Dew Point Calculator
The Temperature Where Air Can't Hold Any More Moisture
Dew point is the temperature air would need to cool to before it becomes fully saturated and water vapor starts condensing out as dew, fog, or frost. Unlike relative humidity, dew point doesn't change with the current air temperature, which makes it a more reliable indicator of how muggy the air actually feels — a dew point of 65°F feels sticky whether the thermometer reads 75°F or 95°F.
The Formula
where γ = ln(RH÷100) + (17.625×T)÷(243.04+T)
T is air temperature in °C and RH is relative humidity in percent. This is the Magnus formula, a widely used approximation for computing dew point from temperature and relative humidity, accurate to within about 0.4°C over typical atmospheric conditions.
Worked Examples
| Air temperature | Relative humidity | Dew point |
|---|---|---|
| 25°C (77°F) | 50% | 13.86°C (56.95°F) |
| 30°C (86°F) | 70% | 23.93°C (75.07°F) |
Dew Point Comfort Scale
| Dew point | Perceived comfort |
|---|---|
| Below 50°F | Dry, comfortable |
| 50–60°F | Comfortable |
| 60–65°F | Slightly humid |
| 65–70°F | Humid, sticky |
| 70–75°F | Very humid, oppressive |
| 75°F+ | Extremely oppressive |
Where This Matters
- Judging real "mugginess" — because dew point doesn't shift with daily temperature swings the way relative humidity does, meteorologists consider it the more reliable comfort indicator for summer weather.
- Fog and frost forecasting — when overnight temperature is expected to drop to or below the dew point, fog or frost becomes likely.
- HVAC and dehumidification — building engineers use dew point to size dehumidification equipment and to predict condensation risk on cold surfaces like windows and pipes.
How to Use This Calculator
- Enter the Air Temperature in °C.
- Enter the Relative Humidity as a percentage (greater than 0, up to 100).
- Select Calculate to see the dew point in both °C and °F.
Related Calculations
Work in the opposite direction with the Humidity Calculator, or see how humidity affects perceived temperature with the Heat Index Calculator.