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WaterVaporPressure

Dew Point Calculator

Enter the air temperature and relative humidity to get the dew point, plus the wet-bulb temperature, actual and saturation vapor pressures, and absolute humidity. The dew point uses the Magnus inversion (±0.01 °C); the vapor pressures use the IAPWS-95 reference. Every constant is cited, and it all runs in your browser.

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Dew point
13.86 °C
Wet-bulb temperature
18 °C
Stull 2011 approximation
Actual vapor pressure
1.5849 kPa
RH × saturation
Saturation vapor pressure
3.1698 kPa
IAPWS-95 at the dry-bulb T
Absolute humidity
11.52 g/m³
vapor mass per m³ of air

Dew point uses the Magnus inversion with the Alduchov–Eskridge constants (±0.01 °C); the saturation and actual vapor pressures use the IAPWS-95 reference; wet-bulb uses Stull (2011). Every value is computed live in your browser.

From relative humidity to dew point

Relative humidity tells you how close the air is to saturation, but it is temperature-dependent — 50% RH at 30 °C holds far more water than 50% RH at 10 °C. The dew point removes that dependence: it is an absolute measure of moisture, the temperature at which the air’s current water-vapor content would saturate. That is why meteorologists track dew point for comfort and fog, and why HVAC and preservation work design to it directly.

This calculator computes the actual vapor pressure as RH × saturation pressure, with the saturation pressure taken from IAPWS-95 (the international reference), then inverts the Magnus relation to recover the dew point. The wet-bulb temperature — the limit of evaporative cooling — comes from the Stull (2011) single-equation approximation and always lies between the dew point and the dry-bulb temperature.

Frequently asked questions

How is dew point calculated from relative humidity?

Dew point is the temperature to which air must cool, at constant pressure and water content, to reach saturation. This calculator uses the Magnus formula inverted analytically with the Alduchov–Eskridge (1996) constants (A = 17.625, B = 243.04): γ = ln(RH/100) + A·T/(B+T), then dew point = B·γ/(A−γ). The pre-factor cancels in the inversion, so only A and B matter, and the result is accurate to about ±0.01 °C between 0 and 50 °C.

What is the difference between dew point and wet-bulb temperature?

Dew point is the temperature at which the air's existing water vapor would start to condense — it depends only on the actual moisture content. Wet-bulb temperature is the lowest temperature reachable by evaporative cooling at the current humidity; it sits between the dew point and the dry-bulb (air) temperature. At 100% relative humidity all three are equal. This tool estimates wet-bulb with the Stull (2011) approximation.

Why does the dew point match the air temperature at 100% humidity?

At 100% relative humidity the air is already saturated, so the actual vapor pressure equals the saturation vapor pressure — no further cooling is needed to reach the dew point. The Magnus inversion returns the input temperature exactly in that case, which is a useful sanity check on any dew-point calculator.

Is this dew point over water or over ice below freezing?

Below 0 °C this calculator reports the dew point over supercooled liquid water, which is the standard WMO convention for relative humidity. The frost point — saturation over ice — is slightly higher. To compare saturation over ice directly, switch the main calculator to its 'over ice' mode, which uses the Murphy–Koop reference equation.

References

Every formula on this page is implemented from, and validated against, the following primary standards and papers.

Reviewed by Jimmy Raymond, Engineer
B.S. Environmental Engineering · B.S. Computer Science · Last reviewed June 4, 2026

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