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

Tx21/35

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.

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

Where

$F_{\text{He}}$: helium fraction, not percentage (0.35 for 35%)
$D$: actual depth (m)
$k$: 10 m of seawater per atmosphere

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.

Result

35.5 m END

Tx21/35at 60.0 m; O₂ + N₂ counted as narcotic.

PPO₂: 1.47 ATA
MOD at 1.4 ATA: 56.7 m

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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