Psychrometric Calculator
Plan your hvac project with our free psychrometric calculator. Get precise measurements, material lists, and budgets.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
W = 0.62198 * Pw / (P - Pw) where Pw = (RH/100) * Pws
Where W is humidity ratio (kg/kg), Pw is partial vapor pressure, Pws is saturation vapor pressure from the Antoine equation, P is atmospheric pressure, and RH is relative humidity percentage.
Worked Examples
Example 1: Office HVAC Design Conditions
Problem:Find all psychrometric properties for indoor design conditions: dry bulb 24 C, 50% RH, at standard atmospheric pressure 101.325 kPa.
Solution:Saturation pressure: Pws = 0.61078 * exp(17.27*24/(24+237.3)) = 2.984 kPa\nVapor pressure: Pw = 0.50 * 2.984 = 1.492 kPa\nHumidity ratio: W = 0.62198 * 1.492 / (101.325 - 1.492) = 0.00930 kg/kg = 9.30 g/kg\nDew point: Td = 237.3 * ln(1.492/0.61078) / (17.27 - ln(1.492/0.61078)) = 12.9 C\nEnthalpy: h = 1.006*24 + 0.00930*(2501 + 1.86*24) = 47.8 kJ/kg
Result:W = 9.30 g/kg | Dew Point = 12.9 C | Enthalpy = 47.8 kJ/kg
Example 2: Hot Humid Summer Conditions
Problem:Determine psychrometric properties for outdoor summer air at 35 C, 80% RH, 101.325 kPa.
Solution:Saturation pressure: Pws = 0.61078 * exp(17.27*35/(35+237.3)) = 5.624 kPa\nVapor pressure: Pw = 0.80 * 5.624 = 4.499 kPa\nHumidity ratio: W = 0.62198 * 4.499 / (101.325 - 4.499) = 0.02889 kg/kg = 28.89 g/kg\nDew point: Td = 237.3 * ln(4.499/0.61078) / (17.27 - ln(4.499/0.61078)) = 31.0 C\nEnthalpy: h = 1.006*35 + 0.02889*(2501 + 1.86*35) = 109.4 kJ/kg
Result:W = 28.89 g/kg | Dew Point = 31.0 C | Enthalpy = 109.4 kJ/kg
Frequently Asked Questions
What is enthalpy in psychrometric calculations and how is it used?
Enthalpy in psychrometric calculations represents the total heat content of moist air per kilogram of dry air, measured in kilojoules per kilogram. It includes both the sensible heat (related to temperature) and the latent heat (related to moisture content). The formula combines the dry air component (approximately 1.006 times the dry bulb temperature) with the water vapor component (humidity ratio multiplied by the sum of the latent heat of vaporization at zero degrees and the specific heat of steam times the temperature). HVAC engineers use enthalpy to calculate cooling and heating loads, design air handling units, and determine the energy required for various air conditioning processes such as heating, cooling, humidification, and dehumidification.
How does atmospheric pressure affect psychrometric calculations?
Atmospheric pressure significantly affects all psychrometric properties because it changes the density of air and the behavior of water vapor within it. At higher altitudes where pressure is lower, the saturation vapor pressure remains the same (it depends only on temperature), but the humidity ratio for a given relative humidity increases because there is less dry air per unit volume. This means air conditioning equipment at high altitudes must handle more moisture per unit mass of dry air. Standard psychrometric charts are drawn for sea-level pressure of 101.325 kPa, but engineers working at different altitudes must use corrected charts or calculations. A 1000-meter increase in elevation reduces atmospheric pressure by roughly 12%, which noticeably changes enthalpy and humidity ratio values.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy