Dew Point Calculator guide
Dew Point Calculator: Temperature, Humidity and Condensation
Use our dew point calculator to work out the dew point from the air temperature and relative humidity, and check comfort and condensation risk.
By the AllCalculator editorial teamUpdated 16 October 20261 min read
The dew point is the temperature at which the water vapour in the air starts to condense. It's a better guide to how muggy it feels than relative humidity, and it explains why your windows steam up. Our Dew Point Calculator works it out.
How to use the calculator (Dew Point Calculator)
- Choose °C or °F.
- Enter the air temperature.
- Enter the relative humidity.
The formula
The calculator uses the Magnus formula, with a = 17.62 and b = 243.12°C:
- γ = ln(RH ÷ 100) + aT ÷ (b + T)
- Dew point = bγ ÷ (a − γ)
It's accurate to within about 0.1°C in normal conditions.
A worked example
With the calculator's default settings, air at 20°C with 65% humidity has a dew point of 13.2°C. Any surface colder than that, like a cold window, will get condensation.
Comfort levels
| Dew point | How it feels |
|---|---|
| Below 10°C | Dry and comfortable |
| 10–13°C | Comfortable |
| 13–16°C | Slightly humid |
| 16–18°C | Humid |
| 18–21°C | Very humid |
| Above 21°C | Oppressive |
Condensation and damp at home
If the inside of your windows or cold walls regularly get wet, the air is reaching its dew point there. To reduce condensation and mould:
- Open windows or use extractor fans when cooking, showering and drying clothes.
- Keep your home steadily heated.
- Use a dehumidifier.
Frequently asked questions
What's the difference between dew point and relative humidity?
Relative humidity is how full the air is compared with its maximum at that temperature. The dew point measures the actual amount of moisture.
Can the dew point be higher than the temperature?
No. When they're equal, the humidity is 100% and you get fog or dew.
Sources and further reading
Links open official and independent websites in a new tab.
This guide is general information to help you understand your options. It isn't financial, tax, legal or building advice and doesn't take your personal circumstances into account. Figures were checked when the guide was last updated and can change, so confirm anything important with GOV.UK, HMRC, a regulated adviser or a qualified tradesperson.