Relative Humidity Calculator
Enter the air temperature with either the dew point or a wet-bulb reading from a psychrometer, and get the relative humidity, vapor pressure and dew point, with every step of the arithmetic shown and a psychrometric lookup table for your temperature. Works in °C, °F or K. Runs entirely in your browser — free, no sign-up.
Results
65.1 %
Relative humidity
18 °C
Dew point
7 °C
Dew point depression
20.6
Vapor pressure e (hPa)
31.62
Saturation vapor pressure (hPa)
15
Absolute humidity (g/m³)
The working
- Saturation vapor pressure at the dew point 18 °C: es(Td) = 20.597 hPa.
- Saturation vapor pressure at the air temperature 25 °C: es(T) = 31.617 hPa.
- Relative humidity = es(Td) / es(T) × 100 = 20.597 / 31.617 × 100 = 65.15 %.
Humidity for other dew point depressions at 25 °C
| Dew point depression | Relative humidity | Dew point |
|---|---|---|
| 1 °C | 94.2 % | 24 °C |
| 2 °C | 88.7 % | 23 °C |
| 3 °C | 83.5 % | 22 °C |
| 4 °C | 78.5 % | 21 °C |
| 5 °C | 73.8 % | 20 °C |
| 6 °C | 69.4 % | 19 °C |
| 8 °C | 61.2 % | 17 °C |
| 10 °C | 53.8 % | 15 °C |
Saturation vapor pressure by the Magnus formula (Alduchov and Eskridge, 1996), fitted for −40 to 60 °C.
How to Use the Relative Humidity Calculator
- 1. Pick the readings you have. Choose air temperature and dew point if a weather report or hygrometer gave you the dew point, or dry-bulb and wet-bulb if you used a psychrometer.
- 2. Enter the temperatures and choose °C, °F or K. The dry-bulb is the ordinary air temperature; the wet-bulb is the reading from a thermometer wrapped in a wet wick and ventilated.
- 3. For a psychrometer, add the air pressure. The wet-bulb equation depends weakly on pressure; sea level is 1013.25 hPa. If the wet bulb is coated with ice below freezing, tick the box.
- 4. Read the humidity and the working. The table underneath shows how humidity changes with the temperature gap, so you can check a reading against a printed psychrometric chart.
What Relative Humidity Measures
Relative humidity (RH) is the vapor pressure of the water in the air divided by the saturation vapor pressure at the air's own temperature, as a percentage: RH = e / es(T) × 100. Warm air can hold much more vapor than cold air, so the same amount of water gives a lower RH on a hot afternoon than on a cold morning. That is why the number is a poor measure of how much moisture is present, and why meteorologists prefer the dew point for that.
From Dew Point
At the dew point the air is saturated, so the vapor pressure of the air equals the saturation vapor pressure at the dew point: e = es(Td). RH is therefore es(Td) / es(T) × 100. The calculator finds both saturation pressures with the Magnus formula in the Alduchov and Eskridge (1996) form, es = 6.1094 × exp(17.625 t / (243.04 + t)) hPa, fitted for −40 to 60 °C.
Worked example: 25 °C with a dew point of 18 °C
es(18 °C) = 20.597 hPa and es(25 °C) = 31.617 hPa.
RH = 20.597 / 31.617 × 100 = 65.1 %.
From a Psychrometer
A psychrometer is two thermometers, one dry and one with a wet wick. Water evaporating from the wick cools the wet bulb below the air temperature, and the drier the air, the bigger the gap. The vapor pressure follows from the psychrometer equation of Ferrel, as given in the Smithsonian Meteorological Tables: e = es(Tw) − 6.60 × 10⁻⁴ × (1 + 0.00115 Tw) × P × (T − Tw), with temperatures in °C and pressures in hPa. Over an ice-coated wet bulb the coefficient is about 5.82 × 10⁻⁴ and es(Tw) is taken over ice.
Worked example: 25 °C dry bulb, 20 °C wet bulb, 1013.25 hPa
es(Tw) = 23.334 hPa.
Psychrometer term: 6.60 × 10⁻⁴ × (1 + 0.00115 × 20) × 1013.25 × (25 − 20) = 3.421 hPa.
e = 23.334 − 3.421 = 19.914 hPa, so RH = 19.914 / 31.617 × 100 = 63.0 %, with a dew point of 17.5 °C.
The equation assumes the wet bulb is ventilated — a sling psychrometer whirled for a minute, or an aspirated one with a fan moving air past it at about 3 m/s. A wet bulb sitting in still air does not cool as far, so the same equation reads too humid. Keep the wick clean and wetted with distilled water, shade the thermometers from the sun and read the wet bulb at its lowest point.
| Dry bulb 25 °C: wet-bulb depression | Relative humidity | Dew point |
|---|---|---|
| 1 °C | 92.0 % | 23.6 °C |
| 3 °C | 76.9 % | 20.7 °C |
| 5 °C | 63.0 % | 17.5 °C |
| 8 °C | 43.9 % | 11.9 °C |
| 10 °C | 32.3 % | 7.3 °C |
At 1013.25 hPa with a ventilated, water-covered wet bulb.
Frequently Asked Questions
- How do you calculate relative humidity from dew point and temperature?
- Divide the saturation vapor pressure at the dew point by the saturation vapor pressure at the air temperature and multiply by 100. For 25 °C and a dew point of 18 °C that is 20.6 hPa divided by 31.6 hPa, or 65 %. The calculator does this with the Magnus formula and shows the steps.
- How do you find relative humidity from wet-bulb and dry-bulb temperatures?
- Subtract the wet-bulb reading from the dry-bulb reading to get the depression, then use the psychrometer equation to turn it into a vapor pressure and divide by the saturation vapor pressure at the dry-bulb temperature. A bigger depression means drier air. At 25 °C dry bulb, a 5 °C depression is about 63 % humidity.
- What is a good indoor relative humidity?
- The US Environmental Protection Agency advises keeping indoor humidity below 60 %, ideally between 30 % and 50 %, to limit mold without drying out the air. Very dry indoor air in winter is common because cold outdoor air holds little moisture.
- Why does relative humidity change during the day if the moisture doesn't?
- Because the air's capacity to hold vapor rises with temperature. As the air warms after sunrise, the same vapor pressure is a smaller fraction of the saturation value, so RH falls; as it cools at night, RH climbs toward 100 %. The dew point stays nearly constant.
- Can relative humidity be above 100 %?
- Briefly, yes: supersaturated air can exist in clouds and fog formation, but in ordinary measurements a reading above 100 % means an instrument error. The calculator treats a dew point or wet-bulb reading above the air temperature as an error for that reason.
- Does air pressure matter for relative humidity?
- Not for the dew-point method. For a psychrometer it matters a little: the same wet-bulb depression at lower pressure, as at altitude, indicates slightly higher vapor pressure. Use your station pressure, or leave the default if you only need a rough value.