END — Equivalent Narcotic Depth

The Equivalent Narcotic Depth (END) is the depth at which a gas mixture feels like air. It measures how much nitrogen narcosis you feel when using different gases.

Divers use this calculation to manage mental clarity. Most divers feel the first signs of narcosis around 30 m when breathing air. If you switch to Trimix, your END might drop to 18 m because the helium reduces the narcotic effect. This keeps your brain sharp while you work at depth.

Always calculate your END before your descent. A common mistake is ignoring how high helium concentrations change your perceived depth. You might feel fine at 50 m with Trimix, although your actual depth is much deeper. Check your gas math twice.