dew point calculator

Dew point calculator

Estimate indoor dew point with the Magnus formula. Use it to see whether cold glass or walls will run with condensation.

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)

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10–25 €

Digital hygrometer (probe if you can)

Without a relative-humidity number, every tip is guesswork. A €10–25 hygrometer is the first useful purchase — before any dehumidifier.

When: Always, as the first diagnostic step.

Not when: It does not replace a moisture meter or a rising-damp survey.

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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.