Relative Humidity Calculator

Enter the air (dry-bulb) temperature and the dew point to get relative humidity (RH = 100 x e / es(T)), plus the saturation and actual vapor pressures.

Quick Facts

Relative humidity formula
RH = 100 × e / es(T)
Ratio of the actual vapor pressure to the saturation vapor pressure at the air temperature.
Saturation vapor pressure (Bolton 1980)
es(T) = 6.112 × e^(17.67T/(T+243.5))
T in °C, result in hPa (millibars); accurate to about 0.1% between -40°C and 50°C.
Dew point rule
Td ≤ T, always
When the dew point equals the air temperature, the air is saturated and RH = 100%.
Comfortable indoor range
30% – 50% RH
General ASHRAE/EPA guidance to limit dryness, static, condensation, and mold risk.

Your Results

Calculated
Relative Humidity
-
RH = 100 × e / es(T)
Saturation Vapor Pressure
-
es(T) at air temperature, hPa
Actual Vapor Pressure
-
es(Td) at dew point, hPa
Comfort Level
-
Interpretation of the RH% result

Ready

Enter the air temperature and dew point, then press Calculate.

Formula and Method for Relative Humidity

Relative humidity (RH) is the ratio of the actual water vapor pressure in the air to the saturation vapor pressure at the same air temperature, expressed as a percentage: RH = 100 × e / es(T). It measures how close the air is to being fully saturated with moisture, not how much water vapor it actually contains — the same absolute amount of moisture gives a much higher RH on a cold day than on a hot one. This calculator estimates RH from two temperature readings you can get with a thermometer and a hygrometer or psychrometer: the air (dry-bulb) temperature and the dew point temperature.

How the calculation works

At the dew point temperature (Td), air is exactly saturated, so the actual vapor pressure e equals the saturation vapor pressure evaluated at Td. The calculator finds the saturation vapor pressure at both temperatures using the Bolton (1980) approximation of the Clausius-Clapeyron relation, es(T) = 6.112 × exp(17.67T / (T + 243.5)) hPa, with T in °C. Relative humidity then follows from RH = 100 × es(Td) / es(T). Because the dew point can never exceed the air temperature, this ratio always stays between 0% and 100%.

Common mistakes to avoid

  • Entering a dew point above the air temperature: this is physically impossible in equilibrium — double-check which reading is which before recalculating.
  • Mixing temperature units: both the air temperature and dew point must use the same unit selection; converting only one of them produces a meaningless RH value.
  • Confusing relative humidity with absolute humidity: RH changes with temperature even when the actual moisture content of the air (grams of water per cubic meter) stays constant.

Real-world applications

  • Weather forecasting and comfort indices use RH alongside temperature to estimate how hot or muggy conditions will feel.
  • HVAC and building design keep indoor RH in a target band to limit mold, dust mites, static electricity, and window condensation.
  • Museums, archives, and labs control RH tightly to protect paper, artwork, electronics, and other moisture-sensitive materials.
  • Agriculture and food storage track RH to manage crop drying, greenhouse conditions, and spoilage risk.

Frequently Asked Questions

What is relative humidity?
Relative humidity (RH) is the ratio of the actual water vapor pressure in the air to the saturation vapor pressure at the same air temperature, expressed as a percentage: RH = 100 × e / es(T). It tells you how close the air is to being fully saturated, not how much water vapor it contains in absolute terms.
How does this calculator find RH from temperature and dew point?
The calculator uses the Bolton (1980) approximation of the Clausius-Clapeyron relation, es(T) = 6.112 × exp(17.67T / (T + 243.5)) hPa, to find the saturation vapor pressure at the air temperature and at the dew point. Air is fully saturated exactly at its dew point, so the saturation vapor pressure at the dew point equals the actual vapor pressure, giving RH = 100 × es(Td) / es(T).
Why can't the dew point be higher than the air temperature?
The dew point is the temperature air would need to cool to, at constant pressure and moisture content, to become saturated. Air cannot hold more water vapor than saturation allows at its current temperature, so the dew point can never exceed the air temperature; if it does, one of the two readings is a measurement error.
What relative humidity is considered comfortable indoors?
Most building-science and health guidance, such as ASHRAE and the EPA, recommends keeping indoor relative humidity between about 30% and 50%. Below that range air feels dry and static electricity increases; above it, condensation, dust mites, and mold growth become more likely.

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