Calculator

Absolute humidity calculator

Air temperature 75°F

Needed in both directions: the same grams of water is a different percentage at every temperature.

Relative humidity 60%

12.94 grams of water per cubic metre · Humid
That is the water actually present, rather than how close the air is to full.

Relative humidity: 60%
At capacity this air holds: 21.57 g/m³
Mixing ratio: 11.08 g/kg
Dew point: 60.2°F

The figures above are the answer for the values this page loaded with. Changing the controls needs JavaScript — without it the calculator cannot recalculate, so read them as one worked example rather than a live result.

How do you convert relative humidity to absolute humidity?

Multiply the saturation vapour pressure at that temperature by the relative humidity to get the actual vapour pressure, then apply the ideal gas law for water vapour. The calculator above does it in both directions. The temperature matters in both, because the same grams of water is a different percentage at every temperature.

Why this conversion is worth doing

Relative humidity is a ratio, and a ratio hides its denominator. 80% tells you the air is four fifths of the way to full without telling you how big "full" is — and "full" varies by a factor of about seven between a freezing morning and a hot afternoon.

Converting to grams per cubic metre removes the denominator. Two readings in g/m³ can be compared directly, which two percentages cannot. This is the whole content of the claim that 80% in winter is drier than 40% in summer, and once you have the converter it stops being a claim and becomes arithmetic:

0°C at 80% → 3.9 g/m³
25°C at 40% → 9.2 g/m³ The winter air, at twice the percentage, is carrying well under half the water.

Working it through

Take 75°F (23.9°C) at 60%.

Saturation vapour pressure at 23.9°C: 6.112 × exp(17.62 × 23.9 / (243.12 + 23.9)) = 29.6 hPa. This is the most vapour pressure that temperature can support.
Actual vapour pressure: 60% of that, so 17.7 hPa, or 1,770 Pa.
Density: 1,770 / (461.5 × 297.05 K) = 0.0129 kg/m³, which is 12.9 g/m³.

The capacity figure the calculator shows is step 1 carried through the same arithmetic at 100% — 21.6 g/m³ here. The gap between the two is how much more water this air could take, which is the figure behind how much water is in the air.

The reverse direction

Going from g/m³ back to a percentage is the same chain run backwards, and it is the direction you need when a datasheet, a sensor or a scientific paper gives you the density and you want to know what it would feel like. The one thing to watch: the answer is meaningless without a temperature, so the calculator keeps that input live in both directions.

Where the units come from

Grams per cubic metre is the SI-adjacent convention and what meteorological instruments report. You will also see g/kg (the mixing ratio, per kilogram of dry air) and occasionally specific humidity (per kilogram of total air, including the vapour). The three differ by under 2% at ordinary conditions, and the mixing ratio calculator covers the distinction properly.

Common questions

What is the formula for absolute humidity?

ρᵥ = e / (Rᵥ × T), where e is the actual vapour pressure in pascals, Rᵥ is 461.5 J/(kg·K) for water vapour and T is the temperature in kelvin. Multiply by 1000 for grams rather than kilograms. The vapour pressure itself comes from the saturation pressure at that temperature times the relative humidity.

Why does absolute humidity need the temperature?

Two reasons, and they are different. Converting from relative humidity needs it because the percentage is relative to the saturation point, which is set by temperature. But even the answer in g/m³ depends on temperature, because it is a density — heat a sealed parcel of air and it expands, so the same water spreads through more cubic metres.

What is a typical absolute humidity?

Roughly 5 to 15 g/m³ in temperate conditions. A cold winter day might be 2 or 3 g/m³ even at 90% relative humidity; a humid summer afternoon can pass 20 g/m³. The ceiling rises steeply with temperature — saturated air at 30°C holds about 30.3 g/m³, at 0°C only about 4.8.

What is the difference between absolute humidity and the mixing ratio?

Absolute humidity is per cubic metre of air, so it changes when the air expands or is compressed. The mixing ratio is per kilogram of dry air, which does not change when a parcel is heated or lifted. That conservation is why forecasters prefer the mixing ratio and why both are shown here.

Can absolute humidity be more than the capacity figure?

Not as vapour. Beyond saturation the surplus condenses into droplets — fog, mist or cloud — which are liquid water suspended in the air rather than water vapour in it. Enter a value above the capacity and the calculator caps the relative humidity at 100% and says so.

Sources

WMO Guide to Instruments and Methods of Observation (WMO-No. 8)

Published 31 July 2026