💧 Psychrometric Conversion

Humidity Converter

Convert between relative humidity, dew point, wet-bulb temperature, absolute humidity, and humidity ratio. Useful for psychrometric calculations.

📋 Project Info (optional, appears on printed report)

Project / Client

Address

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💧 Humidity Inputs

🌡 Dry-Bulb Air Temperature

Dry-Bulb Temperature

°C

🔄 Known Value to Convert From

Relative Humidity (%)

💧 Humidity Conversion Results

°C

All Converted Values

Full Conversion Table

Property	Value	Unit

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Understanding Humidity Conversions

HVAC work constantly moves between different ways of describing how much moisture is in the air, relative humidity for comfort control, dew point for condensation risk, wet-bulb temperature for cooling tower and coil calculations, and humidity ratio for precise psychrometric math. This converter translates between all five at a given dry-bulb temperature, since most of these values only make sense in combination with temperature.

Relative Humidity Depends on Temperature

Relative humidity describes how close air is to saturation at its current temperature, which means the same air parcel shows a different relative humidity reading as temperature changes even with no moisture added or removed. This is why a home can feel humid at night and dry during the day despite no change in actual moisture content, the dry-bulb temperature shift alone changes the relative humidity reading. See the relative humidity calculator for more on this relationship.

Dew Point Reflects Actual Moisture Content

Unlike relative humidity, dew point temperature stays constant as dry-bulb temperature changes, since it directly reflects the actual amount of moisture in the air rather than a ratio relative to current temperature. ASHRAE's official terminology defines it as the temperature to which air must be cooled at constant pressure for water vapor to condense. This makes dew point the right number to check when evaluating condensation risk on a chilled surface, since condensation happens whenever a surface drops to or below the air's dew point regardless of the surrounding dry-bulb temperature.

Wet-Bulb Temperature and Evaporative Cooling Limits

Wet-bulb temperature represents the lowest temperature air can reach through evaporative cooling alone, the physical principle behind cooling towers and the reason wet-bulb rather than dry-bulb temperature governs tower and coil dehumidification performance. The cooling tower calculator relies on wet-bulb temperature for this reason.

Humidity Ratio for Precise Load Calculations

Humidity ratio, the actual mass of water vapor per unit mass of dry air, is the value psychrometric load calculations use directly rather than relative humidity, since it represents an absolute quantity rather than a temperature-dependent ratio, following the psychrometric methods set out in the ASHRAE Handbook -- Fundamentals. Converting between humidity ratio and the more commonly discussed relative humidity or dew point is a routine step when moving between design documentation and field measurements.

Frequently Asked Questions

Relative humidity, wet-bulb temperature, humidity ratio, and absolute humidity all describe the moisture content of air relative to how much moisture that air could hold at a given temperature, so every conversion between these values depends on knowing the dry-bulb temperature. Dew point is the one exception, since dew point directly reflects the actual moisture content of the air regardless of the current dry-bulb temperature.

Relative humidity is a percentage describing how close the air is to saturation at its current temperature, while humidity ratio is an absolute measure of the actual mass of water vapor per unit mass of dry air, typically expressed in grains per pound or grams per kilogram. Relative humidity changes as temperature changes even with no moisture added or removed, while humidity ratio stays constant unless moisture is actually added or removed from the air.

Condensation forms whenever a surface temperature drops to or below the dew point of the surrounding air, since air can't hold moisture beyond its saturation point at that temperature. This is why dew point, not relative humidity alone, is the relevant number when checking condensation risk on chilled surfaces like radiant panels, cold water piping, or duct exteriors in humid spaces. See the radiant ceiling calculator for a practical application of this.

Wet-bulb temperature reflects the theoretical lowest temperature air can reach through evaporative cooling alone, which directly limits how cold a cooling tower can make condenser water and how coil dehumidification performance responds to entering air conditions. Because evaporative and coil heat transfer processes both depend on this evaporative cooling limit, wet-bulb temperature is the standard input for these calculations rather than dry-bulb temperature or relative humidity alone.