Dew Point Calculator
Choose what to find. For the dew point, fill in temperature and relative humidity. For relative humidity, fill in temperature and dew point. The other field is ignored. Uses the Magnus formula with b = 17.62 and c = 243.12 C.
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Formula
Magnus formula with Sonntag (1990) constants b = 17.62 and c = 243.12 C, temperatures in C (F inputs are converted):
gamma = ln(RH ÷ 100) + b × T ÷ (c + T)Td = c × gamma ÷ (b − gamma)- Reverse:
RH = 100 × exp(b × Td ÷ (c + Td) − b × T ÷ (c + T))
Valid from about −45 C to 60 C. Dew point depression is T − Td.
Worked example
25 C at 60% humidity: gamma = ln 0.6 + 17.62 × 25 ÷ 268.12 = −0.5108 + 1.6429 = 1.1321, so Td = 243.12 × 1.1321 ÷ (17.62 − 1.1321) = 16.7 C.
Reverse: air at 20 C with a dew point of 10 C gives 52.6% relative humidity.
Frequently asked questions
What is the dew point?
The dew point is the temperature to which air must cool for water vapour to start condensing as dew or fog. At 25 C and 60% relative humidity it is 16.7 C: cool a surface below that and it fogs up.
Which formula and constants are used?
The Magnus formula with Sonntag (1990) constants b = 17.62 and c = 243.12 C. First gamma = ln(RH/100) + bT/(c+T), then dew point = c × gamma / (b - gamma). It is accurate to a few tenths of a degree from -45 C to 60 C.
How do I get relative humidity from temperature and dew point?
Use RH = 100 × exp(b×Td/(c+Td) - b×T/(c+T)). With air at 20 C and a dew point of 10 C, RH is 52.6%. The dew point can never be above the air temperature.
What dew point feels comfortable?
Roughly: below 10 C (50 F) feels dry and comfortable, 16 to 18 C (60 to 65 F) starts to feel humid, and above 21 C (70 F) feels oppressive. The closer the dew point is to the air temperature, the higher the humidity, and at 100% they are equal.