Henry’s Law Gas Solubility Calculator

Understanding the Henry's law gas solubility calculator — concept and principle

Henry's law estimates how much of a gas dissolves in a liquid when the gas above it has a given partial pressure. It is used to work out dissolved oxygen and carbon dioxide in water, carbonation in drinks, gas transfer in bioreactors and the stripping of volatile compounds. Enter the Henry constant in solubility form, the partial pressure and the molar mass, and the calculator returns the dissolved concentration in mol/L and mg/L.

Only a trace of CO₂ dissolves from air because its partial pressure is tiny.

How it is calculated

c = k_H × p

k_H: Henry solubility constant, mol/(L·atm), for the gas, solvent and temperature.
p: partial pressure of the gas, atm. For a gas in air, p = volume fraction × total pressure.
mg/L = c × M × 1,000, with M the molar mass in g/mol.

The principle

At equilibrium, gas molecules enter and leave the liquid surface at the same rate. The rate of entry is proportional to how often molecules hit the surface, which is proportional to their partial pressure; the rate of escape is proportional to the dissolved concentration. Setting the two equal gives a straight-line relation c ∝ p. The proportionality constant depends on how strongly the gas interacts with the solvent and falls as temperature rises, which is why warm water holds less dissolved gas.

Worked example

Inputs: k_H = 0.034 mol/(L·atm) (CO₂ in water at 25 °C), p = 0.0004 atm (400 ppm), M = 44.01 g/mol.
c = 0.034 × 0.0004 = 0.0000136 mol/L (1.36 × 10⁻⁵ mol/L).
mg/L = 0.0000136 × 44.01 × 1,000 = 0.5985 mg/L.

Each point is the calculator re-run with only “Henry constant k_H (mol/(L·atm))” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Henry constant k_H (mol/(L·atm))	Dissolved concentration (mol/L)
0.0085	0.0000034
0.017	0.0000068
0.0255	0.0000102
0.034	0.0000136
0.0425	0.000017
0.051	0.0000204
0.068	0.0000272

How to read the result

Water in equilibrium with ordinary air holds only about 0.6 mg/L of dissolved CO₂ at 25 °C. In a sealed drink bottle with several atmospheres of CO₂ the concentration is thousands of times higher, and when you open it the partial pressure collapses and the excess gas comes out as bubbles. For oxygen, k_H is much smaller (around 1.3 × 10⁻³ mol/(L·atm) at 25 °C), but its partial pressure in air is about 0.21 atm, giving roughly 8 mg/L.

Common mistakes

Mixing conventions: Henry constants are also published as p/c (atm·L/mol, about 29 for CO₂) or as dimensionless ratios. Check which form your source uses; this calculator needs c/p.
Using total pressure instead of the gas's partial pressure.
Using a 25 °C constant for cold or hot water.

Keep in mind: Henry's law holds for dilute solutions and gases that do not react. CO₂ partly forms carbonic acid, bicarbonate and carbonate, so total dissolved inorganic carbon depends on pH and can be much larger than the value shown. Salts also reduce gas solubility.

Frequently asked questions

How does temperature change k_H?

For most gases k_H decreases as temperature rises. Compilations give a temperature coefficient that lets you correct a 25 °C value.

Why use 0.0004 atm for CO₂?

400 ppm by volume at 1 atm total pressure corresponds to a partial pressure of 0.0004 atm. Today's atmospheric level is somewhat higher, so adjust as needed.

Does Henry's law apply in other solvents?

Yes, but the constant changes with the solvent, so use a value measured for that solvent.

Sources: R. Sander, Compilation of Henry's law constants (version 4.0) for water as solvent, Atmospheric Chemistry and Physics 15 (2015) 4399 · W. Henry, Philosophical Transactions of the Royal Society of London 93 (1803)

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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