Measure room temperature away from a radiator, open window, direct sun or the immediate plume from cooking or showering.
Free · Celsius or Fahrenheit
Dew point and condensation risk calculator.
Enter room temperature and relative humidity to calculate the dew point, then compare it with a measured window, wall or pipe surface. No email required.
Calculated dew point
13.2°Cdew pointCondensation conditions
Entered humidity65%
Surface margin-1.2°C
Surface entered12.0°C
The surface is at or below the calculated dew point. Water can condense there if these readings represent the air touching it. This is a screening calculation, not a damp, mould, ventilation or building diagnosis.
Direct answer
A surface at or below the dew point can collect condensation.
The dew point changes with both air temperature and relative humidity, so there is no single safe surface temperature for every room. Measure the air near the area you are investigating, then measure the coldest relevant surface.
For example, air at 20°C and 65% RH has a calculated dew point of about 13.2°C. A 12°C surface is below that result, so condensation is possible if those readings represent the air touching it.
One reading is a snapshot. Repeat the check at the time the problem normally appears and use the seven-day humidity log to find a pattern.
Three measurements
How to use the condensation calculator.
Record relative humidity in the same area and allow the sensor time to settle. A hygrometer measures air; it does not measure the wall.
Measure the target surface separately. Check glazing edges, corners and behind furniture because they can be colder than the centre.
Interpretation
Read the surface margin, not only the headline.
| Surface margin | Calculator label | What to do next |
|---|---|---|
| 0°C or below | Condensation conditions | The entered surface is at or below the calculated dew point. Recheck the readings and investigate the moisture and cold-surface pattern. |
| More than 0°C, up to 2°C | Very small margin | Small local or overnight changes could cross the dew point. Check colder edges, corners and hidden surfaces. |
| More than 2°C | Above the dew point | The measured spot is above the calculated dew point, but that does not prove every surface is dry or diagnose the building. |
The 2°C band is a Better Home Climate screening convention, not a published building standard or a guarantee against condensation.
Calculation method
How the dew point is calculated.
γ = ln(RH ÷ 100) + (17.62 × T) ÷ (243.12 + T)
Dew point = (243.12 × γ) ÷ (17.62 − γ)
The calculator converts the air temperature to Celsius, applies the Magnus equation with the constants published by the UK's National Physical Laboratory, then converts the result back if Fahrenheit is selected.
NPL describes this approximation for a broad meteorological temperature range. Household sensor accuracy, placement and rapidly changing conditions can create more uncertainty than the displayed decimal suggests.
- National Physical Laboratory: dew point and relative humidity ↗
- UK Government: understanding and addressing damp and mould ↗
Method and page checked 20 September 2026.
Limits
What this calculation cannot diagnose.
Penetrating damp, rising damp, plumbing faults and rain entry need investigation of the source.
Thermal bridges, corners and spaces behind furniture can be colder than the measured surface.
Cooking, showering, laundry, overnight cooling and ventilation can change the dew point and surface temperature.
Common questions
Dew point calculator FAQs.
What is the dew point?
The dew point is the temperature at which air with its current moisture content reaches saturation as it cools. A surface at or below that temperature can collect condensation when the entered air conditions represent the air touching it.
What surface temperature causes condensation?
There is no single temperature for every room. It depends on the air temperature and relative humidity. Enter those readings to calculate the dew point, then compare it with the coldest relevant surface rather than the room thermostat alone.
Is a 2°C margin above the dew point enough?
Not necessarily. This calculator uses 2°C as an editorial caution band because readings and surface temperatures vary; it is not a published building standard or pass/fail threshold. Corners, glazing edges and hidden areas may be colder than the measured spot.
Does high humidity prove what is causing damp?
No. Relative humidity can help explain condensation risk but cannot identify leaks, penetrating damp, rising damp, plumbing faults or construction defects. Persistent damp or mould needs investigation of the underlying cause.
Can I calculate dew point in Fahrenheit?
Yes. Select Fahrenheit and enter all temperatures in °F. The calculator converts the inputs for the calculation and converts the dew point and surface margin back to Fahrenheit.