × 10
Best mix: oxygen
FO2 = (ppO2) ÷ (P)
Best mix: helium, oxygen narcotic
FHe = 1 − (PEND) ÷ (P)
Best mix: helium, oxygen not narcotic
FHe = 1 − FO2 − (0.79 × PEND) ÷ (P)
Blending: helium
He = Pt × FHet − Ps × FHes
Blending: top-up gas for the nitrogen
Top = (Pt × FN2,t − Ps × FN2,s) ÷ (FN2,top)
Blending: oxygen
O2 = Pt × FO2,t − Ps × FO2,s − FO2,top × Top
Blending: top-up to add
Top up = Pt − Ps − He − O2
Where
- FO2, FHe, FN2: oxygen, helium and nitrogen fractions (35% = 0.35), with FN2 = 1 − FO2 − FHe
- P: absolute pressure at depth, (d) ÷ (10) + 1 (bar)
- PEND: absolute pressure at the END target, (END) ÷ (10) + 1 (bar)
- 0.18: lowest ppO2 that is safe to breathe (bar). 0.79: nitrogen fraction of air
- Ps, Pt: pressure in the tank now and the fill pressure (bar). s, t and top mark the gas in the tank, the target mix and the top-up gas
How it's calculated
- Absolute pressure P (bar) = depth ÷ 10 + 1.
- MOD (m) = (ppO₂ limit ÷ FO₂ − 1) × 10. Helium does not change it.
- Shallowest breathable depth (m) = (0.18 ÷ FO₂ − 1) × 10, or the surface when the result is below zero.
- END, oxygen narcotic = ((1 − FHe) × P − 1) × 10. END, oxygen not narcotic = (FN₂ ÷ 0.79 × P − 1) × 10.
- Blending assumes ideal gases: helium is added first, then oxygen, then the top-up gas fills the tank to the target pressure so the final fractions match the target mix.
These tools are educational planning aids; they are not a substitute for dive training, current printed tables or a dive computer. Verify results against your training and always plan within the limits of your certification. Trimix diving is beyond recreational limits and requires technical training and certification. Analyse every tank before use. Partial-pressure blending with pure oxygen requires oxygen-clean equipment and separate training.
How to use the trimix calculator
- 1
Analyse Mix: enter the oxygen and helium in your tank and a depth to see where the gas can be breathed and how narcotic it feels.
- 2
Best Mix: enter your planned depth, ppO₂ limit and the deepest END you accept. The calculator suggests how much oxygen and helium to blend.
- 3
Blending: enter what is already in the tank and the mix you want, follow the fill order and gauge readings, then analyse the finished tank.
What the results mean
The breathable range runs from the shallowest depth where the oxygen partial pressure reaches 0.18 bar down to the depth where it reaches your ppO₂ limit. Mixes with less than 18% oxygen are hypoxic at the surface and need a travel gas.
END is the depth on air that would feel as narcotic as your mix. Helium is not narcotic, so adding helium makes a deep dive feel shallower.
Blending results are the pressures to add, in order. Real gases stop behaving ideally at high pressure, so always analyse the final mix before diving it.