Relative Humidity Calculator
Calculate relative humidity from air temperature and dew point. Enter values in Celsius or Fahrenheit and get an instant humidity percentage with useful moisture information.
Calculate Relative Humidity
Enter the air temperature and dew point to calculate relative humidity.
What Is a Relative Humidity Calculator?
A Relative Humidity Calculator estimates the percentage of moisture in the air relative to the maximum amount of water vapor the air could hold at the same temperature.
This calculator uses air temperature and dew point to determine relative humidity. Because the amount of water vapor that corresponds to saturation changes with temperature, relative humidity is temperature dependent.
How to Use the Relative Humidity Calculator
The calculator requires two values: the current air temperature and the dew point temperature.
Enter Air Temperature
Enter the measured air temperature for the conditions you want to calculate.
Enter the Dew Point
Enter the dew point using the same temperature unit as the air temperature.
Calculate Relative Humidity
Select Celsius or Fahrenheit and click the button to calculate the estimated relative humidity percentage.
How Is Relative Humidity Calculated?
Relative humidity can be estimated from air temperature and dew point by comparing the saturation vapor pressure at the dew point with the saturation vapor pressure at the air temperature.
RH = 100 × [e(Td) ÷ e(T)]
e(T) = exp[(17.625 × T) ÷ (243.04 + T)]
Here, T represents air temperature in Celsius and Td represents dew point in Celsius. If Fahrenheit values are entered, the calculator converts them to Celsius before performing the calculation.
Relative Humidity Calculation Example
Suppose the air temperature is 25°C and the dew point is 15°C. Using the vapor-pressure relationship gives a relative humidity of approximately 54%.
If the air temperature decreases while the amount of water vapor remains similar, relative humidity generally rises. When the air temperature reaches the dew point, relative humidity approaches 100%.
Relative Humidity vs. Dew Point
Relative humidity and dew point are both related to atmospheric moisture, but they describe moisture in different ways.
| Measurement | What It Describes | Unit |
|---|---|---|
| Relative Humidity | Moisture relative to saturation at the current air temperature | % |
| Dew Point | Temperature at which the air reaches saturation when cooled under the assumed conditions | °C or °F |
| Air Temperature | Current thermal condition of the air | °C or °F |
What Affects Relative Humidity?
Air Temperature
Relative humidity changes as air temperature changes, even when the actual water-vapor amount remains approximately constant.
Water Vapor
Adding or removing water vapor directly changes the moisture content of the air and therefore affects relative humidity.
Dew Point
Dew point provides information about atmospheric moisture and can be combined with temperature to estimate relative humidity.
Air Exchange
Ventilation and movement of air between spaces with different moisture conditions can change indoor humidity.
Understanding Relative Humidity Results
Relative humidity ranges from approximately 0% for extremely dry air toward 100% as air approaches saturation. The meaning of a particular value depends on temperature and the application being considered.
| Relative Humidity | General Description |
|---|---|
| Below 30% | Low relative humidity |
| 30%–40% | Moderate-low humidity |
| 40%–60% | Moderate relative humidity |
| 60%–80% | High relative humidity |
| Above 80% | Very high relative humidity |
| 100% | Air is at saturation under the measured conditions |
Why Is Relative Humidity Important?
Weather
Relative humidity is widely used in weather observations because it provides information about how close the air is to saturation at its current temperature.
HVAC and Buildings
Humidity measurements can help evaluate indoor environmental conditions, ventilation, dehumidification, humidification, and potential condensation concerns.
Storage and Manufacturing
Some materials and products are sensitive to moisture. Humidity monitoring can therefore be important in storage areas, laboratories, manufacturing facilities, and other controlled environments.
Agriculture
Temperature and humidity can influence plant environments, evaporation, greenhouse management, and other agricultural processes.
Relative Humidity Calculator FAQs
Relative humidity is the ratio between the actual water-vapor pressure and the saturation water-vapor pressure at the same temperature, expressed as a percentage.
Relative humidity can be estimated by calculating saturation vapor pressure at the dew point and dividing it by saturation vapor pressure at the current air temperature. This calculator performs those steps automatically.
A relative humidity of 100% means the air is at saturation for the measured temperature and pressure conditions.
Yes. Enter both air temperature and dew point in Fahrenheit and select the Fahrenheit option. The calculator handles the internal conversion automatically.
When the air temperature equals the dew point, the air is at or very near saturation and relative humidity is approximately 100%.
Under ordinary unsaturated atmospheric conditions, dew point should not be higher than the current air temperature. If it is, the measurements or inputs should be checked.
The saturation vapor pressure of water changes strongly with temperature. Because relative humidity compares actual vapor pressure with saturation vapor pressure, changing temperature can change relative humidity even without adding or removing much water vapor.