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Thermal insulation

Dew point calculator

Calculate air dew point from temperature and humidity, and compare it with a separately measured surface temperature.

Air dew point

°C
%
°C
Leave blank for air dew point only. Measure the surface separately at the same time and place.

This is air dew point only; surface condensation also depends on surface temperature.

Calculated result

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Enter the measurements, then calculate.

Calculated in this browser.

How to use this calculator

Method and boundary

Enter air temperature in °C and relative humidity in % from the same time and place. The form accepts −45 to 60°C and humidity greater than 0 through 100%. Optionally enter a separately measured surface temperature in °C.

γ = ln(RH/100) + 17.62 × T / (243.12 + T); Td = 243.12 × γ / (17.62 − γ)

Example

At 20°C and 60%, dew point is approximately 11.995°C. A separate surface measurement of 11°C gives surface minus dew point of about −0.995 K. A 1 K difference has the same numerical size as a 1°C difference.

This Magnus approximation describes air dew point. It does not assess interstitial condensation, mould or change over time. Temperature remains °C with imperial preference.

Read guide: Dew point and condensation limitations

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