Humidity Calculator
Relative Humidity Is a Ratio, Not an Absolute Amount
Relative humidity expresses how much moisture the air is holding compared to the maximum it could hold at that temperature — not a fixed quantity of water vapor. That's why the same absolute moisture level reads as high humidity on a cool morning and moderate humidity once the air warms up: warmer air can hold more water vapor before reaching saturation. This calculator derives relative humidity from either a temperature/dew point pair or from measured vapor pressures directly.
The Formulas
RH% = 100 × exp((17.625×Td)/(243.04+Td)) ÷ exp((17.625×T)/(243.04+T))
From vapor pressures:
RH% = 100 × (Actual Vapor Pressure ÷ Saturation Vapor Pressure)
T and Td are air temperature and dew point in °C. The Magnus approximation estimates saturation vapor pressure from temperature; the ratio of actual vapor pressure (implied by the dew point) to saturation vapor pressure at the actual air temperature gives relative humidity directly.
Worked Examples
| Air temperature | Dew point | Relative humidity |
|---|---|---|
| 25°C | 15°C | 53.83% |
| 30°C | 20°C | 55.08% |
A smaller gap between temperature and dew point always means higher relative humidity; when the two are equal, the air is fully saturated at 100% RH.
Where This Matters
- Comfort and health — high relative humidity makes hot weather feel more oppressive because sweat evaporates more slowly, while very low humidity dries out skin, airways, and wood furniture.
- Mold and condensation risk — indoor relative humidity above roughly 60% creates conditions favorable to mold growth and condensation on cold surfaces.
- Agriculture and storage — greenhouse operators and grain storage facilities monitor relative humidity closely since it directly affects plant transpiration rates and spoilage risk.
How to Use This Calculator
- Choose a mode: From Temperature & Dew Point, or From Vapor Pressures.
- For temperature and dew point: enter Air Temperature and Dew Point, both in °C.
- For vapor pressures: enter the Actual Vapor Pressure and Saturation Vapor Pressure, both in hPa.
- Select Calculate to see the relative humidity percentage.
Related Calculations
Work in the opposite direction with the Dew Point Calculator, or see how humidity combines with temperature in the Heat Index Calculator.