Relative Humidity Calculator
Calculate relative humidity from temperature and dew point or vapor pressures
Calculate Relative Humidity
Ambient air temperature
Temperature at which condensation occurs
Absolute Humidity
Specific Humidity
Vapor Pressure Analysis
Formula Used
Magnus Formula: RH = 100 × [e^(17.625×Dp/(243.04+Dp)) / e^(17.625×T/(243.04+T))]
Where: T = air temperature (°C), Dp = dew point (°C)
Example Calculation
Summer Weather Conditions
Air Temperature: 35°C (95°F)
Dew Point: 21.1°C (70°F)
Problem: Calculate the relative humidity
Solution Steps
1. Apply Magnus formula coefficients: β = 17.625, λ = 243.04°C
2. Calculate saturation vapor pressure at 35°C: Pws = 5.627 kPa
3. Calculate vapor pressure at dew point (21.1°C): Pw = 2.504 kPa
4. RH = 100 × (2.504 / 5.627) = 44.5%
Result: Moderate humidity on a hot day
Humidity Comfort Zones
> 70% RH
Too humid - muggy feeling
50-70% RH
Acceptable - slightly humid
30-50% RH
Optimal comfort zone
< 30% RH
Too dry - skin irritation
Quick Facts
100% Relative Humidity
Air is fully saturated with water vapor
Temperature Effect
Higher temperature = lower relative humidity (same moisture)
Dew Point
Temperature when air becomes saturated
Magnus Formula
Uses coefficients β=17.625 and λ=243.04°C
Measurement Methods
Psychrometer - wet/dry bulb temperatures
Hair hygrometer - hair length changes
Capacitive sensors - electrical properties
Chilled mirror - dew point detection
Understanding Relative Humidity
What is Relative Humidity?
Relative humidity is the ratio of the current amount of water vapor in the air to the maximum amount of water vapor the air can hold at that temperature, expressed as a percentage.
Why It Matters
- •Comfort: Affects how temperature feels to our body
- •Health: Impacts respiratory comfort and skin condition
- •Industry: Critical for manufacturing and storage
- •Weather: Key parameter in meteorological forecasting
Calculation Methods
This calculator uses the Magnus formula, which is highly accurate for meteorological applications. The formula relates temperature, dew point, and relative humidity using empirically derived coefficients.
Health Effects
- •Low (<30%): Dry skin, respiratory irritation, static electricity
- •Optimal (30-50%): Maximum comfort and health
- •High (>70%): Mold growth, reduced cooling efficiency
Mathematical Relationships
RH = 100 × (Pw / Pws)
Basic relative humidity definition
Pws = 0.61121 × e^(17.625×T/(243.04+T))
Magnus formula for saturation pressure
H = (Pw × 2165) / (T + 273.15)
Absolute humidity calculation
q = 0.622 × Pw / (P - 0.378 × Pw)
Specific humidity formula
• RH = Relative humidity (%)
• Pw = Vapor pressure (kPa)
• Pws = Saturation vapor pressure (kPa)
• 17.625 = Magnus coefficient β
• 243.04°C = Magnus coefficient λ
• 0.61121 kPa = Base pressure constant
• HVAC system design
• Weather forecasting
• Industrial process control