Dew point calculator

Dew point versus relative humidity

Relative humidity describes how close the air is to saturation at its current temperature. The percentage can change when air warms or cools even if its moisture content does not change. Dew point is useful when comparing the moisture in air at different temperatures.

A surface at or below the dew point can develop condensation. Whether condensation actually forms also depends on the surface and the surrounding conditions. Dew point is not a mold diagnosis or an indoor air-quality assessment.

The calculation used here

This estimate uses the liquid-water saturation-vapor-pressure approximation documented by NWS El Paso, inverted to obtain dew point. With air temperature T in °C and humidity R in percent:

a = 7.5 * ln(10)
gamma = ln(R / 100) + a*T / (237.3 + T)
Td = 237.3*gamma / (a - gamma)

Here ln means natural logarithm, and Td is in °C. This is a Magnus-type approximation, not a direct instrument measurement. At 100% relative humidity, dew point equals air temperature. At 0%, the logarithm is undefined and this calculation cannot return a finite dew point.

Illustrative calculation: at 30°C and 70% relative humidity, the method gives a dew point of about 23.9°C. It does not give a heat index of 23.9°C.

Put the result in context

Use the dew-point chart to compare several humidity levels. For hot-weather apparent temperature, use the heat-index calculator instead. Neither reading can establish a safe exercise duration or diagnose heat illness.

For heating, ventilation or industrial decisions, use a method and instruments appropriate to the conditions and required accuracy, especially near or below freezing.