Better Home Climate data
See how the dew point changes across common room temperatures and 40%–80% relative humidity. Use the table for a quick reference, then measure the actual cold surface before drawing a conclusion about condensation.
The direct answer
At 20°C, increasing the room from 40% to 80% RH raises the calculated dew point from about 6.0°C to 16.4°C. That puts more windows, corners and other cold surfaces within condensation range.
The table is not a diagnosis. It calculates saturation temperature from the entered air condition; it cannot identify a leak, penetrating damp, rising damp, failed glazing or the coldest hidden surface.
Read across your air-temperature row to your relative-humidity column. On small screens, scroll the chart sideways to see every RH level.
| Air temperature | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH |
|---|---|---|---|---|---|
| 10°C | -3.0°C | 0.0°C | 2.6°C | 4.8°C | 6.7°C |
| 12°C | -1.2°C | 1.9°C | 4.5°C | 6.7°C | 8.7°C |
| 14°C | 0.6°C | 3.7°C | 6.4°C | 8.6°C | 10.6°C |
| 16°C | 2.4°C | 5.6°C | 8.2°C | 10.5°C | 12.5°C |
| 18°C | 4.2°C | 7.4°C | 10.1°C | 12.4°C | 14.5°C |
| 20°C | 6.0°C | 9.3°C | 12.0°C | 14.4°C | 16.4°C |
| 22°C | 7.8°C | 11.1°C | 13.9°C | 16.3°C | 18.4°C |
| 24°C | 9.6°C | 12.9°C | 15.8°C | 18.2°C | 20.3°C |
For example, follow the 20°C row to the 60% RH column: the calculated dew point is 12.0°C. A 10°C window surface is below that value even though the room air is warm.
If your readings fall between the listed rows or columns, do not treat a neighbouring cell as an exact answer. Use the free dew point calculator for your temperature, RH and measured surface, in Celsius or Fahrenheit. No contact details are required.
20°C air · 40% RH · 10°C surface
The surface is 4.0°C above the dew point.
This measured surface is above the calculated dew point. Condensation is not predicted at this spot under these air conditions; other surfaces or later readings may differ.
20°C air · 60% RH · 10°C surface
The surface is 2.0°C below the dew point.
This measured surface is below the calculated dew point. Condensation is possible at this spot under these air conditions; the calculation does not identify the cause of a damp problem.
The two examples use the same room and surface temperatures to show what changes when RH rises. They are worked comparisons, not measurements of your home or universal humidity targets. A surface above the dew point is not proof that the home is free from damp or mould. Persistent damp, visible mould or signs of a leak need investigation, not just a different humidity setting.
Reading guide reviewed 7 October 2026. Reference grid unchanged from 20 September 2026.Method and limitations
The Magnus approximation uses constants 17.62 and 243.12 from the UK National Physical Laboratory's dew-point guidance.
The dataset calculates five RH levels at eight common room temperatures. It does not claim that these are statutory indoor targets.
Condensation becomes possible where a surface reaches or falls below the dew point of the air touching it.
Sensor placement, accuracy, changing conditions and hidden cold surfaces can matter more than a displayed decimal.
Sources
The table is generated from one shared calculation function also used by our interactive calculator. That prevents the downloadable data and on-page tool from quietly using different equations.
γ = ln(RH / 100) + (17.62 × T) / (243.12 + T)
Dew point = (243.12 × γ) / (17.62 − γ)
T is air temperature in °C; RH is relative humidity as a percentage. ln is the natural logarithm. This rearranges the NPL-published Magnus approximation over liquid water. Results are rounded to one decimal place, not a claim of sensor accuracy.The generated table may be reused with attribution and a link to this page. Always preserve the calculation and diagnosis limitations.
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