Relative Humidity Calculator
Compute relative humidity from the air temperature and the dew point using saturation vapour pressure — the physical definition of humidity.
How it works
RH = 100 · e(Td) / eₛ(T) · where eₛ(T) = 6.112·exp(17.62·T/(243.12 + T))
Relative humidity is the ratio of the water vapour actually in the air to the maximum it could hold at that temperature, expressed as a percentage. Since the saturation vapour pressure rises steeply with temperature, warm air can hold far more moisture, so the same amount of water gives a lower relative humidity when the air is warm. Computing it from the dew point — the temperature at which the current moisture would saturate the air — is the cleanest physical route, because both quantities depend only on the actual vapour content.
Worked example
At 25 °C, saturation vapour pressure is eₛ = 6.112·exp(17.62·25/268.12) ≈ 31.7 hPa. At a 15 °C dew point the actual vapour pressure is e ≈ 17.1 hPa. So RH = 100 × 17.1/31.7 ≈ 54% — the air holds a little over half the moisture it could at 25 °C.
Sources: Relative humidity — Wikipedia
Frequently asked questions
Why does relative humidity drop when air is heated?
Heating air raises its saturation vapour pressure — its capacity to hold moisture — without adding any water. The same vapour then represents a smaller fraction of a larger capacity, so relative humidity falls. This is why indoor heating in winter makes rooms feel dry.
Can relative humidity exceed 100%?
Briefly and only in special conditions. Above 100% the air is supersaturated and vapour normally condenses into cloud, fog or dew, pulling humidity back to 100%. For everyday purposes it is capped at 100%, which is why this calculator limits the dew point to the air temperature.
Is a higher relative humidity always more uncomfortable?
Not necessarily — 80% humidity at 10 °C feels raw but manageable, while 80% at 30 °C is oppressive. Dew point is a better comfort guide because it reflects the absolute moisture your body must contend with, not just a ratio.
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This calculator is for educational purposes. Double-check important results before acting on them.