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Henry's Law Calculator

Chemistry

Henry's law: at a fixed temperature, c = H(T) × p. The amount of a gas dissolved in water is proportional to that gas's partial pressure above it. Pick a job below.

Quick Examples:

Calculator Settings

Table constants carry 2 significant figures. Near 25 °C expect results within a few percent of measured values.

Input Values

Sander (2023) compilation, 25 °C

Result

Dissolved concentration
8.554 mg/L

Equivalent values:

0.0002673 mol/L

0.2673 mmol/L

267.3 µmol/L

0.008554 g/L

8554 µg/L

0.0002681 mol/kg

0.2681 mmol/kg

x = 4.83 × 10⁻⁶

5.992 mL STP/L

p(O₂) = 0.2029 atmdissolving ⇄ escapingc = 0.2673 mmol/L · 8.554 mg/L

O₂ molecules (other gases not drawn)

Volume for volume, the water holds 3.223 % as many O₂ molecules as the gas above it. Henry's law says this ratio stays fixed, so doubling the pressure doubles both.

Measured data check
Measured saturation data (Benson & Krause 1984): 8.265 mg/L at this O₂ pressure. Henry's law with the 2-figure constant gives 8.554 mg/L (+3.5 %).

Dissolved concentration

Molar concentration0.0002673 mol/L · 0.2673 mmol/L · 267.3 µmol/L
Mass concentration0.008554 g/L · 8.554 mg/L · 8554 µg/L
Molality0.0002681 mol/kg · 0.2681 mmol/kg
Mole fraction x4.83 × 10⁻⁶
Gas volume at STP5.992 mL/L · 0.005992 volumes

mol/kg and mole fraction treat the solution as pure water at 997 kg/m³, which is fine for dilute gases. For other units, use the Solution Concentration Converter.

Conditions

Partial pressure0.2029 atm (20.95 % of the dry gas at 1 atm, after water vapour takes 0.03128 atm)
Water vapour P_H₂O0.03128 atm
Henry's constant at T0.001317 M/atm
Hᶜᶜ at T0.03223
Temperature coefficientC = 1500 K, Δ_sol H = −12.47 kJ/mol
Insights
  • Volume for volume, the water holds 3.223 % as many O₂ molecules as the gas above it (Hᶜᶜ = 0.03223 at 25 °C).
  • From here, each 10 °C of warming lowers the Henry's constant by 15.06 %.

Step-by-Step Solution

Partial pressure: p = y × (P_total − P_H₂O) = 0.2095 × (101300 − 3170) Pa = 20560 Pa = 0.2029 atm

Henry's constant at 25 °C: H = 1.3 × 10⁻⁵ mol/(m³·Pa) (no temperature correction needed)

c = H × p = 1.3 × 10⁻⁵ × 20560 = 0.2673 mol/m³ = 0.2673 mmol/L

Mass concentration: 0.2673 mmol/L × 31.9988 g/mol = 8.554 mg/L

Solubility chart

c (mg/L)

Partial pressure (atm)

Henry line: slope = k_H = 0.001317 M/atm at 25 °C

measured (Benson & Krause), 0–40 °C

Henry's law is a straight line through zero: double the gas's pressure and twice as much dissolves.

Solubility ladder

Table gases at 25 °C. Top scale Hᶜᵖ in mol/(m³·Pa), bottom scale M/atm (log).

10⁻⁶0.00010110⁻⁵0.0010110⁻⁴0.010110⁻³0.101HeN₂H₂O₂Ar, CH₄CO₂

CO₂ is about 89 times more soluble than helium. A constant that lands 100× or 1000× off this scale usually has the wrong unit.

Henry's law covers the dissolved gas; for the vapour pressure of the water itself see the Raoult's Law Calculator, and for partial pressures at depth or altitude the Dalton's Law Calculator.

About This Tool

Henry's Law Calculator – How Much Gas Dissolves in Water

Henry's law says that at a fixed temperature, the amount of a gas dissolved in a liquid is proportional to that gas's partial pressure above it: c = k_H × p. This calculator works out dissolved oxygen and dissolved CO₂ in water. It shows how much gas comes out when a soda is opened or a glass of water warms up, and it converts a Henry's law constant between every common unit.

The law and its partner, Raoult's law

William Henry described the rule in the early 1800s after measuring how much of several gases water absorbs at different pressures. Each gas behaves independently. Air at 1 atm is about 21 % oxygen, so the water only "feels" about 0.21 atm of O₂, and the nitrogen and argon don't change that. Henry's law describes the dilute solute. The solvent follows Raoult's law, which gives the vapour pressure of the water itself. Together they describe a dilute solution from both ends.

