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Humidity Calculator
Use this humidity calculator to find the relative humidity, absolute humidity, and humidity ratio of air from its temperature and dew point. Enter the air temperature and the dew point, and the tool works out how much water vapor the air holds, how close it is to saturation, and whether the air will feel dry, comfortable, or muggy. It also runs in reverse: enter a temperature and a relative humidity to find the absolute humidity.
The dew point can never be higher than the air temperature. If it is, check your numbers — a dew point above the air temperature would mean the air holds more water than it possibly can.
How it works
Humidity is all about water vapor — the invisible water gas mixed into the air. Warm air can hold far more water vapor than cold air, which is why a hot summer day can feel sticky even when the relative humidity number looks modest, and why cold winter air feels so dry once it is heated indoors. This calculator uses the Magnus formula, a well-tested approximation that weather services rely on, to work out how much water vapor the air can hold at any temperature.
Relative humidity compares the water vapor actually in the air with the most the air could hold at that temperature. The dew point is the key: it is the temperature at which the air would become saturated and dew would start to form. At the dew point, the air holds exactly as much water as it can, so the actual vapor pressure equals the saturation vapor pressure at the dew point. Divide that by the saturation vapor pressure at the air temperature and multiply by 100, and you have the relative humidity.
Absolute humidity is simpler to picture: it is the mass of water vapor in each cubic meter of air, in grams per cubic meter. It comes from the ideal gas law applied to the water vapor alone. Take the actual vapor pressure, which you get from the dew point, and divide by the absolute temperature. A typical comfortable room holds about 8 to 12 grams of water vapor per cubic meter, while a muggy summer afternoon can hold more than 20.
The humidity ratio, also called the mixing ratio, measures grams of water vapor per kilogram of dry air. Heating and air-conditioning engineers prefer it because it does not change when air is heated or cooled — only adding or removing water changes it. It is calculated from the vapor pressure and the standard sea-level pressure of 1013.25 hectopascals. In the second mode, you give the temperature and the relative humidity instead, and the tool works backward: it finds the vapor pressure from the relative humidity, then turns that into the absolute humidity.
Enter temperatures in Celsius or Fahrenheit — the tool converts Fahrenheit to Celsius before it calculates. The dew point must be at or below the air temperature, because air cannot hold more water than its saturation limit. If you type a dew point above the temperature, the tool will ask you to fix it rather than showing a meaningless number above 100%.
Worked example
A weather station reports an air temperature of 25 °C and a dew point of 20 °C. What is the relative humidity, and how much water does the air actually hold?
Step 1: Find the saturation vapor pressure at the dew point of 20 °C. Using the Magnus formula, es(20) = 6.112 × exp(17.67 × 20 ÷ 263.5) = 23.37 hPa. This is the actual vapor pressure, because at the dew point the air is saturated.
Step 2: Find the saturation vapor pressure at the air temperature of 25 °C. es(25) = 6.112 × exp(17.67 × 25 ÷ 268.5) = 31.67 hPa. This is the most water vapor the air could hold.
Step 3: Divide and multiply by 100. RH = 100 × 23.37 ÷ 31.67 = 73.8%.
Step 4: Find the absolute humidity. AH = 216.7 × 23.37 ÷ (25 + 273.15) = 17.0 g/m³.
Step 5: Find the humidity ratio. w = 0.62198 × 23.37 ÷ (1013.25 − 23.37) = 0.0147 kg/kg, or 14.7 g/kg.
Try it yourself: enter air temperature 25 °C and dew point 20 °C, and press Calculate. The tool gives exactly the same answers — relative humidity 73.8%, absolute humidity 17.0 g/m³, humidity ratio 14.7 g/kg — and flags the air as very humid, with a mold risk if it stays that damp indoors, since it sits well above the 30–50% comfort band.
FAQ
What is relative humidity?
Relative humidity is the amount of water vapor in the air compared with the maximum amount the air could hold at that temperature, written as a percentage. Air at 73.8% relative humidity holds 73.8% of the water vapor it could possibly hold before condensation begins. Warm air can hold much more water vapor than cold air, so the same amount of water vapor gives a lower relative humidity on a warm day than on a cold day.
What is the difference between relative humidity and absolute humidity?
Relative humidity is a percentage that depends on temperature, while absolute humidity is the actual mass of water vapor in each cubic meter of air, measured in grams per cubic meter. Absolute humidity tells you how much water is really there; relative humidity tells you how close the air is to being saturated. On a hot day, the absolute humidity can be high while the relative humidity looks moderate, because hot air has a much larger capacity for water vapor.
What is a comfortable indoor humidity level?
Most people feel comfortable when indoor relative humidity is between 30% and 50%. Below 30%, the air feels dry: skin cracks, throats get scratchy, and static shocks become common. Above 60%, the air feels sticky and muggy, and above 70% the risk of mold growth and dust mites rises sharply. A dehumidifier helps in damp rooms, while a humidifier helps in dry winter air.
How do you calculate relative humidity from the dew point?
The dew point is the temperature at which the air becomes saturated, so the actual vapor pressure equals the saturation vapor pressure at the dew point. Divide that by the saturation vapor pressure at the air temperature and multiply by 100. For example, with air at 25 degrees Celsius and a dew point of 20 degrees, the vapor pressure is about 23.37 hectopascals and the saturation pressure at 25 degrees is about 31.67 hectopascals, giving 23.37 divided by 31.67 times 100, or about 73.8% relative humidity.
Can relative humidity be over 100%?
In normal outdoor air, relative humidity cannot stay above 100% for long, because once the air is saturated the extra water vapor condenses into dew, fog, or cloud droplets. In a laboratory, very clean air can briefly become supersaturated, reaching a few percent above 100% before droplets form, but in everyday weather the dew point is never higher than the air temperature, so this calculator caps the result at 100%.