Dew point calculator
Find the dew point from air temperature and relative humidity to assess condensation on cold surfaces.
A surface colder than the dew point can collect condensation. Compare the result with the surface temperature, not the room air temperature. This is not a mould diagnosis.
Enter your measurements, then calculate.
Method and limitations
Magnus approximation over liquid water, using Alduchov–Eskridge constants: a = 17.625, b = 243.04 °C. T and Td are in °C; RH is a percentage. Air temperature and computed dew point must be within −40 to 50 °C. Below freezing this remains a liquid-water dew point, not a frost point. Sensor uncertainty and local surface conditions are not included. γ = ln(RH / 100) + a × T / (b + T); Td = b × γ / (a − γ).
Sources
- Improved Magnus Form Approximation of Saturation Vapor Pressure, equation 21 — O. A. Alduchov & R. E. Eskridge — American Meteorological Society (1996), copy hosted by NAU,
- Handbook 44 (2023), Appendix C — Units of measurement — NIST,