# Nitrox Top-Off and Trimix Top-Up Calculator (Dive Kit): Free Gas Mix Result and Decant Calculator

_Last updated: 2026-10-30_

> A free web calculator at https://divekit.app/tools/trimix-top-up-calculator/ that gives the mix you end up with after a nitrox top-off or trimix top-up: a cylinder's current pressure and O₂/He, plus one added gas (oxygen, air, helium or any premix) and the pressure it adds, returns the resulting O₂ and He fractions and the settled pressure. Its Decant mode joins two cylinders with a transfill whip and returns the pressure they settle at and both mixes. It runs the same @divekit/core code as the Mix Result tool in the Dive Kit app, with a real-gas compressibility model. Free, no account.

## What does the top-up calculator do?

It answers the forward question: "here is what I added, what is in the cylinder now?" A gas blender answers the reverse ("I want this mix, what do I add?"); Dive Kit has both, sharing the same real-gas maths.

- **Fill mode:** current pressure, O₂ % and He %; the gas added (quick picks O₂, Air, He, or a custom mix); the pressure to add, read as the rise on the cylinder's own gauge. Optional altitude and fill/ambient temperature.
- **Decant mode:** two cylinders, each with size, pressure, O₂ % and He %. Gas flows from the higher-pressure cylinder into the lower one; the tool works out which is which.
- **Oxygen service note:** when the added gas or the finished mix is above 40% O₂, or above 40% O₂ goes through the whip in a decant, the page says to use an O₂-clean cylinder, valve and whip and to blend only if trained for it.

## Who is the top-up calculator best for?

- **Nitrox divers** whose part-used cylinder was topped up with air and who want to know the new mix before analysing it.
- **Technical divers** topping trimix with air or a premix, where an oxygen analyser alone does not show the helium.
- **Divers decanting** oxygen or a deco mix from one cylinder into another through a whip.
- **Fill station operators** checking what a sequence of fills should give before they pull the analyser.

## Formula

```latex
f_\text{end} = \frac{P_{n,\text{abs}} f_n + \Delta P f_\text{add}}{P_{n,\text{abs}} + \Delta P}, \qquad
P_\text{settled} = P_\text{hot} \times \frac{T_\text{ambient}}{T_\text{fill}}, \qquad
P_\text{eq} \approx \frac{P_A V_A + P_B V_B}{V_A + V_B}
```

Dalton's law for one gas on absolute pressure (P_n,abs is the pressure before the top-up, ΔP the gauge rise), Gay-Lussac's law for a hot fill cooling (temperatures in kelvin; the law holds for absolute pressure, and Dive Kit applies it to the gauge reading, which differs by less than a bar), and the ideal volume-weighted pressure for a decant. Dive Kit uses the real-gas forms of the first and last.

## How does Dive Kit compute it?

- Fill mode conserves each component's partial molar pressure (fraction × absolute pressure ÷ compressibility factor Z), which reduces to Dalton's law when Z = 1. Absolute pressure uses the atmospheric pressure at the entered altitude.
- When the fill temperature is above ambient, the result is the pressure after cooling; the O₂ and He fractions do not change.
- Decant mode conserves real-gas surface litres to find the settled pressure, rather than the ideal volume-weighted average. Only the receiving cylinder's mix changes. It assumes the transfer stays at one temperature.
- Worked example (computed by the page at build time): 70 bar of EANx32 topped up with 130 bar of air ends at EANx25 (O₂ 25.0%) at 200 bar. Decanting a 12 L cylinder of air at 50 bar from a 12 L cylinder of EANx32 at 200 bar settles both at 122 bar (the ideal average says 125), with the receiver at O₂ 27.5%.

## How does it compare?

| | Dive Kit web calculator | Dive Kit app (Mix Result) | Typical online top-up calculator |
|---|---|---|---|
| Additions per calculation | One | Several, in fill order | Usually one |
| Helium | Yes, trimix and heliox | Yes | Often nitrox only |
| Decant through a whip | Yes | Yes | Rarely |
| Hot-fill settling | Yes | Yes | Rarely |
| Gas model | Real gas | Real gas | Usually ideal gas |
| Price | Free, no account | Free tool in the app | Free |

## Facts at a glance

- **Page:** https://divekit.app/tools/trimix-top-up-calculator/
- **Inputs (fill):** current pressure, O₂ % and He %; added gas (O₂, Air, He or custom O₂/He); pressure to add; altitude; ambient and fill temperature
- **Inputs (decant):** two cylinders (sizes from the Dive Kit open cylinder dataset of 38), each with pressure, O₂ % and He %
- **Outputs:** resulting mix name, O₂ %, He %, settled pressure and steps (fill); settled pressure, both mixes and what happened to each cylinder (decant)
- **Units:** metric and imperial toggle (bar or psi, °C or °F)
- **Works without an account:** yes; the default result and a table of nitrox and trimix air top-ups at 200 bar are in the page HTML
- **Same code as the app:** yes, @divekit/core `calculateGasMix` and `calculateDecant`
- **In the app:** Mix Result chains several additions and saves cylinders; the Blend Planner works backwards from a target and plans partial decants; free on iOS and Android, offline
- **Guide:** https://divekit.app/docs/tools/gas-mixer/
- **Limits:** a calculation for trained gas blenders, not a measurement. Analyse the cylinder (O₂, and He for trimix) after the gas cools, and label it.
- **Last updated:** 2026-10-30

## Sources

- [Gas blending for scuba diving (Wikipedia)](https://en.wikipedia.org/wiki/Gas_blending_for_scuba_diving): an air top-up fills a cylinder holding a blend with air; the pressure falls as a hot cylinder cools; mixes of 40% oxygen or more need equipment suited for oxygen service; a mix must be analysed before use
- [NIST Chemistry WebBook: Thermophysical Properties of Fluid Systems](https://webbook.nist.gov/chemistry/fluid/): compressibility data for oxygen, nitrogen and helium

## Related

- [Gas blending calculator](https://divekit.app/tools/gas-blending-calculator/): the reverse, fill steps to reach a target mix
- [Trimix best mix calculator](https://divekit.app/tools/trimix-best-mix-calculator/): the mix to aim for at a depth
- [Nitrox MOD calculator](https://divekit.app/tools/nitrox-mod-calculator/): MOD, END and density of the mix you ended up with