Dew point calculator

At what temperature will your windows fog up?

Find the dew point from temperature and humidity, or the humidity from the dew point.

Find

Air temperature

°C

Relative humidity

More options

Temperature unit

Dew point

12.0 °C

Absolute humidity 10.3 g/m³

T_d = c·γ / (b − γ), γ = ln(RH/100) + b·T / (c + T), b = 17.62, c = 243.12 °C

How to use

Choose what to find: the dew point, the relative humidity or the air temperature.
Type the other two, for example the room temperature and the humidity from a hygrometer. Switch to °F under More options.
Read the answer and the absolute humidity, the grams of water in each cubic metre of air.

Dew point at an air temperature of 20 °C

Relative humidity	Dew point (°C)
30%	1.9
40%	6
50%	9.3
60%	12
70%	14.4
80%	16.4

How it works

Warm air can hold more water vapour than cold air. Relative humidity says how full the air is compared with what it could hold at its temperature. Cool that air and it fills up: the dew point is the temperature at which it is full and water starts to condense, on a cold window, a bottle from the fridge or the grass at night.

The calculator uses the Magnus formula for the pressure of saturated water vapour, eₛ = 6.112 hPa × exp(17.62 T / (243.12 + T)), with T in °C and the coefficients of Sonntag (1990). The dew point is where the vapour in the air would saturate: with γ = ln(RH/100) + 17.62 T / (243.12 + T), T_d = 243.12 γ / (17.62 − γ). For example, a room at 20 °C and 60 % humidity has its dew point at 12.0 °C, so a window colder than that mists up. Absolute humidity follows from the ideal gas law: about 10.3 g of water per m³ here.

The formula is accurate to about 0.35 °C between −45 °C and 60 °C, over liquid water. Below freezing, frost forms at a slightly higher temperature than the dew point shown.

Frequently asked questions

What dew point feels comfortable?

Most people find a dew point below about 13 °C (55 °F) comfortable, while above 18 °C (65 °F) the air feels muggy, whatever the temperature.

Why do my windows fog up in winter?

The glass is colder than the room air's dew point. Lowering the humidity, airing the room or warmer glass (better insulation) stops it.

What is the difference between relative and absolute humidity?

Absolute humidity is the amount of water in the air, in grams per cubic metre. Relative humidity is that amount as a share of the most the air could hold at its temperature, so it changes when the air warms or cools even if the water stays the same.

Can the dew point be above the air temperature?

Not for the inputs accepted here. At 100% relative humidity, dew point equals air temperature.

Is dew point the same as frost point?

No. This calculation is for liquid water; below freezing, frost point is slightly higher.