💦 Absolute Humidity Calculator
Enter the air temperature and relative humidity to get the absolute humidity: the real amount of water vapour in the air, in grams per cubic metre.
Absolute humidity
8.63 g/m³
= 3.771 grains/ft³
Absolute humidity is the actual mass of water vapour in a cubic metre of air (g/m³), unlike relative humidity which is a percentage of the maximum. It's computed from the Magnus saturation vapour pressure at your temperature times the relative humidity. Warm air can hold far more, so the same RH means more water when it's hot. 🔒 In your browser.
How the absolute humidity calculator works
Absolute humidity is the mass of water vapour in a given volume of air (g/m³), regardless of temperature. The tool finds the saturation vapour pressure at your temperature with the Magnus formula, multiplies by the relative humidity to get the actual vapour pressure, then converts to a mass density using the ideal-gas law: AH = 2.1674 × (saturation pressure in hPa) × RH ÷ (273.15 + T). It works in °C or °F.
Unlike relative humidity — a percentage of the maximum the air can hold — absolute humidity is an actual quantity. Because warm air holds far more moisture, 50% RH at 30 °C contains much more water than 50% RH at 10 °C, which matters for greenhouses, museums, drying and condensation control.
Frequently asked questions
What is absolute humidity?
The actual mass of water vapour in a cubic metre of air, measured in grams per cubic metre (g/m³). It doesn't depend on temperature the way relative humidity does — it's a straight amount of water.
How do you calculate absolute humidity?
From temperature and relative humidity: find the saturation vapour pressure (Magnus formula), multiply by the RH to get the actual vapour pressure, then divide by the gas constant times temperature. A common form is AH = 2.1674 × es(hPa) × RH ÷ (273.15 + T °C).
What is the difference between absolute and relative humidity?
Relative humidity is a percentage — how full the air is compared with its maximum at that temperature. Absolute humidity is the actual mass of vapour per volume. The same RH means much more water in warm air than in cold air.
What is a comfortable absolute humidity?
Indoor comfort is often around 7–12 g/m³. Below ~6 g/m³ air feels dry (static, dry skin); above ~13 g/m³ it feels muggy and raises condensation and mould risk. This tool gives the figure so you can compare.
Why does absolute humidity matter?
For anything sensitive to the real amount of water in the air: greenhouse and grow-room control, museum and archive preservation, drying processes, and predicting condensation on cold surfaces — where relative humidity alone can mislead across temperatures.