Dew point calculator
The temperature at which air of this humidity starts giving up moisture.
Fill in the fields and the result will appear here automatically.
Calculate dew point and its gap from air temperature using the Magnus pair a=17.27,b=237.7 °C. Condensation assessment compares dew point with surface temperature: a large gap from air temperature does not rule out a cold wall or pipe.
How it works
Formula and logic
γ=ln(RH/100)+17.27 t/(237.7+t); Td=237.7γ/(17.27−γ). The gap t −Td is also evaluated directly as −(237.7+t)ln(RH/100)/(17.27−γ), preserving a small positive gap near 100% RH.
Example
At 20 °C and 60% RH the dew point is≈11.993 °C and gap≈8.007 °C. A 10 °C surface lies below it, so the example does not imply no condensation risk. At 100% RH the model returns Td=t and a zero gap.
Fields and units
- Air temperature — °C
- Relative humidity — %
How to use
- — Enter temperature and RH for the same air sample.
- — Compare the result with measured surface temperature.
- — RH=0 has no finite logarithm; the form returns an error, not 0 °C.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- TfNSW R 272:17.27/237.7 pair in a road contract, not general accuracy assurance
- Limitation
- t −60 to 60 °C and 0<RH≤100% are this form’s input bounds, not a ±0.4 °C promise. The chosen coefficient pair is approximate; distinguish liquid-water dew from frost. Surface temperature, thermal bridges and individual comfort are not computed.
FAQ
Why does the dew point equal the temperature at 100%?
Because the air is already saturated: no cooling is needed, condensation begins straight away. The logarithm of one is zero, and the formula honestly returns the original temperature.
Why does a wall sweat when the room is warm?
Dew forms by the surface temperature, not the air temperature. A cold corner or window reveal can sit below the dew point while the room air is noticeably warmer.
How accurate is this formula?
t −60 to 60 °C and 0<RH≤100% are this form’s input bounds, not a ±0.4 °C promise. The chosen coefficient pair is approximate; distinguish liquid-water dew from frost. Surface temperature, thermal bridges and individual comfort are not computed.
Why is zero humidity rejected?
Because with zero humidity there is no dew point at all: there is nothing to condense. The logarithm of zero has no value at all, and that is a missing quantity, not an edge of the range.