Why the constant comes in so many forms

The same physical fact can be written as solubility (concentration ÷ pressure, Hᶜᵖ) or as volatility (pressure ÷ concentration, Kᵖᶜ). The concentration can be molarity, molality, a mole fraction or a gas-phase concentration. That gives at least eight conventions, and several are inverses of each other. Take oxygen's textbook value of 1.3 × 10⁻³ mol/(L·atm). Water holds about 55.3 mol/L of water molecules, so as a mole-fraction volatility it becomes Kᵖˣ = 55.3 / 1.3 × 10⁻³ ≈ 4.26 × 10⁴ atm, the large number chemical engineers quote. The two numbers differ by more than 10⁷, and they describe the same gas. The Convert a constant tab lists every form. It also tells you which conventions your number fits, which catches the most common unit mistakes.

Fish, streams and dissolved oxygen

Warm water holds less gas. Dissolving releases heat, so warming pushes the equilibrium back toward the gas. For oxygen, the constant drops by roughly 15 % for every 10 °C. Air-saturated water carries about 11.3 mg/L of O₂ at 10 °C but only about 8.3 mg/L at 25 °C. Many fish need at least 5–7 mg/L, so a warm, still pond can run short of oxygen in summer. Try the "How warm before O₂ drops to 7 mg/L?" example. Measured saturation tables (Benson & Krause, used by water-quality agencies) differ from the two-figure constant by a few percent, and the tool shows both.

Carbonation and "volumes of CO₂"

Drink makers quote carbonation in volumes: litres of CO₂ gas at 0 °C and 1 atm per litre of liquid. A typical soda holds about 3.5 volumes, close to 7 g/L, under roughly 2.5 atm of CO₂ in a cold can. Once the can is open, the CO₂ above the liquid drops to the 0.04 % in air, and almost all of the dissolved gas is now in excess. Henry's law gives the end point but not the speed. Bubbles need somewhere to form, so a soda goes flat over minutes to hours, and faster when it is warm.

Divers and the bends

At 30 m of sea water the total pressure is about 4 atm, so roughly four times as much nitrogen dissolves in the body's fluids. On a fast ascent that extra gas can form bubbles. The Gas released tab includes a rough illustration that treats tissue as plain water. To find the pressure at a given depth, use the Dalton's Law Calculator.

Not for dive planning
The diving example is an educational illustration. Real decompression depends on tissue types, time and blood flow, and needs proper dive tables or a dive computer.

Limits of Henry's law

The law is a dilute, low-pressure limit. Above about 10 bar, real solubility falls below the straight line. Gases that react with water, such as ammonia, sulfur dioxide and hydrogen sulfide, depend on pH, so they are not in the table. For CO₂ the tool counts only the dissolved CO₂(aq), not the bicarbonate that forms above about pH 5. Salt water holds less gas than fresh water ("salting out"), and other solvents need their own constants. To express a concentration in more units, use the Solution Concentration Converter.

Tip
If a constant from a book gives an answer that is 100 or 1000 times off, check the unit before anything else. The solubility ladder in the tool shows where every table gas sits, so a value far outside it stands out.

Frequently Asked Questions

Is the Henry's Law Calculator free?

Yes, Henry's Law Calculator is totally free :)

Can I use the Henry's Law Calculator offline?

Yes, you can install the webapp as PWA.

Is it safe to use Henry's Law Calculator?

Yes, any data related to Henry's Law Calculator only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

What is Henry's law?

At a fixed temperature, the amount of a gas that dissolves in a liquid is proportional to that gas's partial pressure above it: c = k_H × p. Each gas in a mixture follows the law on its own, using only its own partial pressure.

How does this calculator work?

It converts every Henry's constant to Hᶜᵖ in mol/(m³·Pa), applies the temperature correction H(T) = H° × exp[C × (1/T − 1/298.15 K)], and multiplies by the partial pressure. In a mixture it first subtracts water vapour (IAPWS-IF97) from the total pressure if the gas is humid. Gas constants come from Sander's 2023 compilation at 25 °C.

Why are there so many different units for Henry's constant?

Some fields write it as solubility (concentration ÷ pressure) and others as volatility (pressure ÷ concentration), which are inverses. Each can use molarity, molality, mole fraction or gas-phase concentration. The same O₂ constant is 1.3 × 10⁻³ M/atm, or about 4.3 × 10⁴ atm as an engineering Kᵖˣ. The Convert a constant tab lists all of them and tells you which ones your number matches.

Why does warm water hold less dissolved oxygen?

Dissolving these gases releases heat, so warming the water moves the equilibrium back toward the gas phase and Henry's constant falls. For O₂ it drops by about 15 % for every 10 °C near room temperature. Warm water also holds more water vapour, which lowers the O₂ partial pressure a little.

What does "volumes of CO₂" mean for carbonated drinks?

It is the volume the dissolved CO₂ would take up as a gas at 0 °C and 1 atm, divided by the volume of the liquid. A soda at about 3.5 volumes holds roughly 7 g of CO₂ per litre, which needs about 2.5 atm of CO₂ pressure at fridge temperature.

When does Henry's law stop working?

It is a dilute, low-pressure law. It drifts above about 10 bar, and it fails for gases that react with water (NH₃, SO₂, or the bicarbonate that forms from CO₂). It also changes in salt water, which holds less gas. The tool's table constants have 2 significant figures, so expect results within a few percent of measured data near 25 °C.