Relative humidity compares how much water vapour the air holds with the maximum it could hold at that temperature. At 100% the air is saturated: fog forms and evaporation stops.
How to use the Relative Humidity Calculator
- Air temperature
- Dew point
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the air temperature and dew point.
Inputs explained
| Field | What to enter |
|---|---|
| Air temperature | Units: °C, °F, K |
| Dew point | Units: °C, °F, K |
What each result means
- Relative humidity = 100×6.112×exp(17.62×([Dew point])÷(243.12 + ([Dew point])))÷6.112×exp(17.62×([Air temperature])÷(243.12 + ([Air temperature])))
- Vapour pressure = 6.112×exp(17.62×([Dew point])÷(243.12 + ([Dew point])))
- Saturation vapour pressure = 6.112×exp(17.62×([Air temperature])÷(243.12 + ([Air temperature])))
Example values
How the main result changes when Air temperature changes and everything else stays at the example values above:
| Air temperature (°C) | Relative humidity (%) |
|---|---|
| 12.5 | 659.0129 |
| 18.75 | 983.1009 |
| 25 | 1,439.4748 |
| 31.25 | 2,071.4059 |
| 37.5 | 2,932.8215 |
| 50 | 5,623.4887 |
Formula
RH = 100·e_s(T_d)/e_s(T), e_s(T) = 6.112·exp(17.62T/(243.12 + T))
Worked example
Inputs — Air temperature: 25 °C; Dew point: 15 °C.
Results — Relative humidity = 1,439.4748 %; Vapour pressure = 17.0167 hPa; Saturation vapour pressure = 31.6006 hPa; Indoor comfort = Too humid.
Key terms
- Relative humidity
- Ratio of actual to saturation vapour pressure.
Frequently asked questions
What is a healthy indoor humidity?
About 30–60%.
Why does RH change during the day?
Temperature changes while the water content stays similar.
Can RH exceed 100%?
Only briefly (supersaturation) before condensation.
How do I raise indoor humidity?
Use a humidifier or ventilate less in winter.