Relative Humidity Calculator: From Dew Point and Temperature
Relative humidity (RH) describes how much water vapour the air holds relative to its maximum capacity at a given temperature. The dew point is the most precise way to specify atmospheric moisture: it is the temperature at which air must be cooled for condensation to begin. This calculator converts between air temperature, dew point, and relative humidity using the August-Roche-Magnus approximation, the standard empirical formula used by NOAA and the World Meteorological Organization. Enter the air temperature and dew point in degrees Celsius to calculate relative humidity, absolute humidity (grams of water vapour per cubic metre of air), and a comfort classification. The formula is valid for temperatures between roughly -40°C and 60°C and has an uncertainty of about 0.1% across that range. Note that dew point cannot exceed air temperature: 100% relative humidity is the physical upper limit. The calculator also flags potential comfort concerns at very low and very high humidity levels, which are relevant for building management, HVAC design, and general indoor air quality assessment.
Dew point to relative humidity
Dew point cannot exceed air temperature.
Formulas used
August-Roche-Magnus approximation:
RH = 100 × exp((17.625 × Td) / (243.04 + Td)) / exp((17.625 × T) / (243.04 + T))
Absolute humidity (g/m³):
AH = (6.112 × exp((17.67 × T) / (T + 243.5)) × RH × 2.1674) / (273.15 + T)
Where T is air temperature in °C, Td is dew point in °C, and RH is expressed as a percentage (0 to 100).
Comfort classification
| Relative humidity | Classification |
|---|---|
| Below 25% | Very dry (may cause dry skin and static electricity) |
| 25% to 30% | Dry |
| 30% to 60% | Comfortable |
| 60% to 70% | Humid |
| Above 70% | Very humid (may encourage mould growth) |
Note on apparent temperature (heat index)
Apparent temperature (heat index) combines air temperature and relative humidity to estimate how hot conditions feel to the human body. High humidity impairs sweat evaporation, making heat feel more intense. See the temperature converter for related conversions.
Relative humidity calculator: frequently asked questions
What is relative humidity and how is it defined?
Relative humidity (RH) is the ratio of the actual amount of water vapour in the air to the maximum amount the air could hold at the same temperature, expressed as a percentage. Because warm air can hold more water vapour than cold air, the same absolute amount of moisture produces a lower relative humidity at higher temperatures. When relative humidity reaches 100%, the air is saturated and any further cooling causes condensation. Relative humidity is the most commonly reported humidity measure in weather forecasts and indoor air quality standards.
What is dew point and how does it differ from relative humidity?
The dew point is the temperature to which air must be cooled (at constant pressure and moisture content) for condensation to begin. Unlike relative humidity, which changes with temperature even when moisture content is constant, dew point is an absolute measure of atmospheric moisture. A dew point of 20°C (68°F) feels humid regardless of air temperature. Meteorologists often prefer dew point over relative humidity for this reason. When air temperature equals dew point, relative humidity is 100%.
What is the August-Roche-Magnus formula?
The August-Roche-Magnus approximation (also called the Magnus formula) is an empirical equation that relates saturation vapour pressure to temperature. It takes the form: es(T) = 6.1078 × exp(17.625 × T / (243.04 + T)) in hPa. Relative humidity from dew point and temperature is then RH = 100 × es(Td) / es(T), where Td is dew point. The formula is accurate to within about 0.1% for temperatures between -40°C and 60°C and is the standard used by NOAA and WMO for routine meteorological calculations.
What relative humidity is considered comfortable indoors?
ASHRAE Standard 55 and EPA guidance recommend indoor relative humidity between 30% and 60% for human comfort and to minimise health risks. Below 25%, air feels dry, which can cause irritation of mucous membranes, increased static electricity, and cracking of wood furniture. Above 60%, conditions favour dust mite proliferation, mould growth, and structural moisture damage. The comfort sweet spot of 40% to 50% is recommended for most occupied spaces, with slightly drier conditions (30% to 40%) sometimes preferred in cold climates to avoid condensation on windows.
What is absolute humidity?
Absolute humidity is the mass of water vapour per unit volume of air, typically expressed in grams per cubic metre (g/m³). Unlike relative humidity, absolute humidity does not depend on air temperature: it is a direct measure of how much moisture is physically present. Absolute humidity rises when water evaporates into the air and falls when moisture condenses out. For a given dew point, absolute humidity is approximately: AH = (6.112 × exp(17.67 × T / (T + 243.5)) × RH × 2.1674) / (273.15 + T) g/m³.
Official sources
- NOAA: Why the Dew Point is a Better Measure of Moisture than Relative Humidity.
- WMO: Guide to Instruments and Methods of Observation (WMO-No. 8).
- EPA: Indoor Air Quality: Humidity and Moisture.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.