Henry's Law Calculator — Gas Solubility in a Liquid

Background.

This Henry's law calculator relates the amount of a gas dissolved in a liquid to that gas's partial pressure above it: c = H^cp × p. Enter any two of the three quantities — dissolved concentration, partial pressure, Henry's law constant — and it returns the third, together with the constant restated in four different IUPAC conventions so you can check it against whatever table you are reading.

The reason the conventions get so much attention here is that they are where this subject actually goes wrong. IUPAC's 2021 recommendations define **eight** variants of the Henry's law constant. Four are 'solubility' constants, written H^s, whose value goes up as the gas becomes more soluble; four are 'volatility' constants, written H^v, which are their reciprocals and go down. Both are routinely called 'the Henry's law constant'. Worse, several share a unit: H^px_v and H^pw_v are both in pascals but are different quantities, and the two dimensionless variants cannot be told apart by inspection at all. A statement like 'this species has a high Henry's law constant' is genuinely ambiguous — it could mean very soluble or very insoluble. This page therefore makes you name your convention before it will use your number, and reports the answer back in four of them.

**Two scope limits belong right here, next to the number, not in an accordion.** First, Henry's law is a limiting law valid at infinite dilution. A constant measured at a real, finite concentration is what IUPAC calls an 'experimental Henry's law constant', and it is not actually constant — it drifts with concentration. For sparingly soluble gases at ordinary partial pressures this hardly matters; for a soluble gas at high pressure it matters a great deal. Second, and more dangerous: for species that hydrate or ionise on dissolving — carbon dioxide, sulphur dioxide, ammonia, formaldehyde — there are two different constants in circulation. The 'intrinsic' constant counts only the physically dissolved molecule; the 'effective' constant counts everything the molecule turns into, and for a species that mostly converts it can be orders of magnitude larger and pH-dependent. This calculator does the arithmetic your number implies. It cannot tell which kind you gave it.

Temperature deserves its own warning. The calculator has a temperature field, but it uses it for exactly one thing: converting between the pressure-based constant H^cp and the dimensionless form H^cc = H^cp·R·T. **It does not adjust your constant to a different temperature.** A Henry's law constant is temperature-specific, gas solubility falls steeply as water warms, and moving a constant from one temperature to another needs the van 't Hoff equation and an enthalpy of dissolution — which is what the van 't Hoff calculator on this site is for. Enter a constant that already belongs to your working temperature.

The remaining conventions are straightforward. Pressure is the **partial** pressure of the one gas, absolute, in kilopascals — not the total pressure of the mixture above the liquid; use the partial pressure calculator to get there from a composition. Concentration is amount-of-substance concentration in mol/L, restated in mg/L using a molar mass you supply. The molar mass and the Henry constant are both editable inputs rather than baked-in values, because neither is a constant of nature this page has any business fixing on your behalf: published Henry constants for a single well-studied species commonly scatter by ten to twenty percent, and the compilation this page points you at reports its recommended values to only two significant figures. Round your answers accordingly — two significant figures is usually the honest limit, however many decimal places the output shows.

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

Henry's law states that, at equilibrium and in the limit of infinite dilution, the abundance of a volatile solute dissolved in a liquid is proportional to its abundance in the gas phase. William Henry described the observation in 1803: water takes up, of a gas compressed by one, two or more additional atmospheres, a quantity which at ordinary pressure would be twice, three times and so on the volume absorbed under normal pressure. The proportionality factor is the Henry's law constant. IUPAC's 2021 recommendations, which supersede all earlier IUPAC guidance including the Green Book symbol k_H, split it into two named families. If the constant is defined with the liquid-phase abundance on top, it is a Henry's law SOLUBILITY constant H^s and its value rises with solubility; if the gas-phase abundance is on top, it is a Henry's law VOLATILITY constant H^v and its value falls with solubility. Eight variants are recommended, four of each, distinguished by a two-letter superscript naming the numerator and denominator quantities: H^cp_s = c_liquid/p in mol m⁻³ Pa⁻¹, H^xp_s = x/p in Pa⁻¹, H^bp_s = b/p in mol kg⁻¹ Pa⁻¹, H^cc_s = c_liquid/c_gas dimensionless, and their four reciprocals H^pc_v, H^px_v, H^pw_v and H^cc_v. This calculator works internally in H^cp_s and accepts input in four of the eight, converting with the factors tabulated in Sander's compilation: H^cp in mol L⁻¹ atm⁻¹ is 101.325 times the SI value, H^pc is its reciprocal, and H^cc is H^cp × R × T. Henry's law is the solute-side limiting law of a solution, the counterpart of Raoult's law which governs the nearly-pure solvent. The two are not special cases of one another and their proportionality constants are unrelated: Raoult's constant is the solute's own pure vapour pressure, while a Henry constant is a measured property of that solute in that particular solvent.

