Best Mix Calculator

What it is

The Best Mix tool answers two questions that every nitrox and trimix diver asks before a dive. First: for the depth I’m planning, what’s the richest oxygen mix I can safely breathe? Richer is better because more oxygen means less inert gas and less decompression. Second: I have this mix in my cylinder, so how deep can I take it? That deepest safe point is the Maximum Operating Depth, or MOD.

Both answers come from the same physics. Oxygen turns toxic once its partial pressure (PPO₂) climbs too high, and partial pressure rises with depth. Pick a PPO₂ ceiling, and the math fixes the relationship between oxygen fraction and depth. The in-app screen is titled “Best Mix, MOD”, and the home card calls it “Best Mix”.

The tool has two modes you switch with the chip at the top: Nitrox and Trimix.

How to use it (Nitrox)

The two sliders stay linked through your PPO₂ limit, so changing either one keeps the pair consistent. PPO₂ is always shown in bar and is never converted, but depth follows your unit setting (metres or feet), so the tool works the same in imperial.

Nitrox mode starts at EAN28 with a PPO₂ limit of 1.4 bar and an MOD of 40 m. The O₂% slider runs from 21% to 100% and the depth slider from 1 m to 60 m. A Custom PPO₂ value can be anywhere from 0.1 to 10 bar.

How to use it (Trimix)

Trimix mode adds helium and a narcosis target. The controls are different from Nitrox mode: depth is still a slider, but O₂%, He%, equivalent narcotic depth (END), and the Max PPO₂ limit are all typed into number inputs. There is no PPO₂ chip toggle in Trimix mode; you type the PPO₂ limit in bar directly.

The tool links four values in two pairs so you can start from whatever you already know:

• Depth and O₂% are tied together by your PPO₂ limit. Change one and the other follows.
• END and He% are tied together by the narcosis formula. Set the equivalent narcotic depth (END) you’re comfortable with and the tool computes the helium needed. Change He% and it shows the END you’d get.

When the END target is what’s driving the helium recommendation, the hero result labels it “He% driven by END target” so you know which constraint is active.

The result names the mix in the standard O₂/He form, for example TMX 18/45, and shows the full O₂/He/N₂ breakdown. When the mix contains helium, it also points you to the nearest standard mix with its recommended depth range, so you can fall back to a common bank fill instead of a custom blend. It matches against six fixed blends: 25/25, 21/35, 18/45, 15/55, 12/60, and 10/70. With no helium (He% at 0), the nearest-standard card does not appear.

Trimix mode extends the limits: O₂ can drop as low as 5% and depth reaches 300 m. The default target END is 30 m. The Max PPO₂ input accepts 0.1 to 10 bar. Helium can run from 0% up to whatever is left after oxygen (100 minus O₂).

Inputs and outputs

Rounding

The recommended oxygen fraction is always rounded down (floored), never up. Rounding down keeps PPO₂ below your limit rather than nudging it over. So a depth that works out to 17.5% O₂ is reported as 17%, giving you a small safety margin instead of a fractional percentage you can’t fill anyway.

Narcosis and the O₂ setting

In trimix mode the helium recommendation depends on whether you count oxygen as narcotic. There’s a Treat O₂ as Narcotic toggle for this in Dive Settings. If oxygen is treated as narcotic, everything except helium contributes to narcosis, so the tool recommends more helium. If only nitrogen is narcotic, it recommends less. Both conventions are in common use; the tool simply follows your choice. The Gas Specs panel prints a footnote saying whether END was calculated with O₂ as narcotic or non-narcotic, so you can see which convention is active without leaving the screen.

Warnings

• Technical depth / oxygen. Nitrox mode flags when you’ve pushed past technical-diving depth limits, and warns that a PPO₂ above 1.6 bar risks oxygen toxicity, which can cause convulsions and loss of consciousness underwater.
• Hypoxic mix (Trimix only). When the recommended O₂ drops below 18%, the result is hypoxic and not breathable at the surface. This only happens in Trimix mode, since Nitrox mode floors O₂ at 21%. The tool shows the minimum depth where the mix becomes breathable (where PPO₂ reaches 0.16 bar) and reminds you to use a travel gas to reach that depth.
• HPNS (Trimix only). Deep helium dives raise the risk of High Pressure Nervous Syndrome (HPNS). The tool flags it when depth is at or beyond 80 m and helium is at least 10%. It is shown as an informational note, not a red hazard, and reminds you to descend slowly.
• Gas density. The Gas Specs panel colours density yellow or red as it climbs, since dense gas is hard to breathe and drives up CO₂. Yellow starts at about 5.2 g/L and red at about 6.2 g/L. These thresholds come from published research (Anthony and Mitchell, 2016) and are adjustable in your deco defaults.

Behaviour and limits

This tool plans a mix. It does not replace analysing the gas in your actual cylinder with an oxygen (and helium) analyser before you dive.

Related

• Gas Specs analyses a mix you already have at any depth.
• Deco Planner builds the full decompression schedule for the gases you’ve chosen.