Relative Humidity Calculator
Calculate relative humidity from air temperature and dew point using the Magnus formula. Online relative humidity calculator with comfort analysis and charts.
About This Calculator
The Relative Humidity Calculator computes the relative humidity (RH) of air using the widely accepted Magnus formula, given the air temperature and dew point. Relative humidity is a critical metric in meteorology, HVAC design, agriculture, and everyday comfort assessment — it tells you how saturated the air is with water vapor compared to its maximum capacity at that temperature.
The calculator uses the Alduchov and Eskridge revised Magnus coefficients (17.625 and 243.04°C) for accurate saturation vapor pressure calculations across a wide temperature range. The formula is RH = 100 × [exp(17.625 × Dp / (243.04 + Dp)) / exp(17.625 × T / (243.04 + T))], where T is the air temperature and Dp is the dew point temperature, both in degrees Celsius. Results include the relative humidity percentage, a comfort level assessment, and a comparison chart of temperature versus dew point.
Regional Considerations
India: High humidity is common during the monsoon season (June-September), especially in coastal cities like Mumbai and Chennai where RH often exceeds 80%. Indoor comfort typically requires dehumidification in coastal regions. The India Meteorological Department (IMD) uses RH data for weather forecasting and heat index warnings.
United States: The National Weather Service (NWS) issues heat advisories based on the heat index, which combines temperature and RH. Summer in the Southeast and Midwest frequently sees RH above 70%. ASHRAE recommends indoor RH between 30-60% for occupant comfort and building health.
United Kingdom: The UK has naturally high humidity due to its maritime climate, with RH typically 70-90% year-round. Indoor humidity management is important for preventing condensation and mold in older buildings. The Met Office provides RH data for weather forecasting and air quality monitoring.
This tool helps homeowners, HVAC professionals, meteorology students, and agricultural planners determine moisture levels in the air for better decision-making about ventilation, irrigation scheduling, and comfort management.
Frequently Asked Questions
What is relative humidity?
Relative humidity (RH) measures how much water vapor is in the air compared to the maximum amount the air can hold at that temperature. It is expressed as a percentage. Warm air can hold more moisture than cold air, so RH depends heavily on temperature.
How is relative humidity calculated?
RH is calculated using the Magnus formula: RH = 100 × [exp(17.625 × Dp / (243.04 + Dp)) / exp(17.625 × T / (243.04 + T))], where T is air temperature in degrees Celsius and Dp is dew point temperature in degrees Celsius. The formula compares actual vapor pressure to saturation vapor pressure.
What is the ideal relative humidity for comfort?
The ideal indoor relative humidity for human comfort is between 30% and 50%. Below 30%, air feels dry and can cause skin irritation and respiratory discomfort. Above 60%, the air feels muggy and can promote mold growth and dust mites.
How does temperature affect relative humidity?
When temperature rises and moisture content stays the same, relative humidity decreases because warm air can hold more water vapor. For example, heating cold winter air from 0°C to 22°C without adding moisture drops indoor RH from 60% to as low as 15-20%.
What does 100% relative humidity mean?
100% relative humidity means the air is fully saturated with water vapor and cannot hold any more moisture. At this point, fog, dew, or condensation forms. Rain typically occurs when RH reaches 100% and excess moisture falls as precipitation.
What is the difference between relative humidity and absolute humidity?
Relative humidity is a percentage that depends on temperature, while absolute humidity measures the actual mass of water vapor per volume of air in g/m³. Absolute humidity is temperature-independent. RH of 50% at 30°C contains much more water vapor than 50% at 10°C.
How can I measure relative humidity?
RH is measured using a hygrometer. Common types include psychrometers (using wet and dry bulb temperatures), capacitive electronic sensors, and hair hygrometers. Smart home weather stations typically include a digital RH sensor accurate to within 2-3%.
Why does high humidity make us feel hotter?
High humidity slows sweat evaporation from the skin because the air already contains significant moisture. Since evaporation is the body's primary cooling mechanism, less evaporation means you feel hotter than the actual temperature. This is measured by the heat index.