Relative Humidity Calculator
Find relative humidity from temperature and dew point.
Relative humidity compares the moisture in the air to the maximum it can hold.
How the Math Works
The Relative Humidity Calculator uses the fundamental formula RH = 100·e(T_d)÷e(T), where RH represents relative humidity as a percentage. In this equation, e(T_d) is the saturation vapor pressure at the dew point temperature, and e(T) is the saturation vapor pressure at the current air temperature. The saturation vapor pressure values are typically calculated using the Magnus formula or similar approximations, which relate temperature to the maximum amount of water vapor the air can hold before condensation occurs. When you input the temperature and dew point, the calculator determines how much water vapor the air currently contains relative to its maximum capacity at that temperature.
Practical Applications
To use this calculator effectively, first measure or obtain the current air temperature and the dew point temperature using a thermometer or weather instrument. Enter these values into their respective fields, ensuring both are in the same unit (typically Celsius or Fahrenheit). The calculator then applies the formula to instantly compute the relative humidity percentage. This is particularly useful for meteorologists, HVAC professionals, and gardeners who need precise humidity measurements for their work, allowing them to quickly assess atmospheric conditions without manual calculations.
Day-to-Day Use
Understanding relative humidity through this calculation helps you make informed decisions in everyday life. High humidity levels can make temperatures feel hotter and increase discomfort, while low humidity can cause dry skin and static electricity. This knowledge helps you choose appropriate clothing, set your home's humidifier or air conditioner effectively, and determine when to use moisture-absorbing products. Additionally, relative humidity affects how long it takes to dry clothes, influences plant health in indoor gardening, and can signal potential mold growth in damp environments, making this calculation valuable for maintaining comfortable and healthy living spaces.
Worked example
25 °C with a 15 °C dew point → about 54% RH.
FAQ
When is RH 100%?
When the dew point equals the air temperature — the air is saturated.