Humidity Calculator

Relative humidity (RH) is the ratio of the actual amount of water vapor in the air to the maximum amount the air could hold at the same temperature, expressed as a percentage. An RH of 100% means the air is fully saturated and cannot hold any more moisture — condensation or rain will occur. The warmer the air, the more water vapor it can hold.

Relative humidity is calculated using the saturation vapour pressure at both the dry-bulb and wet-bulb temperatures, along with the atmospheric pressure. The wet-bulb temperature is lower than the dry-bulb temperature because evaporation from the wet cloth cools it. The greater the difference between the two readings (known as wet-bulb depression), the lower the relative humidity.

For human comfort and health indoors, a relative humidity between 40% and 60% is generally recommended. Below 40% the air feels dry and can irritate the respiratory system; above 60% it promotes mould growth, dust mites, and general discomfort. The World Health Organization suggests keeping indoor RH below 70% to prevent biological contaminants.

When air temperature rises, the air's capacity to hold water vapor increases, so the same amount of moisture represents a lower relative humidity. In other words, warming air decreases its relative humidity even though the actual amount of water in the air stays the same. This is why heating cold, damp outdoor air can make it feel much drier indoors.

A relative humidity of 100% means the air is completely saturated with water vapor. At this point, any additional cooling or addition of moisture will cause condensation — water droplets will form on surfaces, fog will appear, or rain may fall. This temperature at which saturation occurs is called the dew point.

Relative humidity is commonly measured with a hygrometer or a psychrometer (sling psychrometer). A psychrometer uses two thermometers — one dry (dry-bulb) and one covered with a wet wick (wet-bulb) — and the difference between their readings allows RH to be calculated. Digital sensors in modern hygrometers use capacitive or resistive elements that respond to moisture in the air.

It depends on conditions. When outside air is heated to your indoor temperature, its relative humidity drops. If the resulting indoor RH would be lower than your current indoor RH, opening the windows helps. This calculator includes a Window Opening Check section that compares your inside and outside conditions and gives you a direct recommendation.

The dew point is the temperature at which air must be cooled (at constant pressure) to reach saturation and form dew or condensation. Unlike relative humidity, the dew point is an absolute measure of the moisture content of the air — it does not change when temperature changes. A high dew point (above 18°C) feels muggy; a low dew point (below 10°C) feels dry.