Relative Humidity and Dewpoint Calculator
Relative humidity (RH) is the percentage of water vapor in the air relative to the maximum amount the air can hold at that temperature. The dewpoint is the temperature at which the air must cool to reach saturation (100% RH). Together, these two measurements describe the moisture content of the atmosphere and are fundamental to weather analysis, comfort assessment, and fog prediction. This calculator uses the Magnus formula for saturation vapor pressure to convert between temperature, dewpoint, and relative humidity. Enter temperature and dewpoint to get RH, or enter temperature and RH to get dewpoint.
Magnus formula for relative humidity
es(T) = 6.112 * exp(17.67 * T / (T + 243.5)) [hPa]
RH = 100 * es(Td) / es(T)
Td = 243.5 * ln(RH/100 * es(T)/6.112) / (17.67 - ln(RH/100 * es(T)/6.112))
Temperature T and dewpoint Td are in degrees Celsius for this formula (converted internally from Fahrenheit). es(T) is saturation vapor pressure at temperature T. The Magnus approximation is accurate to within 0.1% for temperatures between -40 C and 60 C.
Interpreting relative humidity and dewpoint
- RH 100%: air is saturated; fog, dew, or precipitation likely to form.
- RH below 30%: air is very dry; static electricity, dry skin, and respiratory irritation common.
- Dewpoint above 70 F: oppressively humid; heat stress greatly elevated.
- Dewpoint 55-65 F: comfortable to slightly humid in summer.
- Dewpoint below 50 F: pleasantly dry conditions.
Relative humidity calculator: frequently asked questions
How is relative humidity calculated from temperature and dewpoint?
Using the Magnus formula, saturation vapor pressure at temperature T is es(T) = 6.112 * exp(17.67*T/(T+243.5)) and at dewpoint Td is e(Td) = 6.112 * exp(17.67*Td/(Td+243.5)). Relative humidity RH = 100 * e(Td) / es(T). When dewpoint equals air temperature, RH is 100%.
What is the dewpoint?
The dewpoint is the temperature to which air must be cooled, at constant pressure and humidity, for water vapor to condense into dew. A dewpoint close to the air temperature means the air is near saturation. Dewpoints above 60 F feel humid; above 70 F feel oppressive.
What is the difference between relative humidity and absolute humidity?
Relative humidity is the ratio of actual water vapor pressure to saturation vapor pressure at the current temperature, expressed as a percentage. Absolute humidity is the mass of water vapor per unit volume of air, in grams per cubic meter. Relative humidity changes with temperature even if the actual moisture content does not change.
Why does relative humidity rise at night even if no moisture is added?
As air cools overnight, saturation vapor pressure falls. If the actual moisture content stays constant, the relative humidity rises toward 100% and may reach the dewpoint, causing dew or fog to form. This is purely a temperature effect, not an addition of moisture.
What dewpoint depression indicates fog risk?
When the dewpoint depression (temperature minus dewpoint) falls to 4 F (2 C) or less, fog is likely. Many aviation weather products use a dewpoint depression of 4 F as a threshold for reporting fog potential. Fog is almost certain when the depression reaches 0 (temperature equals dewpoint).
Official sources
- NOAA/NWS: Dewpoint vs. Relative Humidity.
- WMO Guide to Meteorological Instruments: WMO CIMO Guide.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.