---
title: "Best mix"
description: "Computing a mix for a target depth from an oxygen ceiling, a narcosis limit and a density limit."
---

> Documentation Index
> Fetch the complete documentation index at: https://docs.divelogic.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Best mix

The best-mix calculator runs the gas physics backwards: give it a depth and it
returns the composition that sits exactly on your limits there. It lives in the
gas wizard's **Gas Composition** stage.

1. **Step 1**

   Switch the mode selector from **Manual** to **Calculator**.

2. **Step 2**

   Enter **Target Depth (m)**, the deepest point the mix has to work at, then
   **Max ppO₂**, the oxygen ceiling you want at that depth.

3. **Step 3**

   Press **Calculate Optimal Mix**. The `O₂ %` and `He %` fields fill with the
   result, rounded to whole percentages, and the mix preview updates.

The calculator sets the two fields; it does not lock them. Edit either one
afterwards and the preview follows.

## The three limits, in order

Oxygen is decided first and never revisited. Helium is then whatever the two
remaining limits demand.

- **Oxygen, from the ppO₂ ceiling**: the richest whole percentage whose partial
  pressure at the target depth still fits the limit you gave. It rounds down, so
  the resulting ppO₂ lands at or below what you asked for.
- **Helium, first from narcosis**: the least helium that brings the narcotic
  depth to the target or shallower, rounded up.
- **Helium, then from density**: the least helium that brings gas density to the
  limit or below at that depth, also rounded up.

Helium is the larger of the two helium answers, capped so oxygen and helium
never exceed 100 together. Nitrogen is the remainder.

The result tells you when **density**, rather than narcosis, set the helium
fraction. Past roughly the 50 to 60 m mark density binds first, and where it
does, lowering your narcosis target changes nothing about the mix you are given.

## What the wizard fixes for you

The wizard always asks for **30 m** narcotic depth and **5.2 g/L** density, so
the target depth and the oxygen ceiling are the only two things you control. To
work against different targets, go the other way round: set the composition by
hand and read the gas card until the four numbers sit where you want them.

## What it does not check

- **The surface.** A mix computed for 60 m at 1.3 bar comes to 18 % oxygen,
  0.18 bar at the surface, and nothing raises that. A hypoxic bottom mix needs a
  travel gas, and the planner will not add one for you.
- **Decompression.** The result is a bottom mix for one depth, not a gas plan.
  Ascent gases and their switch depths are separate decisions.
- **What can be filled.** The answer is what the physics asks for, not a mix
  rounded toward one your blender can supply.

## Next

- [Editing a gas](/gases/editing) puts the result back into the builder.
- [Depth limits](/gases/analysing) lists the warnings it can raise.

Source: https://docs.divelogic.ai/gases/best-mix/index.mdx