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Indoor dew point and condensation

Estimate a dew-point temperature from air temperature and relative humidity. Compare a surface only when you have its measured temperature.

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Editable example

An example is filled in. Replace it with your own data to calculate.

Enter air temperature and humidity first. Compare a surface only when you have measured it. Enter 60 for 60%, not 0.6.

What would you like to check?Choose air dew point alone, or compare a surface you have measured.

Indoor air temperature (°C)Enter the air temperature from 0 to 50 °C, not the outdoor temperature or a surface reading.

Relative humidity (%)Enter 5 to 100. If the humidity meter says 60%, enter 60, not 0.6. These are tool limits, not recommended humidity levels.

Results update when inputs are complete. A plan is kept only when you save it. Inputs and results are not uploaded.

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Example result

Estimated dew-point temperature

16.69°C

An approximate threshold for liquid-water condensation on a cooling surface, not a weather forecast, frost point or guarantee of visible water.

Air temperature25 °C

Relative humidity60 %

Air temperature minus dew point8.31 °C

Surface condensation comparisonAir data only; no surface condensation comparison is made.

Worked steps

γ = ln(60/100) + 17.62 × 25 ÷ (243.12 + 25)
Td = 243.12 × γ ÷ (17.62 − γ)

Uses the NPL liquid-water Magnus approximation with constants 17.62 and 243.12. Supported air temperature is 0–50 °C and relative humidity 5–100%; measured surfaces must be 0–50 °C. These are software limits, not health or dehumidifier advice. A surface at or below the unrounded dew point meets the modeled condensation condition; equality includes a 1e-9 °C floating-point tolerance. Measurements and local temperatures have uncertainty. A positive difference does not rule out other dampness or leaks; no water quantity or timing is predicted. Negative liquid-water dew points are possible, but this is not a frost-point calculation. Subzero air or surfaces, wet-bulb temperature, heat index, high-pressure processes and engineering design are excluded. No sensors or location are read.

Method and scope

A surface at or below the unrounded dew point meets the modeled condensation condition; equality includes a 1e-9 °C floating-point tolerance. Measurements and local temperatures have uncertainty. A positive difference does not rule out other dampness or leaks; no water quantity or timing is predicted.

Negative liquid-water dew points are possible, but this is not a frost-point calculation. Subzero air or surfaces, wet-bulb temperature, heat index, high-pressure processes and engineering design are excluded. No sensors or location are read.

References:NPL: dew point, relative humidity and liquid-water Magnus formula

What this helps you do

Understand how cooling a surface can lead to condensation. Enter air temperature and relative humidity to estimate a liquid-water dew point. This does not measure your room or diagnose leaks, mould or building safety.

What to enter

Use indoor air temperature from 0 to 50 °C and relative humidity from 5 to 100%. Enter 60 for 60%, not 0.6. Compare a surface only after measuring it, using the same place and time. Its supported range is 0–50 °C; outdoor air temperature is not a surface measurement.

How the calculation works

With a = 17.62 and b = 243.12, calculate gamma = ln(RH/100) + aT/(b+T), then dew point = b gamma/(a−gamma). These are algebraic forms of the NPL liquid-water Magnus vapour-pressure equation and relative-humidity definition.

Worked example

At 25 °C and 60% relative humidity, the estimated dew point is about 16.69 °C. A measured surface of 15 °C is about 1.69 °C below it and meets the modeled condition; a surface of 20 °C is about 3.31 °C above it. These are calculation examples, not guaranteed condensation outcomes.

Understanding the answer

The main answer is an estimated liquid-water dew point. In surface mode, surface temperature minus dew point is zero or negative when the modeled threshold is met. The comparison uses unrounded values with a 1e-9 °C equality tolerance; your measurement uncertainty may be much larger.

Scope and limitations

This is a near-room-temperature, ambient-pressure approximation. A negative liquid-water dew point is not a frost point. Subzero air or surfaces, wet-bulb calculations, heat index, mould risk, water amount, timing and engineering safety are outside scope. Dampness or leaks may have other causes. Inputs and saved plans stay in this browser.