# Best Mix Calculator for Nitrox and Trimix (Dive Kit): Free Trimix Calculator by Depth

_Last updated: 2026-10-30_

> A free web calculator at https://divekit.app/tools/trimix-best-mix-calculator/ that works both ways. Depth → mix: a planned depth, a maximum PPO₂ and (for trimix) a target END give the best mix, the richest nitrox or the trimix O₂ and He for the dive. Mix → depth: an O₂ % (and He % for trimix) gives the working MOD at the PPO₂ limit and the END there. Both show PPO₂, END, gas density, EAD or minimum depth, EADD and the nearest standard fill. It runs the same @divekit/core code as the Dive Kit app's Best Mix tool. Free, no account.

## What is a best mix?

- **Best mix (nitrox):** the highest oxygen fraction whose PPO₂ at the planned depth stays at or under the limit. More oxygen means less nitrogen and less decompression.
- **Best mix (trimix):** the same oxygen, then enough helium in place of nitrogen to hold the equivalent narcotic depth (END) at the target.
- **The other way round:** for a mix you already have, its MOD is the depth where its PPO₂ reaches the limit; the END and density there show whether narcosis or breathing effort limits it first.

## Who is the best mix calculator best for?

- **Nitrox divers** picking the richest fill for a planned depth at 1.2 to 1.6 bar.
- **Technical divers** working out the oxygen and helium for a trimix dive at a chosen END, and comparing it with standard gases (21/35, 18/45 and so on).
- **Gas blenders and instructors** checking a requested mix against depth, PPO₂, END and density.

## Formula

```latex
F_{\text{O}_2} = \frac{\left\lfloor 100 \times \frac{P_{\text{O}_2,\max}}{P_\text{depth}} \right\rfloor}{100}, \qquad
F_\text{He} = 1 - F_{\text{O}_2} - 0.79 \times \frac{P_\text{END}}{P_\text{depth}}, \qquad
P = P_\text{surf} + \frac{d}{m_\text{bar}}
```

O₂ is floored to the whole percent; He is rounded up to 0.1 %. P_surf is 1.01325 bar at sea level, m_bar the metres of water per bar (10.00 in EN 13319 water, the default). With O₂ counted as narcotic, F_He = 1 − P_END / P_depth.

## How does Dive Kit compute it?

- Depth → mix: core's `bestMixForDepth`, the same `BestMixCalculator` the app runs: O₂ floored so the PPO₂ never passes the limit, He rounded up so the END never passes the target. The mix name rounds He up to the whole percent.
- Mix → depth: the app's `BestMixCalculator` setters (`setFFO2`, then `setFFHe` for trimix). O₂ sets the MOD at the PPO₂ limit, floored once in the display unit (whole metre or foot); He sets the END at that MOD. O₂ + He past 100 % lowers O₂ to fit, as in the app.
- Nitrox mode stops where 21 % O₂ reaches the PPO₂ limit (56 m or 185 ft at 1.4 bar in EN 13319 water, capped at 60 m) and says trimix goes deeper.
- Figures at depth come from `calculateEadPpo2`. Statuses: PPO₂ "deco only" past 1.4 bar, "too high" past 1.6 bar; END "over 30 m" past the app's default END warning; density "elevated" past 5.2 g/L, "too high" past 6.2 g/L.
- Density is reported, not a constraint. When it passes 5.2 g/L the page states the helium that would bring it under.
- Standard fill: 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, 10/70) whose recommended depth range covers the depth, or the richest one that passes when none does.
- Hypoxic mixes show their minimum depth (PPO₂ 0.16 bar). Helium mixes at 80 m or deeper with 10 % He or more carry an HPNS note.
- Worked examples (computed by the page at build time): the best nitrox for 30 m at 1.4 bar is EANx34; the best trimix for 60 m at 1.4 bar and a 30 m END is TMX 19/36 (19 % O₂, 35.8 % He), nearest standard 18/45. The other way round, 21/35 at 1.4 bar has a working MOD of 56 m with an END of 27 m there.
- Static reference table on the page: best nitrox and trimix (END 30 m) at PPO₂ 1.4 for 20 to 100 m, with minimum depth and the nearest standard trimix.

## How does it compare?

| | Dive Kit web calculator | Dive Kit app (Best Mix) | Typical online best mix calculator |
|---|---|---|---|
| Modes | Nitrox, trimix | Same | Usually nitrox only |
| Direction | Both ways (depth in, mix out; mix in, MOD and END out) | Both ways, linked live | Depth in, mix out |
| Figures at depth | PPO₂, END, density, EAD or min depth, EADD, MOD | Same, plus no-stop time (NDL) | O₂ % and MOD |
| Standard gases | Nearest standard fill | Same | Rarely |
| Price | Free, no account | Free tool in the app | Free |

## Facts at a glance

- **Page:** https://divekit.app/tools/trimix-best-mix-calculator/
- **Inputs:** mode (nitrox or trimix), direction (depth → mix or mix → depth), max depth (nitrox 1 to 60 m, trimix 1 to 300 m) or O₂ % (and He % for trimix), max PPO₂ (nitrox 1.2, 1.4 or 1.6 bar; trimix 0.5 to 2.0 bar), target END (trimix, depth → mix), water type, O₂ counted as narcotic
- **Outputs:** best mix name and O₂ %, He %, N₂ % (depth → mix) or working MOD and END (mix → depth), PPO₂, END, gas density, EAD or minimum depth, EADD, standard fill, density and HPNS notes
- **Units:** metric and imperial toggle; PPO₂ always in bar
- **Works without an account:** yes; the default result and the best-mix table are in the page HTML
- **Same code as the app:** yes, @divekit/core `bestMixForDepth`, `BestMixCalculator` and `calculateEadPpo2`
- **In the app:** the Best Mix tool links depth, O₂, He and END both ways and adds the no-stop time; free on iOS and Android
- **Guide:** https://divekit.app/docs/tools/best-mix-calculator/
- **Limits:** a planning aid, not a dive computer. It plans a mix; analyse the gas in the real cylinder before diving.
- **Last updated:** 2026-10-30

## Sources

- GUE General Training Standards v10.4, section 2.2 (https://www.gue.com/files/Standards_and_Procedures/GUE-Standards-v10.4.pdf): END at most 30 m (100 ft) with helium the only non-narcotic gas; planned PPO₂ at most 1.4 bar working, 1.6 bar resting.
- InDepth, "Standard Gases" (https://indepthmag.com/standard-gases-the-advantages-of-having-everyone-singing-the-same-song/): GUE 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 or less, and are blended by adding EAN32 to helium.
- Anthony and Mitchell (2016), Respiratory Physiology of Rebreather Diving (https://www.omao.noaa.gov/sites/default/files/documents/Rebreathers%20and%20Scientific%20Diving%20Proceedings%202016.pdf): 5.2 g/L ideal and 6.2 g/L maximum gas density.

## Related

- [Nitrox MOD calculator](https://divekit.app/tools/nitrox-mod-calculator/): MOD, EAD, END and density of a mix you already have
- [Gas blending calculator](https://divekit.app/tools/gas-blending-calculator/): blend the mix
- [Trimix top-up calculator](https://divekit.app/tools/trimix-top-up-calculator/): what a top-up gives you