Relative Humidity Calculator

Relative humidity (RH) is the ratio of the current amount of water vapor in the air to the maximum amount the air can hold at a given temperature, expressed as a percentage. A value of 100% means the air is fully saturated and dew or fog may form. At 50%, the air contains half the moisture it could potentially hold.

The dry bulb temperature is the standard air temperature measured by a regular thermometer. The wet bulb temperature is measured by a thermometer whose bulb is wrapped in a water-soaked wick — as moisture evaporates, it cools the thermometer. The difference between these two readings (the wet bulb depression) is used to calculate relative humidity using a sling psychrometer.

The dew point is the temperature at which air must be cooled (at constant pressure) for water vapor to condense into dew. The closer the dew point is to the actual air temperature, the higher the relative humidity. When the dew point equals the air temperature, relative humidity is 100%.

For human comfort and health, indoor relative humidity is ideally kept between 40% and 60%. Below 30% the air feels dry, can irritate mucous membranes, and promote static electricity. Above 70%, mold growth, dust mites, and structural moisture damage become concerns. Humans generally feel their best at relative humidity of 40% or above.

The calculator uses the August-Roche-Magnus approximation: RH = 100 × exp((17.625 × Td) / (243.04 + Td)) / exp((17.625 × T) / (243.04 + T)), where T is the dry bulb temperature in Celsius and Td is the dew point temperature in Celsius. For wet bulb inputs, the dew point is first estimated using the psychrometric equation before applying the Magnus formula.

Low relative humidity (below 30%) can cause dry skin, irritated eyes, and increased susceptibility to respiratory infections. High relative humidity (above 70%) promotes mold, bacteria, and dust mite growth, which can trigger allergies and asthma. Maintaining humidity between 40–60% supports optimal respiratory health and overall comfort.

Yes. Warmer air can hold more water vapor, so if you heat air without adding moisture, the relative humidity drops even though the actual water content stays the same. This is why indoor air feels dry in winter — cold outdoor air is brought inside and heated, drastically lowering its relative humidity.

Absolute humidity is the actual mass of water vapor present in a given volume of air, typically expressed in grams per cubic meter (g/m³). Unlike relative humidity, it does not depend on temperature. Relative humidity tells you how close the air is to saturation, while absolute humidity tells you the raw amount of moisture present.