Dew Point Calculator

How the Dew Point Is Calculated

This calculator uses the Magnus formula (the August-Roche-Magnus approximation), the standard method meteorologists use for quick, accurate dew point estimates in typical atmospheric conditions. With temperature T in Celsius and relative humidity RH as a percentage, it first computes an intermediate term alpha = ln(RH/100) + (17.27 × T) / (237.3 + T), then solves for the dew point: T_dp = (237.3 × alpha) / (17.27 − alpha). Fahrenheit inputs are converted to Celsius before this calculation and the result is converted back for display. The constants 17.27 and 237.3°C are the standard Magnus coefficients calibrated against saturation vapor pressure over liquid water, and the approximation is reliable for temperatures between roughly 0°C and 60°C with humidity above about 1%; it grows less accurate below freezing, where the physics of vapor pressure over ice diverge slightly.

Dew Point vs. Relative Humidity

Relative humidity alone is a poor comfort indicator because it's relative to the current temperature — 50% humidity feels very different at 60°F than at 90°F. Dew point, by contrast, is an absolute measure of moisture in the air: it's the temperature air would need to cool to for it to become fully saturated (100% relative humidity) and start forming dew or condensation. That's why meteorologists and HVAC professionals treat dew point, not relative humidity, as the more reliable gauge of how "muggy" the air actually feels. As a rough guide, dew points below 55°F feel comfortable, 55-65°F feels somewhat humid, and above 65°F feels increasingly oppressive.

Related Tools

Dew point is closely tied to condensation risk in building design — pair this with the heat index calculator to see how humidity affects perceived temperature on hot days, or the wind chill calculator for the cold-weather equivalent.