Relative Humidity Calculator

Calculate relative humidity, dew point temperature, and vapor pressure from temperature and humidity data.

Use temperature and dew point or actual vapor pressure to determine relative humidity and related atmospheric parameters using the Magnus formula.

Examples

Click on any example to load it into the calculator.

Comfortable Indoor Conditions

Comfortable Indoor Conditions

Typical comfortable indoor environment with moderate humidity levels.

Temperature: 22 °C

Dew Point: 15 °C

High Humidity Environment

High Humidity Environment

Hot and humid conditions typical of tropical climates or summer days.

Temperature: 30 °C

Dew Point: 25 °C

Low Humidity Conditions

Low Humidity Conditions

Dry conditions typical of desert climates or heated indoor spaces in winter.

Temperature: 20 °C

Dew Point: 5 °C

Saturated Air (100% RH)

Saturated Air (100% RH)

Air at saturation point where relative humidity reaches 100%.

Temperature: 18 °C

Dew Point: 18 °C

Other Titles
Understanding Relative Humidity: A Comprehensive Guide
Explore the science behind humidity, vapor pressure, and atmospheric moisture. Learn how to calculate and interpret relative humidity for various applications in meteorology, HVAC, and environmental monitoring.

What is Relative Humidity?

  • Definition and Concept
  • Importance in Daily Life
  • Measurement Methods
Relative humidity (RH) is a fundamental atmospheric parameter that describes the amount of water vapor present in the air relative to the maximum amount the air can hold at a given temperature. It is expressed as a percentage, where 100% represents air that is completely saturated with water vapor. Understanding relative humidity is crucial for weather forecasting, indoor comfort, industrial processes, and various scientific applications.
The Physics of Water Vapor in Air
Air can hold varying amounts of water vapor depending on its temperature. Warm air can hold more water vapor than cold air. When air reaches its maximum water vapor capacity at a given temperature, it becomes saturated, and any additional water vapor will condense into liquid water (dew, fog, or clouds). The relationship between temperature and the maximum water vapor capacity is described by the saturation vapor pressure curve.
Why Relative Humidity Matters
Relative humidity affects human comfort, health, and various industrial processes. Low humidity can cause dry skin, respiratory irritation, and static electricity, while high humidity can lead to mold growth, condensation problems, and reduced thermal comfort. In meteorology, relative humidity is essential for weather prediction and understanding precipitation patterns.
Measurement and Calculation Methods
Relative humidity can be measured using various instruments including psychrometers, hygrometers, and electronic sensors. It can also be calculated using temperature and dew point data, or by knowing the actual and saturation vapor pressures. The most accurate calculations use the Magnus formula for saturation vapor pressure.

Key Concepts in Humidity Measurement:

  • Absolute Humidity: The actual mass of water vapor per unit volume of air (g/m³)
  • Specific Humidity: The mass of water vapor per unit mass of air (g/kg)
  • Mixing Ratio: The mass of water vapor per unit mass of dry air (g/kg)
  • Vapor Pressure: The partial pressure exerted by water vapor in the air (hPa or Pa)

Step-by-Step Guide to Using the Calculator

  • Input Methods
  • Calculation Process
  • Result Interpretation
The Relative Humidity Calculator offers multiple input methods to accommodate different measurement scenarios and available data. You can calculate relative humidity using temperature and dew point, or by providing actual vapor pressure directly.
Method 1: Temperature and Dew Point
This is the most common method. Enter the current air temperature and the dew point temperature. The calculator will use the Magnus formula to calculate both the saturation vapor pressure (from temperature) and actual vapor pressure (from dew point), then determine the relative humidity as their ratio.
Method 2: Temperature and Actual Vapor Pressure
If you have direct measurements of actual vapor pressure from specialized instruments, enter the temperature and vapor pressure. The calculator will determine the saturation vapor pressure and calculate relative humidity.
Method 3: Temperature and Relative Humidity
If you know the relative humidity and want to find other parameters, enter the temperature and relative humidity percentage. The calculator will determine the actual vapor pressure and dew point.
Understanding the Results
The calculator provides four key outputs: relative humidity percentage, dew point temperature, actual vapor pressure, and saturation vapor pressure. These values are interconnected and provide a complete picture of the atmospheric moisture conditions.

