Dew Point & Condensation Risk Calculator
Enter air temperature and humidity to find the dew point and whether condensation will form on your windows, walls, or floor. Includes mould risk level and a dehumidifier sizing tool. Nothing uploaded.
Results
Mould Risk
Low mould riskMould can grow when relative humidity stays above 70% for extended periods, especially in warm rooms. Reduce humidity or improve ventilation.
Dehumidifier guidance
Dehumidifier Capacity Needed
Estimated for the room volume and conditions above. Actual results vary by insulation, occupancy, and air change rate.
Learn more: dew point, surfaces, and what the outputs assume
Why the tool leads with dew point instead of relative humidity
Relative humidity describes how full the air is compared with what it could hold at its current temperature, so the same percentage means different amounts of water at different temperatures. Air at 20 °C and 50% RH has a dew point of 9.3 °C. Cool it to 10 °C and hold 50% RH and the dew point falls to 0.1 °C, because there is far less water in it.
Building scientist Allison Bailes makes the same case in a piece on dew point as a measure of humidity, writing that "if you're going to give me one number, I'd rather know the dew point," and that air with a given dew point will condense on "any surface with a temperature" at or below it. The calculator uses the Magnus approximation with the standard constants 17.27 and 237.3 °C to get there.
Condensation is a comparison, not a humidity reading
Each surface you list is checked against one rule: condensation is flagged when the surface temperature is at or below the dew point. Room temperature only reaches that comparison through the dew point it helped produce, so a warm room is no protection on its own.
The defaults show why that matters. At 20 °C and 60% RH the dew point is 12.0 °C, so the window at 10 °C is marked as condensing while the wall at 15 °C and the floor at 18 °C are clear. One cold pane sweats in a room that feels perfectly comfortable.
What the mould badge and dehumidifier figure assume
The mould badge is a two-condition rule, not a gradient. High needs 80% RH or more together with 15 °C or warmer; moderate needs 70% RH or more with 10 °C or warmer; anything else reads low. A cold cellar at 8 °C and 85% RH therefore reads low even though damp surfaces there can still support growth, so read the badge as a warm-room screen rather than a verdict on the whole building.
The dehumidifier estimate converts current and target humidity into grams of water per cubic metre, multiplies the gap by room volume, and assumes 12 air changes a day. With the defaults, a 50 m³ room going from 60% to 50% RH at 20 °C gives 1.0 L/day. Warmth matters here too: a 40 m³ room dropping 70% to 55% needs 1.24 L/day at 20 °C but only 0.92 L/day at 15 °C, because colder air holds less water to begin with.
FAQ
Why is the feels-like temperature missing?
The heat index row only appears at 27 °C or warmer with at least 40% relative humidity. The Rothfusz regression behind it was fitted for hot, humid conditions and returns misleading values outside that range, so the tool hides it rather than print a number it does not trust. At 30 °C and 60% RH it shows 32.8 °C.
What is the wet-bulb figure for?
It is the lowest temperature you can reach by evaporating water into the air, calculated here with the Stull approximation. At 20 °C and 60% RH it comes out at 15.0 °C. It tells you how much cooling evaporation can still do, which is why it matters for swamp coolers, drying times, and heat safety.
Why does the dehumidifier number look small?
It sizes only the water held in the room's air at the assumed 12 air changes per day. It does not count moisture added by people, cooking, showers, drying laundry, or a wet wall, all of which can dwarf the air's own load. Treat the figure as a floor for capacity, not a purchase spec.