Dew Point & Relative Humidity Calculator

What is Dew Point and Relative Humidity?

Dew Point is the temperature at which air becomes saturated with moisture and water vapor begins to condense into liquid (dew). Relative Humidity (RH) is the ratio of current moisture content to the maximum moisture the air can hold at that temperature, expressed as a percentage. Understanding the relationship between temperature, RH, and dew point is crucial for HVAC design, mold prevention, and indoor comfort. High dew points (above 65°F/18°C) feel muggy, while low dew points (below 50°F/10°C) feel dry.

Psychrometric Properties

• Dew Point: Temperature at which condensation begins (100% RH)
• Relative Humidity: Current moisture as % of saturation moisture
• Absolute Humidity: Actual mass of water vapor per unit volume of air (g/m³)
• Wet Bulb Temperature: Lowest temperature achievable by evaporative cooling
• Vapor Pressure: Partial pressure of water vapor in air mixture (kPa)
• Saturation Pressure: Vapor pressure at 100% RH for given temperature

How to Use This Calculator

1. Select your preferred temperature unit: Celsius, Fahrenheit, or Kelvin
2. Choose calculation mode: Calculate DP from T/RH or calculate RH from T/DP
3. Enter dry bulb temperature (ambient air temperature)
4. For DP calculation: Enter relative humidity percentage (0-100%)
5. For RH calculation: Enter dew point temperature
6. Click Calculate to see all psychrometric properties
7. Results include dew point, RH, wet bulb, absolute humidity, and vapor pressure

Psychrometric Calculation Formulas

1. Magnus-Tetens Formula (Saturation Vapor Pressure)

e_s(T) = 6.112 × exp[(17.67 × T) / (T + 243.5)] kPa (for T in °C)

2. Actual Vapor Pressure

e = (RH / 100) × e_s(T)

3. Dew Point from RH

T_d = [243.5 × ln(e/6.112)] / [17.67 - ln(e/6.112)]

4. Relative Humidity from Dew Point

RH = 100 × [e_s(T_d) / e_s(T)]

5. Absolute Humidity

AH = (2165 × e) / (T + 273.15) g/m³

HVAC Applications

• Indoor Comfort: Maintain 30-60% RH and DP below 60°F (15°C) for comfort
• Mold Prevention: Keep surface temperatures above dew point to prevent condensation
• HVAC Design: Size dehumidification equipment based on latent load and target DP
• Condensation Control: Ensure cold surfaces (windows, pipes) stay above DP
• Process Control: Monitor DP for drying, painting, coating applications
• Energy Efficiency: Lower DP reduces cooling energy and improves comfort

Comfort & Health Guidelines

• Below 30% RH: Too dry - dry skin, static electricity, respiratory irritation
• 30-60% RH: Ideal range - comfortable, healthy, prevents mold
• Above 60% RH: Too humid - feels muggy, mold risk, dust mites thrive
• DP below 50°F (10°C): Very dry, uncomfortable for extended periods
• DP 50-60°F (10-15°C): Comfortable, ideal for most people
• DP above 65°F (18°C): Muggy, uncomfortable, high latent cooling load

Practical Tips

• Monitor dew point, not just RH - DP is absolute measure of moisture
• RH changes with temperature, but DP remains constant (for constant moisture)
• Condensation forms on surfaces below dew point temperature
• High dew points indicate high moisture content regardless of temperature
• Dehumidification is most efficient when cooling air below its dew point
• Use psychrometric charts to visualize T/RH/DP relationships
• Indoor DP should typically be 10-15°F below indoor temperature
• Window condensation indicates indoor DP exceeds window surface temperature

Common HVAC Scenarios

• Summer cooling: 75°F, 50% RH → DP = 55°F (good comfort)
• Winter heating: 70°F, 30% RH → DP = 38°F (acceptable but dry)
• Muggy conditions: 80°F, 70% RH → DP = 69°F (very uncomfortable)
• Dry winter: 68°F, 20% RH → DP = 24°F (too dry, add humidity)
• Condensation risk: Indoor 70°F DP 50°F, window surface 45°F → condensation occurs