Dew point and condensation calculator
Get the dew point from room temperature and relative humidity. Add the surface temperature of a window, wall or chilled-water pipe to see whether condensation forms and how far to lower the humidity to stop it.
What the dew point is
Air can hold only so much water vapor; when it cools, the extra comes out as droplets. The temperature where that starts is the dew point. At the same temperature, more humid air has a higher dew point.
Below 0 °C the vapor turns to ice (frost) instead of water, so it is called the frost point and is computed with the saturation pressure over ice.
When condensation forms
Water collects on any surface that is at or below the dew point of the air touching it. Window glass and outer wall corners in winter, and chilled-water pipes and ducts in summer, are the usual places. A surface warmer than the dew point stays dry.
Example: window condensation in winter
Room air at 20 °C and 60 % RH has a dew point of about 12.0 °C. If the cold weather keeps the inner glass at 10 °C, that is below the dew point, so condensation forms.
To keep that window dry, lower the room humidity to about 52.5 % or less, or keep the glass warmer than 12.0 °C.
How to reduce condensation
- Ventilate: remove moisture from cooking, showers and drying laundry to lower the room humidity.
- Manage humidity: do not over-humidify in winter, and dehumidify if needed.
- Keep surfaces warm: double or low-e glazing, added insulation, and letting air reach behind curtains. Insulate chilled-water pipes so their outer surface stays above the dew point.
Dew point table by room temperature and humidity
Values in °C at sea-level pressure 101.325 kPa. A surface at or below the value collects condensation.
| Room temperature | RH 40 % | RH 50 % | RH 60 % | RH 70 % |
|---|---|---|---|---|
| 18 °C | 4.2 | 7.4 | 10.1 | 12.5 |
| 20 °C | 6.0 | 9.3 | 12.0 | 14.4 |
| 22 °C | 7.8 | 11.1 | 13.9 | 16.3 |
| 24 °C | 9.6 | 12.9 | 15.8 | 18.2 |
| 26 °C | 11.4 | 14.8 | 17.6 | 20.1 |
Frequently asked questions
How is the dew point calculated?
Find the saturation vapor pressure at the air temperature, multiply by the relative humidity to get the actual vapor pressure, then find the temperature whose saturation pressure equals it. This page solves the Hyland-Wexler equations from ASHRAE Fundamentals 2017 chapter 1 backwards.
What is the dew point at 20 °C and 60 % RH indoors?
About 12.0 °C. Window glass or a wall surface at or below about 12 °C collects condensation. At 50 % RH it drops to about 9.3 °C, and at 40 % to about 6.0 °C.
How do I tell whether a chilled-water pipe needs insulation?
Compare the pipe's outer surface temperature with the dew point of the surrounding air. Air at 26 °C and 50 % has a dew point of about 14.8 °C, so a bare pipe carrying 7 °C water will sweat. Choose an insulation thickness that keeps its outer face above the dew point.
Equations and sources
The same equations as the calculator engine. T = t + 273.15 K, p = pressure in Pa.
| Item | Equation | Notes · basis |
|---|---|---|
| Saturation pressure (water) | ln pws = C8/T + C9 + C10·T + C11·T² + C12·T³ + C13·ln T | ASHRAE Eq. 6 (Hyland-Wexler), 0.01 to 200 °C |
| Saturation pressure (ice) | ln pws = C1/T + C2 + C3·T + C4·T² + C5·T³ + C6·T⁴ + C7·ln T | ASHRAE Eq. 5 (Hyland-Wexler), −100 to 0.01 °C |
| Vapor pressure and humidity ratio | pw = RH/100 · pws(t), W = 0.621945 · pw / (p − pw) | ASHRAE Eq. 20 |
| Dew point (frost point) | pws(td) = pw | Inverse of Eq. 5 and 6 by Newton iteration; ice form (frost point) when td ≤ 0.01 °C |
| Condensation and margin | condensation ⇔ ts ≤ td, margin = ts − td | Condensation when the air at the surface reaches saturation (same definition as the calculator's condensation check) |
| Surface RH and highest RH | RH_s = 100 · pw / pws(ts), RH_max = 100 · pws(ts) / pws(t) | RH_max: the RH at the same room temperature whose dew point equals the surface (same definition as the calculator's condensation check) |
Sources
- ASHRAE Handbook Fundamentals 2017 (SI), chapter 1 Psychrometrics: Eq. 5 and 6 saturation pressure, Eq. 20 humidity ratio
- Hyland, R. W. and Wexler, A. (1983), Formulations for the thermodynamic properties of the saturated phases of H2O from 173.15 K to 473.15 K, ASHRAE Transactions 89(2A)
Last updated 2026-10-07.