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Dew point and relative humidity calculator
Convert between temperature, relative humidity and dew point, and see how much a temperature error shifts an RH reading.
The sensitivity table answers the calibration question: if the sensor's temperature reading is wrong by ±0.2, ±0.5 or ±1 °C while the moisture content is unchanged, what RH does it report?
Result
| Temperature error | Reads (−) | Reads (+) | RH error |
|---|---|---|---|
| ±0.2 °C | 50.6 % | 49.4 % | ≈ ±0.6 %rh |
| ±0.5 °C | 51.6 % | 48.5 % | ≈ ±1.6 %rh |
| ±1.0 °C | 53.2 % | 47.0 % | ≈ ±3.2 %rh |
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Magnus formula over liquid water with Alduchov–Eskridge constants; about 0.4 % in vapour pressure between −40 and 50 °C. Not for frost point over ice.
How the calculation works
- 01Saturation vapour pressure over water: e_s(T) = 6.1094 exp(17.625 T / (243.04 + T)) hPa.
- 02Dew point: T_d = b γ / (a − γ), where γ = ln(RH/100) + a T/(b + T), a = 17.625, b = 243.04 °C.
- 03Absolute humidity = 216.7 e / (T + 273.15) g/m³, with e the actual vapour pressure in hPa.
- 04RH sensitivity at constant vapour pressure: d(RH)/dT = −RH a b / (b + T)².
Limitations
- Valid over liquid water from about −40 °C to +50 °C; not for frost point (over ice).
- Magnus is an approximation (about 0.4 % in e_s); precision hygrometry uses Sonntag or Wexler equations with enhancement factors.
Frequently asked questions
- What is the dew point at 20 °C and 50 % RH?
- About 9.3 °C. Cooling the air to that temperature at constant moisture content brings it to saturation.
- How much does a 1 °C temperature error change RH?
- At 20 °C and 50 % RH, roughly 3 %rh per kelvin — a 1 °C error in the sensor's temperature reads as about 3 %rh. This is why humidity calibration starts with thermal equality between sensor and reference.
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