Henry's Law Calculator — Gas Solubility in a Liquid

Background.

How much oxygen does a litre of water at 25 °C hold when it sits open to the air? Henry's law answers with one multiplication. Oxygen is 20.95% of dry air, so at one standard atmosphere its partial pressure is 21.2276 kPa. Multiply by the recommended Henry's law solubility constant for oxygen in water at 298.15 K, 1.3 × 10⁻⁵ mol m⁻³ Pa⁻¹, and the dissolved concentration comes out at 2.760 × 10⁻⁴ mol/L, about 8.8 mg/L.

The law, described by William Henry in 1803, says that at equilibrium the amount of a gas dissolved in a liquid is proportional to that gas's partial pressure above it. Double the pressure and the dissolved amount doubles. It is why a sealed bottle of fizzy drink holds far more carbon dioxide than an open glass, and why a diver who surfaces too fast can have nitrogen come out of solution in the body.

The arithmetic is easy; the constant is where mistakes happen. IUPAC's 2021 recommendations define eight forms of it: four solubility constants, which get larger as a gas becomes more soluble, and four volatility constants, their reciprocals, which get smaller. All eight have been called "the Henry's law constant", and some share a unit. So you state which form yours is, and the result restates it in four of them, which makes a wrong choice easy to spot against your source.

A constant also belongs to one temperature. The temperature you enter is used only to convert between the pressure-based and dimensionless forms; it does not shift your constant to a different temperature.

What is henry's law calculator (gas solubility)?

Henry's law is a limiting law: at equilibrium, and as the dissolved amount tends to zero, the concentration of a volatile solute in a liquid is proportional to its partial pressure in the gas above. The constant depends on the gas, the solvent and the temperature. It works best for sparingly soluble gases at modest pressures. For gases that react with water, such as CO₂, SO₂ and ammonia, published constants come in an intrinsic form, counting only the dissolved molecule, and an effective form, counting everything it turns into, and the two can differ by orders of magnitude.

How to use this calculator.

1. 01Choose what to solve for: the dissolved concentration, the partial pressure, or the constant itself from your own measured pair.
2. 02Pick the convention that matches the unit of your constant: mol m⁻³ Pa⁻¹ or mol L⁻¹ atm⁻¹ for solubility forms, Pa m³ mol⁻¹ for the volatility form, or dimensionless. Sander's compilation at henrys-law.org lists 46,434 values for 10,173 species with their sources.
3. 03Enter the partial pressure of that one gas in kPa, not the total pressure. For a mixture it is the gas's mole fraction times the total; for humid air, subtract the water vapour pressure first.
4. 04Enter the temperature your constant applies to, in kelvin, and the gas's molar mass for the mg/L result: 31.998 g/mol for O₂, 28.014 for N₂, 44.009 for CO₂.
5. 05Round the answer to two significant figures unless your constant is known better than that; most are not.

The formula.

The working equation is c = H^cp × p, with c in mol m⁻³, p in Pa and H^cp in mol m⁻³ Pa⁻¹. Your mol/L and kPa are each multiplied by 1,000 on the way in and divided on the way out. Rearranged, p = c ÷ H^cp and H^cp = c ÷ p. Whatever form you entered is first turned into H^cp: a mol L⁻¹ atm⁻¹ value is divided by 101.325, which is exactly 101,325 Pa per atmosphere over 1,000 L per cubic metre; a volatility constant H^pc is inverted, 1 ÷ H^pc, because it is a reciprocal, not a rescaling; and a dimensionless H^cc, the ratio of the concentration in the liquid to that in the gas, is divided by R × T, with R = 8.314462618 J mol⁻¹ K⁻¹ (CODATA 2022). At 298.15 K, R × T is 2,478.96, which is why 1 mol m⁻³ Pa⁻¹ corresponds to a dimensionless 2,478.96 at that temperature. Zero is refused for every input the chosen "solve for" uses, because several of the reported forms divide by it; a box the calculation does not use is ignored, so it can hold 0 or be left as it is. The law assumes an ideal gas phase, a dilute solution and no reaction between gas and solvent.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Sander, R., Acree, W. E., De Visscher, A., Schwartz, S. E. & Wallington, T. J. (2022). Henry's law constants (IUPAC Recommendations 2021). Pure and Applied Chemistry 94(1), 71–85. The governing definition set used by this page: the split into Henry's law solubility constant H^s = Q_liquid/Q_gas and volatility constant H^v = Q_gas/Q_liquid, the eight recommended variants with their two-letter superscripts and coherent SI units (Fig. 1 and §2.1), the conversion table (Tab. 2), the van 't Hoff temperature dependence (§4.2), and the intrinsic-versus-effective distinction (§4.4). Explicitly supersedes all earlier IUPAC recommendations including the Green Book symbol k_H. Text verified from the open-access author manuscript (BNL-220956-2021-JAAM) 2026-07-29.
2. [2]Sander, R. (2023). Compilation of Henry's law constants (version 5.0.0) for water as solvent. Atmospheric Chemistry and Physics 23, 10901–12440. 46 434 values for 10 173 species from 995 references. Source of this page's conversion factors (Table 1: H^cp in M/atm = H^cp(SI) × 101.325; H^pc = 1/H^cp; H^cc = H^cp·R·T; Table 2: 1 mol m⁻³ Pa⁻¹ ⇔ 101.325 M/atm ⇔ H^cc 2478.96 at 298.15 K) and of the worked example's constant. Supersedes the obsolete version 4.0 (2015). Open access; retrieved and text-verified 2026-07-29.
3. [3]Sander, R. Henry's Law Constants — species entry for oxygen (CAS RN 7782-44-7), henrys-law.org, version 5.0.0. Lists H^cp_s = 1.3 × 10⁻⁵ mol m⁻³ Pa⁻¹ with d ln H^cp_s / d(1/T) = 1500 K at the reference temperature 298.15 K from Burkholder et al. (2019), together with more than thirty independent determinations ranging from 1.1 to 1.4 × 10⁻⁵. This is the page from which the worked example's constant and its quoted uncertainty spread were taken. Open access; table read directly 2026-07-29.
4. [4]Tiesinga, E., Mohr, P. J., Newell, D. B. & Taylor, B. N. CODATA Recommended Values of the Fundamental Physical Constants: 2022. NIST Standard Reference Database 121, 'molar gas constant' R = 8.314 462 618… J mol⁻¹ K⁻¹, listed with standard uncertainty '(exact)'. Used in the H^cp ↔ H^cc conversion. Independent national metrology institute; open access; retrieved 2026-07-29.
5. [5]National Institute of Standards and Technology. NIST Chemistry WebBook, SRD 69 — Thermophysical Properties of Fluid Systems, water saturation table computed from the IAPWS Formulation 1995 (Wagner & Pruss, J. Phys. Chem. Ref. Data 31, 387, 2002). At 25.0000 °C: saturation pressure 3.169 93 kPa and saturated-liquid molar density 55.3421 mol/L. Both figures are used on this page — the first for the humid-air correction in the worked example, the second in the mole-fraction/molality conversion note. Values pulled directly from the data endpoint 2026-07-29.