What this calculator does
An absolute humidity calculator works out the actual mass of water vapour in a given volume of air, in grams per cubic metre, rather than the percentage figure that a weather forecast usually quotes. Relative humidity only tells you how close the air is to saturation at its current temperature; absolute humidity tells you how much water is actually there, which is what matters for things like dehumidifier sizing, greenhouse climate control, or comparing moisture levels across different temperatures.
The calculation starts from the saturation vapour pressure at the given air temperature, using the Magnus formula, a standard meteorological approximation. That saturation pressure is scaled down by the relative humidity to get the actual vapour pressure present in the air, which is then converted to a mass per volume using the ideal gas law for water vapour.
The formula
Saturation vapour pressure is estimated from temperature using es = 6.1094 x exp(17.625T / (T + 243.04)), in hectopascals, with T in degrees Celsius. Multiplying that by the relative humidity (as a fraction) gives the actual vapour pressure. Absolute humidity in grams per cubic metre then follows from AH = (actual vapour pressure x 216.74) / (T + 273.15), which converts the vapour pressure and temperature in kelvin into a vapour mass density.
| Term | Meaning |
|---|---|
| Absolute humidity | The mass of water vapour present per cubic metre of air, in grams per cubic metre, independent of the air temperature. |
| Relative humidity | The current vapour pressure expressed as a percentage of the saturation vapour pressure at that same temperature. |
| Saturation vapour pressure | The maximum vapour pressure the air can hold at a given temperature before water starts condensing out. |
The inputs explained
| Field | What to enter |
|---|---|
| Air temperature (°C) | The air temperature the humidity reading was taken at, in degrees Celsius. |
| Relative humidity (%) | The relative humidity reading, as a percentage from 0 to 100. |
When to use it
Sizing a dehumidifier
Dehumidifier capacity is usually rated in litres of water removed per day at a reference condition; converting a room's actual temperature and relative humidity to absolute humidity gives a clearer picture of how much moisture is really present to remove.
Comparing indoor and outdoor air
Two air masses can share the same relative humidity reading yet hold very different amounts of water vapour if their temperatures differ, since warmer air can hold more vapour at the same relative humidity. Absolute humidity makes that comparison fair.
Greenhouse or grow-room climate control
Plant transpiration and mould risk relate more closely to the actual moisture content of the air than to the relative humidity percentage alone, so tracking absolute humidity alongside temperature gives a fuller picture of growing conditions.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How absolute humidity changes with temperature at a fixed relative humidity
The same 50% relative humidity reading, at a range of air temperatures.
How absolute humidity changes with relative humidity at a fixed temperature
A range of relative humidity readings, all at 20°C.
Questions
What is the difference between absolute and relative humidity?
Relative humidity is a percentage showing how close the air is to saturation at its current temperature. Absolute humidity is the actual mass of water vapour per cubic metre of air, which does not depend on how close the air is to saturation.
Why does absolute humidity rise so much with temperature at the same relative humidity?
Warmer air can hold exponentially more water vapour before saturating, so a fixed relative humidity percentage of that larger saturation capacity corresponds to a much larger actual vapour mass.
How accurate is the Magnus formula used here?
The Magnus approximation used for saturation vapour pressure is accurate to within a fraction of a percent across the ordinary range of everyday air temperatures, which is well within the precision of typical humidity sensors.
Can absolute humidity exceed 100%?
Absolute humidity is a mass per volume, not a percentage, so it has no upper cap of 100. It simply keeps rising with temperature and relative humidity; it is relative humidity that is bounded at 100%.
For moisture content in solid materials rather than air, see the moisture content calculator.