Henry's Law Calculator (Gas Solubility in Water)
By Santaji GadepatilinUpdated November 4, 2026
The amount of gas that dissolves in a liquid is proportional to the partial pressure of that gas above it. This Henry's law calculator turns a Henry's constant and a partial pressure into a dissolved concentration — the same relationship that sets the saturation value an optical dissolved oxygen sensor reads against.
How to use the Henry’s law calculator
Enter the gas’s Henry’s law constant (solubility form, mol/(L·atm)), its partial pressure above the liquid and its molar mass. The calculator returns the equilibrium dissolved concentration in mmol/L and mg/L. Clear the partial pressure and enter a concentration to find the pressure needed instead.
Henry’s law formula
c = kH × p · c[mg/L] = c[mmol/L] × M
Example
Oxygen from air (p = 0.2095 atm) dissolving in water at 25 °C, k_H = 1.3 × 10⁻³ mol/(L·atm):
- c = 1.3 × 10⁻³ × 0.2095 = 2.723 × 10⁻⁴ mol/L = 0.2723 mmol/L
- c = 0.2723 × 31.998 = 8.71 mg/L
Measured air-saturation values for fresh water at 25 °C are about 8.3 mg/L; the difference comes from the rounded constant and the water vapour that slightly lowers the O₂ partial pressure.
Henry’s law constants in water at 25 °C
| Gas | k_H (mol/(L·atm)) | 1/k_H (L·atm/mol) | Solubility at 1 atm partial pressure | In equilibrium with air |
|---|---|---|---|---|
| Oxygen (O₂) | 0.0013 | 769 | 41.6 mg/L | 8.71 mg/L |
| Nitrogen (N₂) | 0.00061 | 1,639 | 17.1 mg/L | 13.3 mg/L |
| Carbon dioxide (CO₂) | 0.034 | 29 | 1,496.3 mg/L | 0.628 mg/L |
| Argon (Ar) | 0.0014 | 714 | 55.9 mg/L | 0.522 mg/L |
| Hydrogen (H₂) | 0.00078 | 1,282 | 1.6 mg/L | — |
| Helium (He) | 0.00037 | 2,703 | 1.5 mg/L | — |
| Carbon monoxide (CO) | 0.00095 | 1,053 | 26.6 mg/L | — |
Constants from standard compilations (Sander); the air column uses sea-level partial pressures (CO₂ at 420 ppm).
Notes
- Henry’s law holds for dilute solutions of gases that do not react with the solvent. CO₂ partly reacts to carbonic acid, so total dissolved carbon is higher at high pH.
- Partial pressures in a gas mixture follow Dalton’s law.
Frequently asked questions
What is Henry's law?
At constant temperature, the concentration of a gas dissolved in a liquid is proportional to its partial pressure above the liquid — c = k_H × p. Doubling the partial pressure doubles the dissolved amount.
Why are there different forms of Henry's constant?
Some tables give solubility constants k_H = c/p (mol/(L·atm), used here); others give volatility constants k_H = p/c (L·atm/mol) or p/x (atm). The volatility form is simply the reciprocal, so check the units before using a table value.
How does temperature affect gas solubility?
Most gases become less soluble as water warms — k_H for oxygen falls by roughly a third from 10 °C to 30 °C. That is why warm water holds less dissolved oxygen.
Why does a soda fizz when opened?
It is bottled under several atmospheres of CO₂. Opening it drops the CO₂ partial pressure to about 0.0004 atm, so the dissolved CO₂ is far above its new equilibrium and comes out as bubbles.