About the Relative Humidity Calculator
This relative humidity calculator works out how close the air is to saturation. Give it the air temperature together with either the dew point (from a weather report or hygrometer) or a wet-bulb reading (from a sling or aspirated psychrometer) and it returns the relative humidity, the actual and saturation vapour pressures, the dew point and the absolute humidity.
It is useful for students checking psychrometry homework, HVAC technicians verifying a humidistat, greenhouse and grow-room operators, museum and archive staff, woodworkers watching moisture, and anyone who logged a dew point and wants the matching RH. The table shows how the RH would change for other dew points at the same temperature.
Saturation vapour pressure uses the Magnus formula with the Alduchov–Eskridge constants, accurate to about 0.4% between −40 °C and 50 °C over liquid water. The wet-bulb method uses the standard psychrometer equation for a well-ventilated wet bulb at the station pressure you enter; an unventilated or dirty wick reads high and gives an RH that is too high.
With the default inputs, the relative humidity is 53.8%. Change any value above to recalculate instantly.
How to use the relative humidity calculator
- 1Choose whether you have a dew point or a wet-bulb reading.
- 2Pick °C or °F and enter the air temperature.
- 3Enter the dew point, or the wet-bulb temperature and station pressure.
- 4Read the relative humidity, vapour pressures and dew point.
- 5Use the table to see how RH changes with more or less moisture.
Formula and method
Relative humidity is the ratio of the water vapour pressure actually in the air (e) to the maximum it could hold at that temperature (eₛ), expressed as a percentage. Saturation vapour pressure rises steeply with temperature, which is why the same moisture gives a high RH on a cool morning and a low RH in the afternoon.
With a dew point, the actual vapour pressure is simply the saturation pressure at the dew point. With a psychrometer, evaporation cools the wet bulb; the psychrometer equation subtracts a term proportional to the wet-bulb depression (T − T_w) and the station pressure P from the saturation pressure at the wet-bulb temperature. Temperatures are in °C and pressures in hPa; the calculator converts °F automatically.
- RH
- Relative humidity (%)
- e
- Actual vapour pressure (hPa)
- eₛ(T)
- Saturation vapour pressure at temperature T (hPa)
- T, T_d, T_w
- Air, dew-point and wet-bulb temperatures (°C)
- P
- Station air pressure (hPa)
Worked examples
25 °C air with a 15 °C dew point
eₛ(25) = 31.62 hPa and e = eₛ(15) = 17.02 hPa, so RH = 100 × 17.02 ÷ 31.62 = 53.8%. A pleasant summer day.
Muggy 30 °C with a 25 °C dew point
eₛ(30) = 42.37 hPa and eₛ(25) = 31.62 hPa, giving RH = 74.6%. The air holds about 22.6 g of water per cubic metre, which feels oppressive.
Indoor 86 °F with a 60 °F dew point
86 °F = 30 °C and 60 °F = 15.56 °C. The ratio of saturation pressures gives RH = 41.6%.
Sling psychrometer: 25 °C dry, 18 °C wet at sea level
eₛ(18) = 20.65 hPa. The psychrometer term is 0.00066 × 1.0207 × 1013.25 × 7 = 4.78 hPa, so e = 15.82 hPa and RH = 15.82 ÷ 31.62 = 50.0%, with a dew point of 13.9 °C.
Psychrometer at altitude: 20 °C dry, 15 °C wet, 850 hPa
Lower pressure means faster evaporation, so the same depression implies more moisture than at sea level: e = 17.05 − 0.00066 × 1.0173 × 850 × 5 = 14.17 hPa and RH = 60.7%.
Frequently asked questions
How do you calculate relative humidity from dew point?+
Compute the saturation vapour pressure at the dew point and at the air temperature with the Magnus formula, then divide: RH = 100 × eₛ(T_d) ÷ eₛ(T). When the dew point equals the air temperature, RH is 100%.
How does a wet and dry bulb thermometer measure humidity?+
Water evaporating from the wet wick cools it. The drier the air, the faster the evaporation and the bigger the gap between dry- and wet-bulb readings. The psychrometer equation converts that gap and the pressure into vapour pressure and RH.
What is a comfortable indoor relative humidity?+
Most guidance, including the US EPA, recommends keeping indoor humidity roughly between 30% and 50%, and below 60% to limit mould and dust mites. Below about 30% skin, eyes and wood furniture dry out.
Why does relative humidity drop when air warms up?+
Warming air does not remove water, but it raises the saturation vapour pressure. The same amount of vapour is then a smaller fraction of the maximum, so RH falls even though the dew point stays the same.
Is dew point or relative humidity better for comfort?+
Dew point is a better guide to how muggy it feels because it measures the actual moisture. RH depends on temperature: 50% RH is dry-feeling at 10 °C but sticky at 35 °C.