Input Validation and Error Handling:

  • Temperature Range: Valid inputs are between -40°C and 60°C
  • Relative Humidity: Must be between 0% and 100%
  • Dew Point: Cannot exceed the air temperature
  • Vapor Pressure: Must be positive and reasonable for the given temperature

Real-World Applications of Relative Humidity

  • Meteorology and Weather
  • HVAC and Building Science
  • Industrial Processes
Relative humidity calculations are essential in numerous fields and applications, from weather forecasting to industrial manufacturing processes.
Weather Forecasting and Meteorology
Meteorologists use relative humidity data to predict weather patterns, precipitation, and severe weather events. High humidity levels often precede rain or storms, while low humidity can indicate clear, dry conditions. Understanding humidity patterns helps in climate modeling and long-term weather prediction.
HVAC and Indoor Air Quality
Heating, ventilation, and air conditioning (HVAC) systems rely on humidity control for occupant comfort and health. Proper humidity levels (typically 30-60%) prevent mold growth, reduce respiratory problems, and maintain comfortable indoor environments. Humidity calculations are crucial for system design and operation.
Industrial and Manufacturing Processes
Many industrial processes are sensitive to humidity levels. Paper manufacturing, pharmaceutical production, food processing, and electronics manufacturing all require specific humidity conditions. Precise humidity control ensures product quality and process efficiency.
Agriculture and Greenhouse Management
Plant growth and crop yields are significantly affected by humidity levels. Greenhouses and agricultural facilities use humidity monitoring and control systems to optimize growing conditions and prevent plant diseases.

Optimal Humidity Ranges for Different Applications:

  • Human Comfort: 30-60% RH at 20-25°C
  • Data Centers: 40-60% RH to prevent static electricity
  • Museums: 45-55% RH to preserve artifacts
  • Greenhouses: 60-80% RH for most plants

Common Misconceptions and Correct Methods

  • Humidity vs. Temperature
  • Measurement Accuracy
  • Calculation Precision
Several misconceptions exist about relative humidity and its relationship with temperature and comfort. Understanding these helps in proper interpretation and application of humidity data.
Misconception: Higher Temperature Always Means Higher Humidity
While warm air can hold more water vapor, the relative humidity can actually decrease with increasing temperature if the absolute moisture content remains constant. For example, air at 20°C with 50% RH will have lower relative humidity when heated to 30°C, even though the actual water vapor content hasn't changed.
Misconception: Relative Humidity is the Best Measure of Comfort
While relative humidity is important, other factors like absolute humidity, air movement, and personal factors also affect comfort. The heat index and wet-bulb temperature provide better measures of how humidity affects human comfort and health.
Misconception: All Humidity Calculations are Equally Accurate
Different formulas for calculating vapor pressure have varying levels of accuracy. The Magnus formula used in this calculator provides good accuracy for temperatures between -40°C and 60°C. For extreme temperatures or high precision requirements, more complex equations may be needed.
Misconception: Dew Point is Always Lower Than Air Temperature
The dew point can never exceed the air temperature. When the dew point equals the air temperature, the relative humidity is 100%, and the air is saturated. This condition often leads to fog, dew, or precipitation formation.

Accuracy Considerations:

  • Magnus Formula Accuracy: ±0.1% RH for most practical applications
  • Temperature Measurement: ±0.5°C accuracy recommended
  • Dew Point Measurement: ±1°C accuracy typical for most instruments
  • Vapor Pressure: ±1 hPa accuracy for most meteorological applications

Mathematical Derivation and Examples

  • Magnus Formula
  • Vapor Pressure Relationships
  • Practical Calculations
The mathematical foundation of relative humidity calculations is based on well-established physical relationships and empirical formulas that have been validated through extensive research and practical applications.
The Magnus Formula for Saturation Vapor Pressure
The Magnus formula is an empirical equation that accurately describes the relationship between temperature and saturation vapor pressure: es = 6.1094 × exp((17.625 × T) / (T + 243.04)), where es is in hPa and T is in °C. This formula is valid for temperatures between -40°C and 60°C and provides accuracy within ±0.1% for most practical applications.
Relative Humidity Calculation
Relative humidity is calculated as RH = (e / es) × 100%, where e is the actual vapor pressure and es is the saturation vapor pressure. The actual vapor pressure can be determined from the dew point temperature using the same Magnus formula, since the dew point represents the temperature at which the air would become saturated with its current water vapor content.
Dew Point Calculation
The dew point can be calculated from relative humidity and temperature by first determining the actual vapor pressure (e = RH × es / 100), then solving the Magnus formula for temperature using the actual vapor pressure as the saturation vapor pressure. This requires iterative methods or approximation formulas.
Practical Calculation Examples
Consider air at 25°C with a dew point of 15°C. The saturation vapor pressure at 25°C is 31.7 hPa, and the actual vapor pressure (from dew point) is 17.0 hPa. The relative humidity is (17.0 / 31.7) × 100% = 53.6%. This represents comfortable indoor conditions.

Mathematical Relationships:

  • RH = (e/es) × 100% - Basic relative humidity formula
  • e = es(Td) - Actual vapor pressure equals saturation at dew point
  • es = 6.1094 × exp((17.625×T)/(T+243.04)) - Magnus formula
  • Td ≈ T - ((100-RH)/5) - Approximate dew point from RH (for RH > 50%)