Relative Humidity Calculator
Calculate relative humidity instantly in your browser — the published formula, worked locally with no data sent anywhere.
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What this tool does
Calculate relative humidity instantly in your browser — the published formula, worked locally with no data sent anywhere. Uses the Magnus/Bolton saturation curve over liquid water; below 0 °C the value over ice would be slightly different. Absolute humidity assumes ideal-gas behaviour of water vapour.
How to use the Relative Humidity Calculator
- Enter or select air temperature (°c).
- Enter or select dew point (°c).
- Read the calculated result; change any measurement to compare alternatives.
Formula
Magnus vapour pressures: RH = 100 × e_s(dew point) / e_s(temperature), where e_s(T) = 6.112 × exp(17.67·T / (T + 243.5)) hPa (Bolton form of the Magnus equation).
- temp
- Air temperature (°C)
- dew
- Dew point (°C)
Uses the Magnus/Bolton saturation curve over liquid water; below 0 °C the value over ice would be slightly different. Absolute humidity assumes ideal-gas behaviour of water vapour.
Worked example
For relative humidity calculator, the following measurements illustrate the exact method: Air temperature (°C): 25; Dew point (°C): 15.
Inputs
- Air temperature (°C)25
- Dew point (°C)15
Result
- Relative humidity (%)53.8
- Actual vapour pressure (hPa)17.04
- Saturation vapour pressure at this temperature (hPa)31.67
- Absolute humidity (g/m³)12.39
- Dew point depression (°C)10
Results explained
- Relative humidity (%)
- Relative humidity (%) from the formula above. Uses the Magnus/Bolton saturation curve over liquid water; below 0 °C the value over ice would be slightly different. Absolute humidity assumes ideal-gas behaviour of water vapour.
- Actual vapour pressure (hPa)
- Actual vapour pressure (hPa) from the formula above. Uses the Magnus/Bolton saturation curve over liquid water; below 0 °C the value over ice would be slightly different. Absolute humidity assumes ideal-gas behaviour of water vapour.
- Saturation vapour pressure at this temperature (hPa)
- Saturation vapour pressure at this temperature (hPa) from the formula above. Uses the Magnus/Bolton saturation curve over liquid water; below 0 °C the value over ice would be slightly different. Absolute humidity assumes ideal-gas behaviour of water vapour.
- Absolute humidity (g/m³)
- Absolute humidity (g/m³) from the formula above. Uses the Magnus/Bolton saturation curve over liquid water; below 0 °C the value over ice would be slightly different. Absolute humidity assumes ideal-gas behaviour of water vapour.
- Dew point depression (°C)
- Dew point depression (°C) from the formula above. Uses the Magnus/Bolton saturation curve over liquid water; below 0 °C the value over ice would be slightly different. Absolute humidity assumes ideal-gas behaviour of water vapour.
Frequently asked questions
Magnus vapour pressures: RH = 100 × e_s(dew point) / e_s(temperature), where e_s(T) = 6.112 × exp(17.67·T / (T + 243.5)) hPa (Bolton form of the Magnus equation).
Uses the Magnus/Bolton saturation curve over liquid water; below 0 °C the value over ice would be slightly different. Absolute humidity assumes ideal-gas behaviour of water vapour.
Saturation vapour pressure roughly doubles for every 10 °C of warming. The same vapour pressure that saturates 10 °C air is only about half of saturation at 20 °C, so RH halves even though no moisture left the air.
Every field is labelled with its unit — enter values in exactly the labelled unit and read the result in the labelled output unit. Fields with a unit symbol also support the site-wide imperial/metric switch, which converts before the formula runs.
It uses double-precision arithmetic and the published constants and formulas named on this page. The last displayed digit may be rounded, and real conditions can differ from the idealized model described in the assumptions.
Yes. Every calculation runs entirely in your browser; nothing you enter is uploaded, stored or shared.