END calculator — equivalent narcotic depth

Narcosis doesn't announce itself; it just quietly takes your judgment while you're deep. Equivalent narcotic depth (END) is how technical divers put a number on the effect: the air depth that would feel the same as your mix at your depth. DecoLog computes it for any mix — and is explicit about the convention it uses.

What END actually measures

Inert-gas narcosis scales with the partial pressure of narcotic gas you breathe. Air at 40 m is markedly impairing; air at 20 m much less so. When you add helium — essentially non-narcotic at diving depths — you dilute the narcotic gases, and the honest question becomes: which air depth does this mix at this depth feel like? That's END. A trimix with an END of 30 m at 60 m means the narcotic load is the same as air diving at 30 m, however deep the water actually is.

The convention question: is oxygen narcotic?

Here the field genuinely splits. By the Meyer–Overton correlation — narcotic potency tracks lipid solubility — oxygen should be roughly as narcotic as nitrogen, and several agencies teach END counting both. Others compute END from nitrogen alone. The same mix therefore gets two very different numbers depending on the convention, and neither is "wrong" — but you must know which one your tool uses.

DecoLog uses the oxygen-narcotic convention: everything except helium counts. END = (fO₂ + fN₂) × P ÷ 0.79, converted back to depth — the 0.79 being the narcotic fraction of air. Counting oxygen yields a deeper (more conservative) END for the same mix.

Worked example — the convention matters

Trimix 18/45 (18% O₂, 45% He, 37% N₂) at 60 m in seawater (~7.2 ata):

• O₂-narcotic (DecoLog): narcotic fraction 0.18 + 0.37 = 0.55 → END = (0.55 × 7.2 ÷ 0.79 − 1) × 10 ≈ 40 m.
• N₂-only: narcotic fraction 0.37 → END ≈ 24 m.

Same mix, same depth — a 16 m gap purely from bookkeeping. If your training says "keep END at 30 m," 18/45 at 60 m passes under one convention and fails under the other. This is why serious planners state their convention up front.

Second example: trimix 21/35 at 45 m (~5.6 ata): narcotic fraction 0.65 → END ≈ 36 m counting oxygen, ≈ 21 m counting nitrogen only.

Choosing a helium fraction

In practice you work backwards from ceilings, not forwards from a mix:

• Pick your END ceiling (commonly 30 m or shallower — an agency and personal choice).
• Pick your density ceiling — the 5.2 g/L guidance often demands more helium than narcosis alone does.
• Set fO₂ from your ppO₂ limit at target depth (the MOD arithmetic).
• Helium replaces whatever nitrogen those ceilings can't tolerate.

Related: EAD (equivalent air depth) is the nitrox cousin of END — same idea, used to read nitrogen loading of a nitrox mix against air tables. DecoLog's gas lab computes both, and the blending calculator turns the chosen mix into actual fill pressures.

Run your mix

Enter mix and depth; get END, EAD, MOD, ppO₂ and density in one screen — the complete picture of what that gas does to you at that depth. Free, offline, no signup.

⚠ DecoLog is a planning aid, not a substitute for a dive computer or proper training. Narcosis tolerance varies between divers and days — choose END limits with your instructor and dive within your certification.