Calculate Equivalent Narcotic Depth for any trimix blend, or find the ideal mix for your target depth and narcosis limit. Uses the conservative method — O₂ treated as narcotic.
| Mix | FO₂ | FHe | FN₂ | MOD (PO₂ 1.4) | END at 60m |
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Trimix is a breathing gas consisting of three components: oxygen (O₂), helium (He), and nitrogen (N₂). It is the standard gas choice for technical divers venturing beyond 40–50 metres (130–165 ft), where breathing air or nitrox becomes dangerous due to nitrogen narcosis and oxygen toxicity.
The helium fraction replaces some of the nitrogen and oxygen in the mix. Because helium is essentially non-narcotic at recreational and technical diving pressures, replacing these gases reduces narcosis at depth. The result is a clearer head, better decision-making, and improved safety margins on deep technical dives.
Trimix comes in several categories:
Common trimix blends used in technical diving include TMx 21/35 (normoxic, suitable to ~50m), TMx 18/45 (hypoxic, typical for 60–80m dives), TMx 15/55, and TMx 10/70 for very deep dives beyond 100m.
Nitrogen narcosis — sometimes called "rapture of the deep" or the Martini Effect — is an altered state of consciousness caused by breathing inert gases under pressure. It is similar in effect to alcohol intoxication and affects cognitive function, reaction time, and decision-making at depth.
The exact mechanism is not fully understood, but narcosis follows the Meyer-Overton correlation: the more lipid-soluble a gas, the more narcotic it tends to be. Nitrogen is moderately narcotic; helium has essentially no narcotic effect at depths achievable by scuba divers. This is why helium is added to the mix.
Some divers notice narcosis as shallow as 20m (66ft), while others may tolerate it well to 40m (130ft) on air. Individual variation is large, and narcosis can be unpredictable — cold, stress, fatigue, and CO₂ build-up can all worsen the effect. The accepted rule of thumb is that the effects of narcosis roughly double with every additional 10m of depth.
This is a debated topic. Some agencies (such as NOAA) historically treated only nitrogen as narcotic. However, a growing body of evidence and the conservative approach adopted by most modern technical agencies — including PADI TecRec, SSI, IANTD, and GUE — treats oxygen as narcotic for END calculations. This calculator uses the conservative (O₂-narcotic) method, which most professional dive planners and computers use.
Equivalent Narcotic Depth (END) is the hypothetical depth at which a diver breathing air would experience the same narcotic load as the diver breathing the trimix at the actual depth. It provides a simple way to quantify and compare the narcosis effect of different gas mixes.
For example: a diver breathing TMx 18/45 at 60m has an END of approximately 28m. That means the narcotic experience at 60m on that mix feels the same as diving to 28m on air — a much more comfortable and manageable state.
Using the conservative O₂-narcotic method:
A lower END means less narcosis. The practical target for most technical divers is an END of 30m (100ft) or less at the planned bottom depth. Some divers are comfortable to 40m (130ft) END; very conservative divers target 25m (82ft).
Choosing the right trimix blend requires balancing three constraints: narcosis limits, oxygen toxicity limits, and the practicalities of gas blending and decompression.
Most technical training agencies recommend an END of no more than 30m (100ft) for bottom gas. Some divers use 35m (115ft) as a limit if they have significant deep air experience. For very deep dives (80m+), 25m END or lower may be appropriate.
For bottom gas, a PO₂ of 1.4 bar is the standard limit used during the working portion of a dive. 1.6 bar is the absolute ceiling and is sometimes used for decompression stops. This calculator defaults to 1.4 bar — the widely recommended standard.
Enter your planned depth, target END, and PO₂ limit. The calculator will output the recommended FO₂ and FHe percentages, the FN₂, and the MOD for both common PO₂ limits. You can then confirm the mix with your gas blender and technical instructor.
Trimix bottom gases require careful decompression planning. You will typically need one or more decompression gases (e.g., 50% nitrox at 21m, 100% O₂ at 6m) planned in advance. Gas blending calculations for these can be done with the Gas Blending Calculator.
Trimix diving requires formal technical training and certification. The typical progression is:
Major agencies offering trimix courses include PADI TecRec, SSI Xtend, IANTD, TDI, and GUE. Always complete your training with an experienced and active technical instructor.