# Relative Humidity Calculator

URL: https://speedycalculator.com/calculators/physics/relative-humidity/
Last updated: 2026-10-30

> The relative humidity calculator finds the relative humidity of air from its temperature and dew point using the Magnus formula.

## Inputs

- Air temperature (°C, °F, K)
- Dew point (°C, °F, K)

## Results

- Relative humidity
- Vapour pressure
- Saturation vapour pressure
- Indoor comfort

## Formula

```
RH = 100·e_s(T_d)/e_s(T),  e_s(T) = 6.112·exp(17.62T/(243.12 + T))
```

## Explanation

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

FieldWhat to enterAir temperatureUnits: °C, °F, KDew pointUnits: °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 (%)12.5659.012918.75983.1009251,439.474831.252,071.405937.52,932.8215505,623.4887

## 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.

## FAQ

**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.


## Sources

- [NOAA / National Weather Service – Weather calculators](https://www.weather.gov/epz/wxcalc)
- [Vaisala – Humidity conversion formulas](https://www.vaisala.com/en/lp/humidity-conversion-formulas)