dew point calculator

Dew Point Calculator

The dew point is the temperature at which air, at that humidity, starts to leave water. If glass or a wall is colder, condensation appears; if the surface stays wet for long, mould has a favourable condition. Enter temperature and relative humidity: the Magnus formula gives an estimate, not a certificate.

You want to know whether cold glass or walls will run with condensation.

Measure temperature and relative humidity in the middle of the room, away from radiators and open windows.
Enter the values. The result is an estimate, not a certificate.
If a surface (glass, corner, window reveal) is near or below that figure, water can appear.

Dew point

12.0 °C

Magnus-formula estimate. Not a laboratory measurement and not a building certificate.

At 20.0 °C and 60 % RH the dew point is 12.0 °C. If a surface (single glazing, reveal, behind a wardrobe) nears that figure, condensation can appear. At 20 °C and 60 % RH the expected result is about 12.0 °C.

Magnus is an approximation. Cheap sensors are often several RH points off.

Useful next step

Measure the coldest surface and compare it with the dew point. Then, if needed, estimate mould risk.

Surface mould-risk estimate Guided damp diagnosis Window condensation: when it is normal and when it is a warning Damp at home: diagnose it before you buy anything

What may help (after diagnosis)

Some links go to Amazon.it and are affiliate links: we may earn a commission, at no extra cost. They are category examples, not lab reviews. Amazon.com is not connected on the English edition.

12–40 €

TempPro TP53

Thermo-hygrometer (T + RH together), category example. Dew point needs both numbers. Not a lab review.

When: When you want the calculators on your numbers, not a guess.

Not when: It does not measure moisture in the wall.

View on Amazon.it

Amazon.it affiliate link (tag qordac21-21): we may earn a commission, at no extra cost to you. A category example, not a lab review. Amazon.com is not used.

40–90 €

TFA Dostmann 30.5060.02

Example of a higher-spec thermo-hygrometer. Still a household instrument: it does not certify a building.

When: If you want a more stable indoor reading, once you know what the figure is for.

Not when: It is not a survey instrument.

View on Amazon.it

Amazon.it affiliate link (tag qordac21-21): we may earn a commission, at no extra cost to you. A category example, not a lab review. Amazon.com is not used.

FAQ

At 20 °C and 60 % RH, what is the dew point?+

With the Magnus formula used here it is about 12.0 °C. It is not a static value: if you change humidity or temperature, the result changes. Rounded tables sometimes print 12–13 °C.

If the glass is 11 °C and the dew point is 12 °C?+

Water can appear on the glass. That is not a joinery survey: it is moist air meeting a cold surface.

Is a €15 hygrometer enough?+

For an order of magnitude, yes. For a dispute, no. Cheap sensors are often several RH points off.