Gas Management

END & Trimix Calculator

Calculate partial pressures and Equivalent Narcotic Depth under a named convention. Results explain one model; they do not recommend a trimix or safe depth.

Depth and breathing gas

Nitrogen is calculated as the remainder after oxygen and helium. All pressure outputs are absolute ATA.

Quick examples

Load a complete depth-and-gas scenario, then adjust any value below.

Actual depth

Oxygen

Helium

PPO₂ threshold for MOD

O₂ + N₂ narcotic; helium non-narcotic

Equivalent Narcotic Depth

35.5 m

Named planning convention

PPO₂ at depth

1.47 ATA

1.4 ATA selected threshold

MOD at 1.4 ATA

56.7 m

Absolute pressure: 7.00 ATA
Calculated nitrogen: 44.0%
PPN₂: 3.08 ATA
PPHe: 2.45 ATA
Narcotic partial pressure (PPO₂ + PPN₂): 4.55 ATA

Selected PPO₂ threshold exceeded

The selected PPO2 threshold is exceeded at this depth.

Theoretical helium for a target END

This reverses only the named END equation while holding oxygen constant. It does not recommend a mix.

Target END

Theoretical helium

42.86%

Before operational rounding

Remaining nitrogen

36.14%

Educational inversion only

END does not evaluate gas density, work of breathing, decompression, thermal effects, helium-related tradeoffs, available gases, training, or whether a rounded mix is appropriate.

The convention used

This calculator counts oxygen and nitrogen as equally narcotic and helium as non-narcotic. Another convention can give a different END.

\mathrm{END}=(1-F_{\mathrm{He}})(D+k)-k

$F_{\mathrm{He}}$ is the helium fraction, $D$ is actual depth, and $k$ is 10 m or 33 ft. The model assumes 1 ATA at the surface.

What END cannot tell you

A lower calculated END is not proof of clear thinking and is not a safe-depth result. Susceptibility, carbon dioxide, workload, cold, and individual response still matter.

Frequently Asked Questions

What convention does this END calculator use?

It treats oxygen and nitrogen as equally narcotic and helium as non-narcotic. That is a conservative mixed-gas planning convention, not a settled measurement of how narcotic each gas is for every diver.

What is the END of Tx21/35 at 60 meters?

Under the oxygen-plus-nitrogen narcotic convention and the 10 meters per ATA approximation, Tx21/35 at 60 meters has an END of 35.5 meters. Its PPO2 is 1.47 ATA, so it exceeds a selected 1.4 ATA threshold at that depth.

Does a calculated target END determine the right trimix?

No. The inverse result only solves one END equation. It does not assess gas density, work of breathing, decompression, helium-related tradeoffs, supply, operational rounding, training, or whether the mix is appropriate.

Is END the same as Equivalent Air Depth?

No. END models narcotic gas pressure under a named convention. Equivalent Air Depth compares nitrogen loading for decompression use and follows a different equation and purpose.

Related Guides

Skills & safety

Nitrogen Narcosis: Signs and Management

Understand what causes narcosis, calculate trimix END under an explicit convention, and learn the habits that keep impairment manageable.

Nitrox & oxygen

Equivalent Air Depth (EAD) Explained

Work out the air depth a Nitrox dive is decompression-equivalent to, and see why EAD never changes your oxygen limit.

Nitrox & oxygen

Diving Safety: Essential PPO2 Limits Every Diver Should Know

Understand partial pressure of oxygen limits, oxygen toxicity risks, and how to stay safe at depth.

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