Relative Humidity Calculator
Enter any two of the three values (air temperature, dew point, or relative humidity) and this calculator works out the third using the Magnus formula. Switch between Celsius, Fahrenheit, and Kelvin. Results update as you type, and the steps panel shows the full working.
What is relative humidity?
Relative humidity (RH) is the percentage of water vapor in the air compared with the maximum amount the air could hold at the same temperature. When RH is 100%, the air is saturated and any additional moisture causes condensation or rain. When RH is 50%, the air contains half the water vapor it could hold at that temperature. Because warmer air can hold more moisture than cooler air, the same absolute amount of water vapor produces a lower RH reading in warm air than in cold air.
How the Magnus formula works
This calculator uses the revised Magnus formula to compute the saturation vapor pressure (the maximum water vapor pressure at a given temperature). The formula is: e(T) = 6.1078 * exp(17.625 * T / (243.04 + T)), where T is temperature in degrees Celsius and e is in hectopascals (hPa). Relative humidity is then RH = 100 * e(dew point) / e(air temperature). The formula can be inverted algebraically to solve for the dew point or the air temperature when the other two quantities are known, which is how the three solve modes work.
What is dew point and why does it matter?
The dew point is the temperature to which air must be cooled (at constant pressure and moisture content) for condensation to begin. Unlike relative humidity, the dew point is an absolute measure of how much moisture is in the air, regardless of temperature. A dew point above 20 degrees Celsius feels muggy to most people; below 10 degrees feels dry and comfortable. Weather forecasters use the dew point alongside air temperature to communicate actual moisture conditions more reliably than RH alone, because RH changes throughout the day as temperature rises and falls while the dew point stays roughly constant.
Indoor humidity: comfort and health ranges
ASHRAE Standard 55 recommends indoor relative humidity between 30% and 60%, with most comfort studies finding the sweet spot at 40-50%. Below 30%, air becomes noticeably dry: static electricity builds up, wooden furniture can crack, and mucous membranes in the nose and throat dry out, reducing the body's natural defenses against airborne pathogens. Above 60%, mold and mildew become a serious risk, dust mite populations surge (a common asthma and allergy trigger), and condensation can form on windows and cold surfaces, leading to damp and structural damage. In winter, indoor air often drops below 30% because cold outside air holds little moisture and heating systems warm it without adding water. In summer, humid outdoor air can push indoor levels above 60% if air conditioning or dehumidifiers are not used.
Relative Humidity Comfort and Health Guide
| RH Range | Description | Effects |
|---|---|---|
| 0-20% | Very Dry | Dry skin, static electricity, irritated airways |
| 20-30% | Dry | Slightly dry air, minor discomfort in winter |
| 30-50% | Comfortable | Ideal indoor range per ASHRAE and WHO guidance |
| 50-60% | Slightly Humid | Acceptable, but mold risk increases above 60% |
| 60-70% | Humid | Mold growth likely, dust mites thrive, discomfort |
| 70-100% | Very Humid | High condensation risk, structural damage, health concerns |
Typical effects of indoor relative humidity on comfort, health, and building materials.
Frequently asked questions
What is the difference between relative humidity and dew point?
Relative humidity expresses moisture as a percentage of the maximum the air can hold at its current temperature, so the same dew point gives a higher RH reading in cold air than in warm air. Dew point is an absolute value: the temperature at which condensation starts, regardless of the current air temperature. Meteorologists prefer dew point as a direct indicator of how much moisture is actually present, while RH is more useful for describing how air feels to the body.
Can relative humidity exceed 100%?
In practice, RH does not meaningfully exceed 100% in the open atmosphere because condensation (dew, fog, or rain) removes the excess moisture. In very clean air with few condensation nuclei, supersaturation above 100% can occur briefly in a lab or high atmosphere, but for everyday purposes RH tops out at 100%, meaning the air is fully saturated.
What relative humidity is ideal indoors?
The ASHRAE Standard 55 comfort range is 30-60%, with most people finding 40-50% optimal. Below 30%, dryness causes skin and airway irritation. Above 60%, mold and dust mites become a health concern. The exact ideal varies by season: slightly lower humidity (around 30-40%) is often more practical in winter because very cold outdoor air carries little moisture.
Why does the dew point stay constant while RH changes throughout the day?
Dew point depends only on the actual amount of water vapor in the air. As the sun warms the air during the day, temperature rises but the water vapor content stays roughly the same, so the dew point barely changes. However, because warmer air can hold more moisture, the ratio of actual to maximum moisture (RH) drops. Then in the evening as temperatures cool, RH climbs back toward 100% even though no moisture has been added.
How do I lower humidity in a room?
The most effective methods are: running an air conditioner (which cools air below the dew point to condense moisture, then reheats it), using a standalone dehumidifier, increasing ventilation with drier outdoor air, and fixing moisture sources such as leaks, condensation on cold pipes, or unvented cooking and showering. In humid climates, keeping windows closed during the hottest and most humid parts of the day also helps.