Relative Humidity Calculator

What is Relative Humidity?

Relative humidity (RH) is the ratio of the actual amount of water vapor present in the air to the maximum amount the air can hold at a given temperature, expressed as a percentage. When RH reaches 100%, the air is fully saturated and condensation begins. Relative humidity is one of the most important meteorological variables, affecting human comfort, weather patterns, agriculture, and numerous industrial processes.

Unlike absolute humidity (which measures actual moisture content), relative humidity is temperature-dependent. The same amount of water vapor produces a higher RH at lower temperatures because cold air has a lower saturation capacity. This is why mornings often feel damp and afternoons feel dry even when the actual moisture content remains unchanged.

The Relative Humidity Formula

Vapor Pressure Method:

$$RH = \frac{e}{e_s} \times 100\%$$

Where:

• $RH$ = Relative humidity (percent)
• $e$ = Actual vapor pressure (mbar)
• $e_s$ = Saturated vapor pressure at the air temperature (mbar)

Magnus Formula (Dewpoint Method):

The Magnus formula provides an accurate approximation for relating temperature, dewpoint, and relative humidity:

$$RH = 100 \times \frac{\exp\left(\frac{17.271 \cdot T_d}{237.7 + T_d}\right)}{\exp\left(\frac{17.271 \cdot T}{237.7 + T}\right)}$$

Where:

• $T$ = Air temperature in degrees Celsius
• $T_d$ = Dewpoint temperature in degrees Celsius

How to Use the Calculator

Our calculator supports two methods:

• Vapor Pressure Method: Enter actual and saturated vapor pressure values to calculate RH, or solve for either pressure value.
• Dewpoint Method (Magnus): Enter temperature and dewpoint to calculate RH, or solve for dewpoint or air temperature from known RH and the other variable.

Understanding the Results

• Below 30%: Very dry air. Can cause dry skin, irritated eyes, static electricity, and respiratory discomfort.
• 30-50%: Comfortable range for most people. ASHRAE recommends 30-60% for occupied spaces.
• 50-60%: Slightly humid but acceptable. May feel stuffy in poorly ventilated areas.
• Above 60%: Humid conditions. Encourages mold growth, dust mites, and condensation on cold surfaces.
• 100%: Saturation point. Fog, dew, and precipitation occur.

Why Relative Humidity Matters

• Human comfort: High RH slows evaporative cooling, making warm air feel hotter (heat index). Low RH accelerates evaporation, making cool air feel colder.
• Health: Low RH increases transmission of airborne viruses; high RH promotes mold and dust mites.
• Building moisture control: High RH leads to condensation in walls and attics, causing structural damage.
• Agriculture: Greenhouse humidity affects plant transpiration, disease susceptibility, and fruit quality.
• Museum conservation: Artwork, documents, and artifacts require stable RH (typically 45-55%) to prevent deterioration.
• Electronics manufacturing: Low RH increases static discharge risk; high RH causes corrosion.

Common Misconceptions

A common mistake is comparing RH values at different temperatures. 50% RH at 30°C contains far more moisture than 50% RH at 10°C because warm air holds more water vapor. For absolute comparisons, use dewpoint temperature or mixing ratio. Another common error is confusing relative humidity with absolute humidity — RH is a percentage of capacity, not a measure of actual moisture content.

Also check: STP Calculator, Pressure Calculator, Specific Gravity Calculator, Density Calculator, Thermal Conductivity Calculator, and Unit Converter.