Relative Humidity Calculator from Air Temperature and Dew Point

How to calculate relative humidity from temperature and dew point

Relative humidity compares actual vapor pressure, represented by the dew point, with the saturation vapor pressure possible at the current air temperature. Dividing the two pressures and multiplying by 100 produces relative humidity as a percentage.

The same dew point produces a lower relative humidity when air warms and a higher relative humidity when air cools. That is why an unchanged amount of atmospheric moisture can produce very different humidity readings through the day.

How to use the relative humidity calculator with dry-bulb temperature

Enter the ordinary dry-bulb air temperature first, then enter the dew point measured for the same air sample. Select Celsius, Fahrenheit, or Kelvin independently for each reading; the calculator converts both values before comparing vapor pressures.

Choose Calculate to return RH as a percentage. The supporting result shows actual vapor pressure at the dew point, saturation vapor pressure at the air temperature, and absolute humidity so you can inspect the relationship instead of relying on a percentage alone.

• Use air temperature, not a wall, window, roof, or pavement surface temperature.
• Use dew point and temperature observations from roughly the same time and location.
• Confirm that dew point is less than or equal to air temperature.
• Enter humidity as a percentage only when using the main multi-value calculator.

Relative humidity example from 25°C air and a 15°C dew point

At an air temperature of 25°C and a dew point of 15°C, the Magnus vapor-pressure ratio gives approximately 53.8% relative humidity. In Fahrenheit, the same inputs are 77°F and 59°F and produce the same percentage.

If the dew point remains 15°C while the air warms, relative humidity falls because the warmer air has a higher saturation vapor pressure. If the air cools toward 15°C, relative humidity rises toward 100% even though the water-vapor amount has not increased.

Input checks when calculating relative humidity from dew point

For ordinary atmospheric conditions, dew point should not exceed air temperature. If it does, the implied relative humidity is greater than 100%, so the calculator stops and asks you to verify the values and units.

Use observations taken at roughly the same place and time. Pairing an afternoon air temperature with a morning dew point can produce valid arithmetic but may not represent either observation period.

Relative humidity versus absolute humidity and dew point

Relative humidity describes how close the air is to saturation at its current temperature. Dew point tracks the actual moisture state more directly, while absolute humidity reports the mass of water vapor per volume of air under the calculator's stated assumptions.

Use RH when checking current indoor or outdoor saturation conditions. Use dew point when comparing moisture across changing temperatures or evaluating whether a surface could cool enough for condensation. Neither value alone measures every aspect of comfort, mold risk, or air quality.

Relative humidity calculator questions using dew point