🌫️ Relative Humidity Calculator
Enter the air temperature and dew point to get the relative humidity as a percentage.
Relative humidity
59.6%
Relative humidity is the ratio of the actual water vapour to the maximum the air can hold at that temperature. It's derived from the temperature and dew point via the Magnus saturation-vapour equation. When the dew point equals the temperature, the air is saturated — 100%. 🔒 In your browser.
How the relative humidity calculator works
Relative humidity is the amount of water vapour in the air compared with the most it could hold at that temperature. The tool derives it from the temperature and dew point using the Magnus saturation-vapour-pressure relationship: it compares the saturation pressure at the dew point with that at the air temperature. When the two are equal, the air is saturated at 100%.
Because warm air holds much more moisture, the same amount of water gives a lower relative humidity when it's hot and a higher one when it's cold — which is why humidity climbs overnight as the temperature falls toward the dew point.
Frequently asked questions
How do I calculate relative humidity?
From the air temperature and dew point: RH = 100 × saturation pressure at the dew point ÷ saturation pressure at the air temperature, using the Magnus formula. This tool does the calculation for you.
What is relative humidity?
The ratio, as a percentage, of the water vapour currently in the air to the maximum the air can hold at that temperature. 100% means the air is saturated; 50% means it holds half its capacity.
Why does humidity change with temperature?
Warmer air can hold far more moisture, so for a fixed amount of water the relative humidity falls as the air warms and rises as it cools. That's why relative humidity is often highest at dawn.
What is a comfortable relative humidity?
Indoors, roughly 30–50% is comfortable and healthy. Much higher feels clammy and encourages mould; much lower dries out skin and airways. Outdoor comfort depends more on the dew point than on relative humidity.
Can relative humidity be over 100%?
Briefly and locally, air can be slightly supersaturated, but for practical purposes it caps at 100% — the point of saturation, where excess vapour condenses into dew, fog or cloud.