Absolute Humidity Calculator
Convert temperature and relative humidity into the real mass of water vapour in the air — the number behind dehumidifier sizing, museum control, and grow rooms.
Two inputs, real moisture
Enter the air temperature in °C and the relative humidity in % and the calculator returns the absolute humidity in grams of water vapour per cubic metre.
Temperature does the heavy lifting
Warm air holds far more moisture, so the same relative humidity means very different absolute amounts: 60 % at 25 °C carries roughly twice the water of 60 % at 10 °C.
What is an absolute humidity calculator?
Temperature and relative humidity in, g/m³ out
An absolute humidity calculator converts the two everyday readings on a hygrometer — air temperature and relative humidity — into the actual mass of water vapour in the air, measured in grams per cubic metre. Relative humidity only tells you how full the air is compared with its capacity at that temperature, and that capacity rises steeply as the air warms. Absolute humidity strips out the temperature dependence and gives the real moisture content, which is what matters for sizing a dehumidifier, controlling a museum or server room, managing a grow tent, or comparing the moisture of two rooms at different temperatures. The calculator uses the standard meteorological approximation built on the Magnus saturation-pressure formula.
Enter the air temperature and relative humidity to get the absolute humidity in g/m³ instantly.
The formula converts temperature into a saturation vapour pressure, scales it by the relative humidity, and turns the result into a mass per cubic metre.
AH = (6.112 · exp(17.67·T / (T + 243.5)) · RH · 2.1674) / (273.15 + T)The exponential term is the Magnus formula for the saturation vapour pressure at temperature T. Multiplying by RH/100 gives the actual vapour pressure, the factor 2.1674 converts pressure into a vapour mass using the molar mass of water and the gas constant, and dividing by the absolute temperature (273.15 + T) completes the ideal-gas conversion to grams per cubic metre.
Suppose the air temperature is 25 °C and the relative humidity is 60 %.
Saturation vapour pressure
6.112 · exp(17.67 · 25 / (25 + 243.5)) ≈ 31.7 hPa — the maximum vapour the air could hold at 25 °C.
Scale by relative humidity
At 60 % the actual vapour pressure is about 0.6 of that, then multiplied by 2.1674 to convert to a vapour mass.
Divide by absolute temperature
Dividing by 273.15 + 25 = 298.15 gives about 13.82 g/m³ of water vapour.
Absolute humidity is the figure to use whenever you care about the real amount of water in the air rather than how "full" it feels. A reading of 13.82 g/m³ is typical of a warm, moderately humid indoor space; comfortable indoor air is usually around 6–12 g/m³, while a muggy tropical day can exceed 20 g/m³. The single most useful insight is the temperature dependence: because warm air holds so much more vapour, a heated winter room at 21 °C and 40 % RH actually contains less water than a cool 15 °C cellar at 70 % RH, even though the cellar's relative humidity is far higher. Use absolute humidity to size a dehumidifier, decide whether ventilating with outside air will dry or dampen a room, or keep a stable environment for instruments, artwork, or plants.
The formula is the standard approximation, but a few points are worth keeping in mind.
An approximation at standard pressure
The calculation uses the Magnus saturation-pressure approximation and assumes air at roughly standard sea-level pressure; at high altitude or unusual pressures the result shifts slightly. It also assumes the air is not supersaturated and that your relative-humidity reading is accurate — a small error in RH carries straight through to the answer. Over water and over ice the saturation pressure differs near and below freezing, so very cold, near-saturated air can be a little off. For everyday indoor and outdoor use, though, the result is well within the accuracy of a typical hygrometer.