How to Calculate Dew Point
The dew point (Td) is the temperature air would need to cool to, at constant pressure and constant moisture content, for it to become fully saturated with water vapor (100% relative humidity). Cool the air any further and excess vapor condenses into liquid water — dew on grass, fog, or condensation on a cold glass. This calculator finds dew point from air temperature and relative humidity using the Magnus-Tetens approximation of the Clausius-Clapeyron relation, the standard method used in meteorology and HVAC engineering.
How the calculation works
First the calculator finds the saturation vapor pressure at the current air temperature: es = 6.1094 × exp(17.625T / (243.04 + T)), where T is in °C and es is in hectopascals (hPa). Multiplying by relative humidity gives the actual vapor pressure already in the air: e = es × RH / 100. Inverting the Magnus formula for that actual vapor pressure yields the dew point: γ = ln(RH/100) + 17.625T / (243.04 + T), then Td = 243.04γ / (17.625 − γ). If you entered temperature in Fahrenheit, the tool converts to Celsius for the calculation and converts the result back.
Common mistakes
- Confusing dew point with relative humidity: 50% RH at 35°C (95°F) is much more humid in absolute terms than 50% RH at 10°C (50°F) — dew point captures actual moisture content, RH does not.
- Expecting a dew point above the air temperature: physically impossible — the maximum dew point can reach is the current air temperature, at exactly 100% RH.
- Entering RH as a decimal instead of a percentage: use 50 for 50% humidity, not 0.5 — the formula expects RH on a 0-100 scale.
- Using the formula far outside its tested range: the Magnus-Tetens constants used here (b = 17.625, c = 243.04) are validated for air temperatures roughly between -45°C and 60°C; extreme cold uses a separate ice-based formula.
Real-world applications
- Meteorologists use dew point rather than relative humidity to describe how muggy the air will feel, since it does not change just because the temperature does.
- HVAC and building engineers use dew point to size dehumidifiers and to predict condensation risk on cold surfaces like windows, ductwork, or pipes.
- Agriculture and viticulture use overnight dew point forecasts to anticipate frost, dew formation, and fungal disease risk on crops.
- Aviation uses the spread between air temperature and dew point to estimate cloud base height and fog formation risk.