Dew point calculator
The dew point is the temperature at which the water vapour in your room air starts turning back into liquid. Any surface colder than it gets wet. Enter your room conditions:
How to use the result
Compare the dew point with the temperature of your coldest surfaces. An infrared thermometer (from about £15) pointed at the glass edge, the window reveal or the corner where mould appears tells you directly. If that surface is at or below the dew point, it condenses. If it's a few degrees above, it stays visibly dry — but may still be damp enough for mould.
That second number in the results matters more than most people realise. Mould doesn't need visible water. Building standards such as BS EN ISO 13788 use a surface humidity of 80%, sustained over time, as the threshold for mould risk. The "80% temperature" shown above is the surface temperature at which that happens for your room air.
Worked example: why heating alone can mislead you
Take a bedroom at 18 °C and 70% humidity with a cold wall corner at 10 °C.
| Change | Result | What it means |
|---|---|---|
| Starting point: 18 °C, 70% | Dew point ≈ 12.4 °C | The 10 °C corner is below dew point — it gets wet. |
| Remove moisture: 18 °C, 55% | Dew point ≈ 8.8 °C | The corner is now above dew point. |
| Heat only the air to 20 °C | Humidity reads ≈ 62%; dew point still ≈ 12.4 °C | The reading improved, but the same water is in the air. A corner that stays at 10 °C is still wet. |
| Warm the corner to 14 °C | Above dew point, but surface humidity ≈ 90% | No droplets — but still mould-risk humidity. |
The lesson: a lower humidity reading after turning the heating up doesn't mean moisture has gone. You need to compare dew point or water content, which is exactly what this calculator does.
Typical dew points in UK homes
| Room | 50% | 60% | 70% | 80% |
|---|---|---|---|---|
| 15 °C | 4.7 °C | 7.3 °C | 9.6 °C | 11.6 °C |
| 18 °C | 7.4 °C | 10.1 °C | 12.4 °C | 14.5 °C |
| 20 °C | 9.3 °C | 12.0 °C | 14.4 °C | 16.4 °C |
| 22 °C | 11.1 °C | 13.9 °C | 16.3 °C | 18.4 °C |
On a 0 °C night, the inside of single glazing is typically only 2–5 °C, so it condenses at almost any normal humidity. Modern double glazing usually sits around 13–16 °C at its centre but colder at the edges, which is why condensation often starts as a strip along the bottom of the glass.
The formula
We use the Magnus approximation for saturation vapour pressure over water, which is accurate to well within 0.1 °C across normal indoor temperatures:
es(T) = 6.112 × exp(17.62·T / (243.12 + T)) hPa
dew point = 243.12·γ / (17.62 − γ), where γ = ln(RH/100) + 17.62·T / (243.12 + T)
Absolute humidity is 216.7 × e / (T + 273.15) g/m³, where e is the actual vapour pressure. See the methodology page for the full model.
Next steps
- Not sure what humidity you have? A digital hygrometer with min/max memory is the cheapest upgrade to your diagnosis.
- Want to know which surfaces in your room are at risk, and what will fix it? Run the full assessment.
- Wondering whether opening the window will help on a damp day? Use the open-window calculator.