Humidity Calculator
This relative humidity calculator finds the relative humidity from the air temperature and the dew point. Enter both temperatures and the relative humidity calculator returns the relative humidity as a percentage. It uses the Magnus form of the relative humidity formula, so an rh calculator like this one gives the same reading a psychrometric chart would.
Results
The Humidity Calculator is part of our Cooling & Heating collection. Browse more cooling & heating to size, convert, and check your work alongside this tool.
What the relative humidity calculator shows
Relative humidity is the amount of water vapor in the air compared with the most it could hold at that temperature, written as a percentage. This relative humidity calculator compares the saturation vapor pressure at the dew point with the saturation vapor pressure at the air temperature. Whether you look for an rh calculator, a rel humidity calculator, or an rh humidity calculator, the reading is the same, since they all apply the same relative humidity formula.
The relative humidity formula
The relative humidity formula divides the saturation vapor pressure at the dew point by the saturation vapor pressure at the air temperature:
Each vapor pressure comes from the Magnus equation, e(T) = exp(17.27 × T / (237.7 + T)), with T in degrees Celsius. The relative humidity calculator runs this twice, once for the dew point Td and once for the air temperature T, then takes the ratio. This is how determining relative humidity from a dew point works, and it is the method behind every rh calculator.
Relative humidity formula example
Air at 25°C with a dew point of 16.7°C. The relative humidity calculator applies the relative humidity formula:
So the air holds about 60 percent of the moisture it could carry at 25°C. Enter your own numbers and the relative humidity calc updates at once.
How to use the relative humidity calculator
Enter the air temperature and the dew point, both in the same unit, and the relative humidity calculator returns the relative humidity as a percentage. The dew point is the temperature at which the air would become fully saturated, so the closer it sits to the air temperature, the higher the humidity. To compute relative humidity you need only these two values, which is why a temperature and humidity calculator is faster than reading a chart by hand.
Reading the humidity calculator result
A reading near 100 percent means the air is close to saturation and dew or fog can form; a reading near 30 percent means dry air that pulls moisture from skin and wood. Comfortable indoor air usually sits between 40 and 60 percent. The humidity calculator gives the percentage; if the dew point equals the air temperature the result is 100 percent, and it can never read higher. Searches like relative humidity calculate or determining relative humidity all point to this same reading.
When to use the relative humidity calculator
- Checking indoor comfort with the relative humidity calculator
- Determining relative humidity for HVAC sizing
- Comparing readings from a temp humidity calculator
- Finding when dew or condensation will form
Common relative humidity calculator mistakes
- Dew point above air temperature. The dew point cannot exceed the air temperature; the relative humidity calculator caps the reading at 100 percent.
- Mixing units. Enter both temperatures in the same scale before you compute relative humidity.
- Confusing relative and absolute humidity. This tool reports a percentage, not the grams per cubic meter an absolute humidity calculator gives.
Tips for accurate readings
- Measure the dew point and air temperature at the same spot and time.
- Indoor comfort sits near 40 to 60 percent; aim there in winter to cut condensation.
- Round the dew point to one decimal, since small shifts move the percentage.
How the relative humidity formula uses dew point
The relative humidity formula leans on one fact: warm air holds more water vapor than cold air, and the dew point marks the temperature where the air you have would be full. When the dew point is far below the air temperature, the air is only partly full and the humidity is low; when the two temperatures meet, the air is saturated and the reading is 100 percent. The relative humidity calculator turns this into a number by comparing saturation vapor pressures, so a rise in dew point at a fixed air temperature always raises the humidity. This is why a muggy day has a high dew point, and the heat load calculator adds that moisture to the cooling total.
Compute relative humidity from temperature and dew point
To compute relative humidity by hand, you run the Magnus equation twice and divide, which is slow and easy to slip on. The relative humidity calculator does both passes at once, so determining relative humidity takes a second. If you only have a dry bulb and a wet bulb temperature instead of a dew point, you first convert those to a dew point, then use this tool; a dedicated wet bulb method covers that less common case, while a temperature and humidity calculator like this one handles the everyday temperature and dew point pair.
Relative humidity calculator reference chart
Relative humidity at an air temperature of 25°C for several dew points, from the relative humidity calculator:
| Dew point | Air temperature | Relative humidity |
|---|---|---|
| 10°C | 25°C | 39% |
| 15°C | 25°C | 54% |
| 20°C | 25°C | 74% |
| 25°C | 25°C | 100% |
Humidity Calculator FAQ
What is the relative humidity formula?
The relative humidity formula is RH = 100 × e(Td) / e(T), the saturation vapor pressure at the dew point divided by the one at the air temperature. The relative humidity calculator applies it for you.
How do you compute relative humidity from dew point?
Run the Magnus equation at the dew point and at the air temperature, then divide and multiply by 100. That is how determining relative humidity works, and what this rh calculator does.
What is a comfortable relative humidity?
Indoor air between 40 and 60 percent feels comfortable and limits mold and static. Below 30 percent the air is dry; above 70 percent it feels muggy.
Can relative humidity be over 100 percent?
In this humidity calculator the reading tops out at 100 percent, which is saturation. Air can briefly supersaturate in nature, but for everyday use 100 percent is the ceiling.