Henry's Law Calculator
Use Henry's law, C = kH × P, to find how much oxygen, nitrogen, CO₂ or another gas dissolves in water at a given partial pressure and temperature — in mol/L, mmol/L and mg/L.
How it works
C = kH × P · kH(T) = kH(298.15 K) × exp[ C (1/T − 1/298.15) ] · mg/L = C (mol/L) × M (g/mol) × 1000 · 1 atm = 101.325 kPa = 1.01325 bar = 760 mmHg
Henry's law (William Henry, 1803) states that at a constant temperature the amount of a gas that dissolves in a liquid is proportional to its partial pressure above the liquid. It is why carbonated drinks fizz when opened and why divers absorb more nitrogen at depth. Most gases become less soluble as water warms, which the van ’t Hoff temperature factor captures. Constants are Sander’s compiled values at 25 °C for fresh water; salt water dissolves less gas, and gases that react with water (CO₂ forming carbonic acid, ammonia) follow the law only for the dissolved molecular form.
Worked example
Air contains 21% oxygen, so at sea level the partial pressure of O₂ is 0.21 atm. At 25 °C, kH = 1.3 × 10⁻³ mol/(L·atm), so C = 1.3 × 10⁻³ × 0.21 = 2.730 × 10⁻⁴ mol/L (0.2730 mmol/L). Multiplying by 31.998 g/mol gives 8.735 mg/L of dissolved oxygen.
Sources: Sander, R. — Compilation of Henry’s law constants (ACP, 2015) · Wikipedia — Henry's law
Frequently asked questions
What is Henry's law?
The concentration of a dissolved gas is proportional to its partial pressure: C = kH·P. Double the pressure of a gas above water and twice as much dissolves, at the same temperature.
Why are there different units for Henry's constant?
Some tables give solubility constants (kH = C/P, e.g. mol/(L·atm)), others volatility constants (P/C, e.g. L·atm/mol, or atm per mole fraction). Check which one your source uses; the volatility form is the inverse of the one used here.
How does temperature affect gas solubility?
For most gases solubility falls as temperature rises. Oxygen’s Henry constant drops by about 18% between 20 °C and 30 °C, which is why warm water holds less oxygen for fish.
How much CO₂ is in a can of soda?
Cans are filled at a CO₂ pressure of about 2–3 atm. At 3 atm and 25 °C, C = 0.034 × 3 ≈ 0.10 mol/L, roughly 4.5 g of CO₂ per litre.
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This calculator is for educational purposes. Double-check important results before acting on them.