Best Mix Calculator: Nitrox and Trimix by Depth

The best mix for a dive is the richest oxygen the gas can carry at the planned depth without passing your PPO₂ limit. For trimix, helium then replaces enough nitrogen to hold the equivalent narcotic depth (END) at your target, so this doubles as a trimix calculator. It works both ways: set a depth to get the mix, or set a mix (O₂, and He for trimix) to get the depth it is good to, with the PPO₂, END and gas density there.

The formula

F_{O₂} = \frac{⌊ 100 \times \frac{P_{O₂,max}}{P_{depth}} ⌋}{100}

The oxygen fraction, floored to the whole percent. P_O₂,max is your limit in bar.

P_{depth} = P_{surf} + \frac{d}{m_{bar}}

The absolute pressure at the planned depth d: P_surf is 1.01325 bar at sea level and m_bar the metres of water per bar (10.00 in EN 13319 water).

F_{He} = 1 - F_{O₂} - 0.79 \times \frac{P_{END}}{P_{depth}}

P_END is the absolute pressure at the target END. Rounded up to 0.1 %. With O₂ counted as narcotic it becomes F_He = 1 − P_END/P_depth, which adds helium.

Worked example

Nitrox first: the richest mix for a dive to 30 m at a 1.4 bar working limit, in EN 13319 water.

1Ambient pressure at 30 m is 4.013 bar.
2FO₂ = 1.4 ÷ 4.013 = 34.88 %, floored to 34 %: EANx34, with a working MOD of 31 m.

Trimix next: 60 m, the same 1.4 bar limit, and a target END of 30 m.

1FO₂ = 1.4 ÷ 7.013 = 19.96 %, floored to 19 %.
2For narcosis to match air at 30 m (4.013 bar), the nitrogen may be 0.79 × 4.013 ÷ 7.013 = 45.2 %.
3Helium fills the rest: 100 − 19 − 45.2 = 35.8 % (rounded up to 0.1 %), so the best mix is TMX 19/36.
4At 60 m it reads PPO₂ 1.33 bar, END 30.0 m and gas density 6.0 g/L. The nearest standard fill is 18/45.

In imperial: the best nitrox for 100 ft at 1.4 bar is EANx34 (working MOD 101 ft), and the best trimix for 200 ft with a 100 ft END is TMX 19/36 (19 % O₂, 35.9 % He).

The other way round, with Mix → depth: the standard 21/35 at 1.4 bar has a working MOD of 56 m, where its END is 27 m and its gas density 5.8 g/L.

Best mix by depth

The richest nitrox and the trimix for a 30 m END at a 1.4 bar working limit, with the shallowest depth each hypoxic mix may be breathed and the standard trimix fill nearest to it.

Depth	ft	Nitrox	Trimix	He %	Min depth m	Standard
20 m	66	EANx46	EANx46	0.0	–	–
30 m	98	EANx34	EANx34	0.0	–	–
40 m	131	EANx27	TMX 27/10	9.8	–	25/25
50 m	164	EANx23	TMX 23/25	24.3	–	21/35
60 m	197	–	TMX 19/36	35.8	–	18/45
70 m	230	–	TMX 17/44	43.5	–	15/55
80 m	262	–	TMX 15/50	49.9	1	12/60
90 m	295	–	TMX 13/56	55.4	3	12/60
100 m	328	–	TMX 12/60	59.3	4	10/70

EN 13319 water, oxygen not counted as narcotic. Oxygen is floored and helium rounded up, so neither the PPO₂ nor the END passes its target; the mix name rounds helium up to the whole percent. Nitrox stops at 56 m, where the leanest nitrox (21 %) reaches 1.4 bar.

How Dive Kit computes this

Oxygen is floored to the whole percent, so the PPO₂ at depth stays at or under your limit; helium is rounded up to 0.1 %, so the END stays at or under the target. The mix name rounds helium up to the whole percent for the same reason. This is the app's best-mix code from @divekit/core, run in your browser.

