Relative Humidity Calculator
Use our relative humidity calculator to calculate relative humidity (%) from air temperature and dew point in Fahrenheit. Includes the relative humidity formula, how to calculate relative humidity with temperature, and what a good relative humidity range is.
What Is Relative Humidity?
Relative humidity (RH) describes how close the air is to being saturated with water vapor. It is expressed as a percentage: 100% RH means the air is fully saturated at that temperature, while lower values mean the air can hold more moisture.
Because warmer air can hold more water vapor, relative humidity depends on temperature. That means the same amount of moisture in the air can produce different RH values at different temperatures.
This calculator finds relative humidity using air temperature and dew point (both in °F). Dew point is especially useful because it reflects the actual moisture content of the air.
Relative Humidity Formula
Relative humidity can be computed from temperature and dew point by comparing saturation vapor pressures at those temperatures.
Td is dew point and T is air temperature. e(x) is the saturation vapor pressure at temperature x.
This form expects T in °C. Convert °F to °C first, then compute RH and return the final RH as a percent.
Apply this conversion to both air temperature and dew point before using the vapor pressure equation.
A bigger gap between temperature and dew point usually means lower relative humidity.
How to Calculate Relative Humidity
- 1
Enter the air temperature (°F).
- 2
Enter the dew point (°F).
- 3
The calculator converts both values to Celsius internally and applies the relative humidity formula.
- 4
Read your result as relative humidity (%).
Frequently Asked Questions
One common method uses temperature and dew point: compute vapor pressure at dew point and divide by saturation vapor pressure at air temperature, then multiply by 100.
Not automatically. If the amount of moisture stays the same, increasing temperature usually lowers relative humidity because the air’s moisture capacity increases.
Many people find about 30% to 50% comfortable for indoor air. Very low RH can feel dry, while very high RH can feel muggy and may encourage mold.
Outdoor relative humidity varies widely by climate, season, and time of day. It can range from very dry values in deserts to near 100% in fog, rain, or tropical conditions.
Dew point tracks actual moisture in the air more directly than RH. Using temperature and dew point together gives a consistent way to compute relative humidity.