Indoor Dew Point Calculator

Dew point is the temperature at which air reaches saturation. If a window or wall surface is at or below the calculated dew point, condensation can form on that surface.

Room temperature (°C)Relative humidity (%)

Estimated dew point13.2°C

If a window, wall or other surface falls to roughly this temperature or below, condensation can form on that surface. This estimate uses the Magnus formula for ordinary indoor conditions.

What the result means

This is an estimate based on the Magnus formula for ordinary indoor conditions.
Surface temperature matters: relative humidity alone cannot tell you exactly where condensation will occur.
Persistent damp can have structural causes that a humidity calculation cannot diagnose.

Related guides

Read more about the topics covered by this calculator:

Condensation on windows UK

How this calculator works

Formula

Magnus formula: Td = (b × α(T,RH)) / (a − α(T,RH)), where α = ln(RH/100) + aT/(b+T)

Inputs

Room temperature in °C
Relative humidity in %

Outputs

Estimated dew point temperature in °C

Assumptions

Uses the Magnus formula with a = 17.62, b = 243.12, valid for ordinary indoor conditions.
Surface temperature must be known separately to assess condensation risk.
This is an estimate; actual dew point can vary slightly with atmospheric pressure.

Example calculation

At 20°C and 65% RH, the estimated dew point is approximately 13.1°C. If a window surface is at or below this temperature, condensation can form.

Tariff source

N/A — this calculator does not use energy rates.

Limitations

Relative humidity alone does not tell you exactly where condensation will occur.
Surface temperature of windows, walls or mirrors determines actual condensation risk.
This tool does not diagnose building defects or structural damp.