Work out the dew point from air temperature and relative humidity, or relative humidity from the dew point, with absolute humidity.
Relative humidity
87.6%
Air at 12.0 °C and a dew point of 10.0 °C.
Check the values to see a result.
Any surface colder than the dew point collects condensation.
Td = 243.04 γ ÷ (17.625 − γ), where γ = ln(RH ÷ 100) + 17.625 T ÷ (243.04 + T)
At 12.0 °C with a dew point of 10.0 °C, the relative humidity is 87.6%.
Tip: A cold-water pipe, a duct or the inside of an enclosure lid collects condensation whenever its surface is below the dew point of the air around it.
The dew point is the temperature at which air becomes saturated and water starts to condense. Air at 20 °C and 50% relative humidity has a dew point of 9.3 °C: any surface colder than that, such as a chilled water pipe, collects condensation.
Relative humidity changes with temperature even when the amount of water in the air does not. The dew point and absolute humidity do not, which makes them the better measures for comparing air in different places.
Td = 243.04 γ ÷ (17.625 − γ), γ = ln(RH ÷ 100) + 17.625 T ÷ (243.04 + T)
This is the Magnus formula with the coefficients of Alduchov and Eskridge (1996), fitted to the saturation vapour pressure over water between −40 °C and 50 °C, where it stays within about 0.4% of the reference values. T and Td are in °C and RH in percent.
RH = 100 × es(Td) ÷ es(T), es(T) = 6.1094 × exp(17.625 T ÷ (T + 243.04))
es is the saturation vapour pressure in hPa. The actual vapour pressure is the saturation pressure at the dew point, so their ratio is the relative humidity.
ρv = e ÷ (Rv × TK)
The mass of water vapour in each cubic metre of air, from the vapour pressure e and the gas constant for water vapour, 461.5 J/(kg·K). Saturated air at 20 °C holds about 17.3 g/m³.
A quick rule for humid air, from Lawrence (2005): above 50% RH, the dew point is about T − (100 − RH) ÷ 5. At 20 °C and 80% RH that gives 16 °C against 16.4 °C from the full formula.
The dew point is 12.9 °C. Cooling coils and chilled water pipes below that temperature will be wet, which is why data hall guidance is written in dew point rather than relative humidity alone.
Dew point 12.9 °C Open in the calculator
The dew point is 26.2 °C, only 3.8 K below the air. Cold-water pipework and the inside of an enclosure lid on a cool night will run with water; the IP rating checker covers water from outside, not condensation inside.
Dew point 26.2 °C Open in the calculator
Outdoor air at 12 °C with a dew point of 10 °C has a relative humidity of 87.6%. The margin to condensation is only 2 K, so any surface slightly colder than the air will be damp.
Relative humidity 87.6 % Open in the calculator
Dew point in °C for air temperatures and relative humidities, from the Magnus formula.
| Air (°C) | 30% RH (°C) | 50% RH (°C) | 70% RH (°C) | 90% RH (°C) |
|---|---|---|---|---|
| 0 | −15.5 | −9.2 | −4.8 | −1.4 |
| 5 | −11.2 | −4.6 | 0.0 | 3.5 |
| 10 | −6.8 | 0.0 | 4.8 | 8.4 |
| 15 | −2.4 | 4.7 | 9.6 | 13.4 |
| 20 | 1.9 | 9.3 | 14.4 | 18.3 |
| 22 | 3.6 | 11.1 | 16.3 | 20.3 |
| 24 | 5.4 | 12.9 | 18.2 | 22.3 |
| 26 | 7.1 | 14.8 | 20.1 | 24.2 |
| 28 | 8.8 | 16.6 | 22.0 | 26.2 |
| 30 | 10.5 | 18.4 | 23.9 | 28.2 |
| 35 | 14.8 | 23.0 | 28.7 | 33.1 |
| 40 | 19.1 | 27.6 | 33.5 | 38.0 |
Warm air can hold more water vapour before it saturates, so the same water is a smaller share of the maximum. Air at 10 °C and 80% heated to 20 °C without adding water drops to about 42%. Its dew point stays at 6.7 °C throughout.
No. At 100% relative humidity they are equal; below that, the dew point is lower.
Below 0 °C, water vapour turns straight to ice at the frost point, which is slightly above the dew point over water. This calculator gives the dew point over water at all temperatures.
An RH sensor accurate to ±2% RH gives a dew point within about ±0.6 °C at 20 °C and 50%. The error grows at low humidity, where a small absolute error is a large fraction of the reading.