Indoor Humidity & Condensation Calculator
Compare a room’s dew point with an actual surface-temperature measurement.
Your indoor moisture inputs
Approximate indoor moisture calculation over the supported air-temperature range.
Indoor moisture report
13.88 °C · Magnus dew-point estimate
Surface at/below estimated dew point; condensation possible
Full result table
| Scenario RH % | Dew point °C | Surface margin °C |
|---|---|---|
| 10 | -11.051201 | 23.051201 |
| 20 | -2.002847 | 14.002847 |
| 30 | 3.624945 | 8.375055 |
| 40 | 7.779988 | 4.220012 |
| 50 | 11.100565 | 0.899435 |
| 60 | 13.879654 | -1.879654 |
| 70 | 16.277201 | -4.277201 |
| 80 | 18.390516 | -6.390516 |
| 90 | 20.283389 | -8.283389 |
| 100 | 22 | -10 |
Approximate indoor moisture calculation over the supported air-temperature range. A surface at or below dew point suggests condensation potential; this is not a mold test, a health diagnosis or a building inspection. Local surface conditions can differ from an air sensor.
Let’s work it out.
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How to use Indoor Humidity & Condensation Calculator
- Compare a room’s dew point with an actual surface-temperature measurement.
- The difference between surface temperature and calculated dew point is shown as a signed margin.
- The comparison curve changes relative humidity while holding air temperature fixed.
Worked example
At 100% relative humidity, the calculated dew point equals air temperature. Lowering RH at fixed temperature lowers the calculated dew point.
Formula, accuracy & limitations
Magnus approximation: γ = ln(RH/100) + 17.625T/(243.04+T); dew point = 243.04γ/(17.625−γ). Surface margin = surface temperature − dew point. Saturation pressure at T = 6.1094 exp(17.625T/(243.04+T)) hPa.
Approximate indoor moisture calculation over the supported air-temperature range. A surface at or below dew point suggests condensation potential; this is not a mold test, a health diagnosis or a building inspection. Local surface conditions can differ from an air sensor.
The primary result is rounded to two decimal places. Calculation tables show up to four and supporting metrics up to six. Very small nonzero values use scientific notation. Display rounding is not fed back into the arithmetic.
Official source and method context ↗Separate room air from the cold surface
Compare a room’s dew point with an actual surface-temperature measurement. The air and the surface are different inputs; a cold window or wall can be below room temperature. Use Celsius and a humidity reading from the same general time instead of combining unrelated observations.
Read the surface margin
The difference between surface temperature and calculated dew point is shown as a signed margin. At or below zero, the model indicates condensation potential under those input assumptions. A positive margin is not proof that all materials are dry or that a room has no moisture problem.
Inspect humidity sensitivity
The comparison curve changes relative humidity while holding air temperature fixed. It explains why RH and temperature must be interpreted together. The calculator uses an approximate saturation-vapour relationship, not a measurement of water within materials or a prediction of how long dampness will persist.
Keep mold and health claims out of the result
A dew-point calculation cannot identify mold, determine exposure or diagnose symptoms. Recurring moisture can require investigation beyond this worksheet. The result remains a clearly labelled physical comparison rather than a fabricated mold-risk percentage.
Frequently asked questions
Does a low margin prove mold is present?
The calculation concerns condensation potential, not identification of mold or a health diagnosis.
Why measure the surface separately?
A cold surface can differ substantially from room air temperature.
What happens at 100% RH?
The formula returns a dew point equal to the supplied air temperature.
Can this inspect hidden wall moisture?
No material-moisture or building-envelope measurement is inferred.
