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.
Switch the mode selector from Manual to Calculator.
Enter Target Depth (m), the deepest point the mix has to work at, then Max ppO₂, the oxygen ceiling you want at that depth.
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 puts the result back into the builder.
- Depth limits lists the warnings it can raise.