Dew Point Calculator: Condensation & Mould Risk
Condensation on your windows or black mould on your walls? Enter your room temperature and humidity to get the dew point, and see whether your windows, walls and cold corners will get condensation or mould, and what humidity keeps them dry.
No sign-up required. Free to use, with every formula shown on this page. Last reviewed October 2026.
Check your room
Dew Point & Mould Calculator
Enter your room's temperature and humidity to get the dew point, see whether the windows, walls and cold corners will get condensation or mould, and the humidity that keeps them dry.
1 Your room
Quick fillRead it from a hygrometer (a cheap digital one is fine). Check in the morning, when it's usually highest. No meter? A lived-in room in winter is often 50–65%.
2 Outside
WeatherUse a typical cold night or morning for your area. The colder it is outside, the colder your windows and walls get.
3 Your windows and walls
Not sure? Houses built before about 1930 usually have solid walls; later ones have a cavity, which may have been filled since.
Dew point
0 °C
Water in the air
0 g/m³
Stop mould below
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Coldest surface
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Your surfaces
| Surface | Temperature | Risk |
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Mould can start once the air touching a surface stays at about 80% humidity, before any water is visible (70–80% is borderline). At 100%, the dew point, condensation forms.
What to aim for
What you'll need
What to look for, not a brand.
What's next?
How it's worked out: dew point from room temperature and humidity (Magnus formula). Surface temperature, unless you measured it = outside + fRsi × (room − outside), where fRsi = 1 − Rsi × U (BS EN ISO 13788), using a typical U-value for the window or wall and an inside surface resistance Rsi of 0,13 for glass, 0,25 for walls and 0,50 for corners or behind furniture. Surface humidity = water vapour in the room air ÷ the most the air can hold at the surface temperature. Mould risk from 80% (ISO 13788).
Estimates only. Real walls and windows vary (the edges of the glass and the frames are colder than the middle of the pane), and humidity changes through the day. Damp that keeps coming back, spreads, or appears without condensation (for example from a leak or rising damp) needs a professional survey.
How it works
Why cold surfaces get damp
Warm air holds more water than cold air. When room air touches a cold window or wall it cools down, and the water it carries ends up on the surface: as condensation if it gets cold enough, or as the damp film that mould feeds on. The sources and assumptions behind it are on our How we calculate page.
The dew point
From your room temperature and humidity, the checker works out the dew point: the temperature at which the air can’t hold its water any more. Any surface at or below it gets wet.
How cold the surface gets
Windows and walls sit between the warm room and the cold outside. The checker estimates their inside surface temperature from how well they insulate, or you can enter a reading from an infrared thermometer.
The 80% rule
Mould can grow once the air right at a surface stays at about 80% humidity, before any water appears. The checker shows the humidity at the surface and the room humidity that keeps it below 80%.
Examples
Which surfaces get damp first?
The same room can be fine on an insulated wall and wet on an old window. These examples use the checker’s own formula on a cold evening.
Illustrative only: a room at 19 °C and 65% humidity with 5 °C outside (dew point 12,3 °C). Surface temperatures are estimated from typical U-values with the BS EN ISO 13788 method. Real walls and windows vary.
What to do about it
How to stop condensation and mould
There are two ways to fix it: take water out of the air, or make the cold surface warmer. Start with the cheap habits.
Let the moisture out
Use the extractor fan or open a window while cooking and showering, and for 20 minutes after. Dry laundry outside or in a ventilated room, and keep trickle vents open.
Keep air moving
Leave a 5–10 cm gap between furniture and outside walls, and keep heating steady rather than letting rooms get very cold. Corners and the backs of wardrobes are where mould starts.
Dry the air or warm the surface
A dehumidifier brings humidity down quickly in rooms that stay damp. For a lasting fix, better glazing or wall insulation keeps surfaces warm. Use our Dehumidifier Picker to size one.
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Common questions
Frequently asked questions about dew point, condensation and mould
Common questions about the dew point, condensation on windows, mould on walls and what humidity to aim for.
What is the dew point?
The dew point is the temperature at which air can’t hold its water any more. Any surface colder than the dew point gets wet with condensation. Room air at 20 °C and 60% humidity has a dew point of about 12 °C, so a window pane at 10 °C will mist up.
How do I calculate the dew point?
From the air temperature (T, in °C) and relative humidity (RH), with the Magnus formula: first γ = ln(RH ÷ 100) + 17.62 × T ÷ (243.12 + T), then dew point = 243.12 × γ ÷ (17.62 − γ). The calculator above does it for you, and shows which of your windows and walls fall below it.
Why do my windows get condensation every morning?
Overnight the outside gets colder, so the glass gets colder, while people breathing and sleeping add moisture to the air. Once the glass falls below the dew point, water forms on it. Older double glazing and single glazing get much colder than modern glazing, so they run with water first.
What humidity should I keep my home at?
Around 40–60% for most homes. In winter, the right level depends on how cold your windows and walls get: the checker shows the highest room humidity that keeps a particular surface below 80%, which is where mould can start.
Why does mould grow on walls with no condensation?
Mould doesn’t need running water. If the air touching a wall stays at about 80% humidity for long periods, mould can grow. Cold corners and walls behind furniture often reach that level while the rest of the room feels dry.
Is condensation the same as damp?
Condensation is the most common cause of damp patches and mould in homes, but not the only one. Leaks, rain getting through walls and rising damp also cause damp. If a patch is there even in dry, warm weather, keeps growing, or has a tide mark, get it checked by a professional.
Does a dehumidifier stop mould?
It helps by lowering the humidity, so cold surfaces stay below the level where mould grows. It works best together with ventilation and steady heating. Our Dehumidifier Picker suggests a size for your room.
How do I measure the temperature of a wall or window?
An infrared thermometer (from about 15–20) reads a surface temperature instantly. Measure on a cold morning, in the coldest spots: corners, window frames and behind furniture. Tick “I measured a surface temperature” in the calculator and enter the reading.
Are the results exact?
No. They are estimates based on typical U-values and the BS EN ISO 13788 method. Real walls and windows vary, humidity changes through the day, and damp from leaks or rising damp needs a professional survey.