Pressures start from the real surface pressure (1.01325 bar) and your water type. Nitrox mode stops where 21 % oxygen reaches your PPO₂ limit (capped at 60 m, as in the app) and says so: past that depth, trimix goes deeper. With O₂ counted as narcotic the END is matched on nitrogen plus oxygen, so the mix carries more helium.

Gas density is reported, not used as a constraint: best mix answers PPO₂ and END only. When density passes 5.2 g/L the page says how much helium would bring it under. The standard-fill line names, for nitrox, the richest of Air, EANx28, 32, 36, 40 and 50 that passes at the depth; for trimix, the standard mix (25/25, 21/35, 18/45, 15/55, 12/60 or 10/70) whose recommended depth range covers the depth, or the richest one that passes when none does.

Mix → depth runs the app's linked inputs the other way: the oxygen you set fixes the MOD at your PPO₂ limit (floored to the whole metre or foot), and the helium you set gives the END at that MOD. If oxygen and helium add up past 100 %, the oxygen is lowered to fit, as in the app. The app adds a no-stop time at depth; to read any mix at any depth, use the nitrox MOD calculator.

The app screen for this tool is documented in the user guide: Best Mix in the Dive Kit guide.

How is the best mix calculated?

Work out the absolute pressure at your planned depth, then divide your PPO₂ limit by it: that is the highest oxygen fraction you can breathe there. Round it down. For trimix, find the nitrogen fraction that gives the target END at that pressure and fill the rest with helium.

What END should I plan a trimix dive for?

GUE's standards, for one, cap the END at 30 m (100 ft) and count every gas except helium as narcotic; set O₂ counted as narcotic to Yes to plan that way, which takes more helium for the same END. 30 m is also the app's default END warning. Other agencies set their own limits, so follow the rule your training uses.

How deep can I take a mix I already have?

Switch the calculator to Mix → depth and enter the oxygen (and, for trimix, the helium). The answer is its working MOD at your PPO₂ limit, with the END and gas density at that depth, so you can see whether narcosis or density stops you before oxygen does.

Why is the best mix rounded down?

Because rounding the oxygen up would put the PPO₂ over your limit at the planned depth. A floored mix keeps you at or under it, and gas from a fill station carries its own tolerance, so analyse the cylinder and plan from the analysed figure.

When do I need trimix instead of nitrox?

When narcosis or gas density on nitrox gets too high for the depth, or when the depth passes the point where even 21 % oxygen reaches your PPO₂ limit. Switch the calculator to Trimix to see how much helium your END target takes; the nitrox MOD calculator shows the END and density of any nitrox at depth.

Should I use the best mix or a standard gas?

Teams that dive standard gases (such as 21/35 or 18/45) fill from a bank and plan around each mix's depth range. The best mix squeezes the most oxygen out of one dive. The page shows both: the computed mix and the nearest standard fill that passes at your depth. To blend either, use the gas blending calculator.

Sources

GUE General Training Standards, Policies, and Procedures, Version 10.4 (section 2.2) : An END limit of 30 m (100 ft) with helium the only non-narcotic gas, and a planned PPO₂ of at most 1.4 bar while working and 1.6 bar while resting.
InDepth: Standard Gases, the Simplicity of Everyone Singing the Same Song : GUE's standard bottom gases (21/35, 18/45, 15/55 and others) are designed for a working PPO₂ of 1.2 to 1.3 bar and an END of 30 m (100 ft) or less, and are blended by adding EAN32 to helium.
Anthony and Mitchell (2016), Respiratory Physiology of Rebreather Diving, in Rebreathers and Scientific Diving Proceedings : The 5.2 g/L ideal maximum and 6.2 g/L absolute maximum gas density behind the density notes.

Last updated 30 October 2026 · English page

Best Mix Calculator for Nitrox and Trimix