How to use this calculator.

1. 01Decide the temperature you are working at, and find a Henry's law constant for your gas, your solvent and that temperature. Sander's compilation at henrys-law.org lists 46 434 values covering 10 173 species and gives the source of each.
2. 02Read the units of the constant carefully and pick the matching convention. mol m⁻³ Pa⁻¹ and mol L⁻¹ atm⁻¹ are solubility forms; Pa m³ mol⁻¹ is the reciprocal volatility form; a dimensionless value needs you to check the source, because the unit cannot tell you which of the two it is.
3. 03Enter the partial pressure of your gas in kPa — its own share of the pressure above the liquid, not the total. If you have a composition instead, get the partial pressure from the partial pressure calculator first.
4. 04Enter the temperature the constant belongs to, and the molar mass of the gas if you want the mg/L output.
5. 05Read the dissolved concentration in mol/L and mg/L, and check the four restated forms of the constant against your source to confirm you picked the right convention.
6. 06To go the other way — you measured a concentration and want the equilibrium partial pressure, or you measured both and want the constant — switch the first menu.
7. 07Round to two significant figures unless your source justifies more. Most tabulated Henry constants do not.

The formula.

The working equation is c_aq = H^cp_s × p, with the concentration in mol m⁻³ and the partial pressure in Pa. The calculator converts your mol/L to mol m⁻³ (× 1000) and your kPa to Pa (× 1000) at the boundary, does the arithmetic in coherent SI, and converts back.

The three rearrangements are:

c = H^cp × p solve for the dissolved concentration p = c / H^cp solve for the partial pressure H^cp = c / p solve for the constant from your own measurement

Whatever convention you entered is first normalised to H^cp in SI, using the conversion factors tabulated in Sander (2023) Table 1:

from mol L⁻¹ atm⁻¹ : H^cp(SI) = value ÷ 101.325 from Pa m³ mol⁻¹ : H^cp(SI) = 1 ÷ value (a RECIPROCAL, not a rescaling) from dimensionless : H^cp(SI) = value ÷ (R × T)

The 101.325 is exact: 101 325 Pa per atmosphere divided by 1000 L per cubic metre. Sander's own conversion table states it as 1 mol m⁻³ Pa⁻¹ ⇔ 101.325 M atm⁻¹, and the dimensionless conversion at 298.15 K as 1 mol m⁻³ Pa⁻¹ ⇔ 2478.96 — which is R × T = 8.314462618 × 298.15 = 2478.957, and which this calculator reproduces.

Rounding stage: FINAL ONLY. Every conversion and division is carried at 30-digit Decimal.js precision, with a single rounding at the return boundary — to twelve decimal places rather than the usual ten, because Henry solubilities in SI units are of order 10⁻⁵ and dissolved concentrations of order 10⁻⁴, and ten places would throw away significant figures that the inputs actually carry.

Directional behaviour, read off the equations. A larger SOLUBILITY constant means more gas dissolves at the same pressure: doubling H^cp doubles the concentration. A larger VOLATILITY constant means the opposite, because it is the reciprocal — this is the single most common error in this area, and the calculator's test suite asserts it explicitly by entering the same number under both conventions and confirming the resulting concentrations differ by a factor of four rather than being equal. Concentration is strictly proportional to partial pressure, which is why a carbonated drink fizzes when opened: the CO₂ partial pressure above the liquid drops from a few hundred kilopascals to the atmosphere's 0.04 kPa, and the equilibrium dissolved concentration collapses with it.

Invalid domain. Every input must be strictly positive. A zero Henry constant, a zero pressure or a zero concentration each make at least one of the four reported forms a division by zero — the volatility form is a reciprocal and the 'solve for the constant' mode divides by the pressure. Temperature must be absolute and greater than zero, and molar mass must be positive. Each rejection names the field rather than returning a NaN or an infinity.

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.