Indoor humidity
Dew point and condensation calculator
Enter your room temperature and humidity. Add a measured surface temperature if you want to check a window or wall.
Example result
Estimated dew point
12.0°C
Surfaces below 53.6°F can collect condensation from this air.
This estimates the condensation threshold, not how much water will collect or whether mold is present.
Why a window can get wet at 50% humidity →What the result means
Dew point is the temperature at which the entered air reaches saturation when cooled at constant pressure without losing moisture. Compare it with a surface temperature: glass below the dew point can collect water even when the middle of the room has a moderate humidity reading.
Example: air at 68°F (20°C) and 60% RH has a dew point of about 53.6°F (12°C). A window at 50°F (10°C) is below that threshold; a window at 59°F (15°C) is above it.
Getting useful readings
A room thermometer and hygrometer provide the two required inputs. If you do not know the inside glass temperature, leave the surface check off: outdoor temperature alone does not determine it.
To explore changes without measuring a room, try the adjustable room illustration. To compare moisture indoors and outdoors, use the ventilation calculator.
Formula, supported range and limitations
The calculation uses the National Physical Laboratory’s liquid-water Magnus approximation: γ = ln(RH/100) + 17.62T/(243.12 + T), then dew point = 243.12γ/(17.62 − γ), with T in °C. Temperature conversion is exact; results are rounded only for display.
Room and surface inputs cover 32–104°F (0–40°C); RH covers 5–100%. The optional surface check labels values within 1.8°F (1°C) of the dew point as close to the threshold. This is a display convention, not sensor accuracy or a safe limit. Subfreezing dew-point values use the liquid-water relationship, not a frost-point calculation.
The tool does not measure your room, predict drying time or diagnose a building problem. Fix leaks and investigate persistent wetness. EPA moisture guidance discusses both reducing moisture and warming cold surfaces.