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Calcimator

Trimix Blend Calculator

Calculate MOD, Equivalent Narcotic Depth (END), partial pressures, and hypoxic safety for Trimix gas blends used in technical deep diving.

About this calculator

Trimix adds helium to the oxygen/nitrogen mix so technical divers can go deep without the oxygen toxicity risk of straight air or the crushing narcosis of breathing high nitrogen partial pressures at depth. This calculator first normalizes your entered oxygen and helium percentages so they never sum past 100%, with nitrogen filling whatever fraction remains. Maximum Operating Depth (MOD) is derived from your oxygen fraction and your chosen ppO2 ceiling: MOD = (ppO2 limit ÷ fO2 − 1) × 10, giving the deepest point where oxygen partial pressure still stays within your set limit. Equivalent Narcotic Depth (END) estimates how narcotic the mix feels at depth by treating both nitrogen and oxygen as narcotic gases — the more conservative modeling choice, since some agencies and divers assume oxygen carries no narcotic effect and would get a shallower END for the same blend.

Helium is treated as non-narcotic throughout, consistent with why it's added in the first place. The tool also checks whether your finished mix is hypoxic (ppO2 below 0.16 at the surface, meaning it's unsafe to breathe before descending) and, if so, calculates the minimum depth at which it becomes safe to switch to. Keep in mind this uses simplified formulas suitable for planning, not gas-blending certification: always verify final partial pressures with an analyzer before diving, and never treat MOD or END here as a substitute for training in mixed-gas procedures and their real decompression obligations.

Inputs

%
%
m

Results

MOD (metres)

56.7

END (metres)

35.5

MOD (feet)186
END (feet)116
Nitrogen (%)44
ppO2 at depth1.47
ppN2 at depth3.08
ppHe at depth2.45
Hypoxic at surface?0
Min switch depth (m)0
How to Use This Calculator
  1. Enter the oxygen percentage in your Trimix blend (e.g., 21% for normoxic, lower for hypoxic mixes).
  2. Input the helium percentage — helium replaces nitrogen to reduce narcosis at depth.
  3. Set your planned maximum depth in meters.
  4. Set your ppO2 limit in bar (1.4 for bottom gas, 1.6 for decompression).
  5. Read MOD in meters to confirm your dive plan stays within safe oxygen limits.
  6. Check END (Equivalent Narcotic Depth) in meters — ideally keep END below 30 m to minimize narcosis risk.

How the result changes with Oxygen (%)

Oxygen (%)MOD (metres)END (metres)
11117.335.5
1677.535.5
3233.835.5
5316.435.5

What each input means

Oxygen (%)
Oxygen percentage. Below 18 % is hypoxic trimix.
Helium (%)
Helium percentage. Reduces narcosis and density.
Planned depth (m)
Maximum planned depth in metres.
ppO2 limit (bar)
Maximum partial pressure of oxygen (1.4 recreational, 1.6 deco).

What each result means

MOD (metres)
Maximum Operating Depth where ppO2 reaches your limit.
MOD (feet)
Maximum Operating Depth in feet.
END (metres)
Equivalent Narcotic Depth — effective depth for narcosis risk.
END (feet)
Equivalent Narcotic Depth in feet.
Nitrogen (%)
Balance nitrogen percentage.
ppO2 at depth
Partial pressure of oxygen at planned depth.
ppN2 at depth
Partial pressure of nitrogen at planned depth.
ppHe at depth
Partial pressure of helium at planned depth.
Hypoxic at surface?
1 = mix is hypoxic at surface (ppO2 < 0.16), 0 = safe to breathe at surface.
Min switch depth (m)
If hypoxic, the shallowest depth where this mix is safe to breathe.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Oxygen (%) = 21, Helium (%) = 35, Planned depth (m) = 60, ppO2 limit (bar) = 1.4 = 4 input(s) provided
  2. Calculate MOD
    MOD = (ppO2max / fO2 - 1) * 10
    56.7 = 56.7
  3. Calculate END
    END
    35.5 = 35.5
  4. Calculate MOD
    MOD = modM * 3.281
    186 = 186
  5. Calculate END
    END = endM * 3.281
    116 = 116

Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does this calculator treat oxygen as narcotic when computing END?

That's the more conservative modeling choice: some agencies and divers treat oxygen as non-narcotic and would compute a shallower END for the same blend, but this calculator sums nitrogen and oxygen fractions together (fN2 + fO2) before multiplying by ATA. Helium is always excluded from the narcotic fraction, consistent with why it's added to the mix in the first place.

What happens if I enter oxygen and helium percentages that add up to more than 100%?

The calculator automatically scales both percentages down proportionally so they never exceed 100% combined, then fills the remainder with nitrogen. So entering, say, 40% oxygen and 70% helium doesn't produce an invalid result — it normalizes both figures before running the MOD and END formulas.

Why would my blend get flagged as hypoxic at the surface?

A mix is flagged hypoxic when its oxygen partial pressure at 1 ATA (the surface) falls below 0.16 bar — the widely used minimum for a breathable surface gas. Trimix blends built for deep dives often run oxygen percentages under 18% to control MOD, which can push ppO2 at the surface below that threshold, making the gas unsafe to breathe before descending to the calculated minimum switch depth.

Does adding helium change my MOD?

No — MOD depends only on your oxygen fraction and chosen ppO2 ceiling (MOD = (ppO2 limit ÷ fO2 − 1) × 10), so two blends with the same oxygen percentage but different helium content share the same MOD. Helium instead affects END (by diluting the narcotic fraction) and gas density, not the oxygen-toxicity depth limit.

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