Physics of Buildings

Psychrometric calculator

Dry bulb and one more reading in; dew point, wet bulb, RH, humidity ratio, enthalpy and volume out.

Default: 25 °C at 50 % RH at sea level. Change any box or the second reading; impossible combinations (a wet bulb above the dry bulb, RH over 100 %) are explained instead of computed. The working is below the results.

How it is calculated

The calculator follows the moist-air equations of the ASHRAE Handbook—Fundamentals (2017, chapter 1) as implemented and documented in the open-source PsychroLib [1][2]. It treats air as an ideal mixture of dry air and water vapour at a total pressure p, set from the altitude by the U.S. Standard Atmosphere [3] or entered directly.

The saturation vapour pressure pws comes from the Hyland–Wexler formulas, over ice at or below 0.01 °C and over liquid water above, valid from −100 to 200 °C. Whichever second reading you give fixes the vapour pressure pw: relative humidity directly (pw = φ·pws), the dew point as the temperature where pws equals pw, the wet bulb through the adiabatic-saturation balance. Then:

W = 0.621945 · pw / (p − pw)(1)
h = 1.006·t + W·(2501 + 1.86·t) kJ/kg dry air(2)
v = 287.042·(t + 273.15)·(1 + 1.607858·W) / p(3)

with t in °C and p in Pa. The wet bulb t* is the temperature that satisfies ASHRAE's adiabatic-saturation equation for the air's humidity ratio (equation 33 above freezing, 35 below), found by bisection between the dew point and the dry bulb; the dew point is found the same way from pws. IP enthalpy is computed on its own scale, 0.240·t + W·(1061 + 0.444·t) Btu/lb with t in °F, which is zero for dry air at 0 °F rather than 0 °C, so SI and IP enthalpies are not simple multiples of each other [2]. Units convert by the NIST factors [4].

Worked example

Air at 25 °C and 50 % RH at sea level (101.325 kPa). Over water at 298.15 K, pws = 3169 Pa (NIST's IAPWS-95 table gives 3169.9 Pa at 25 °C [5]). So pw = 0.5 × 3169 = 1585 Pa and W = 0.621945 × 1585 / (101 325 − 1585) = 0.00988 kg/kg, 9.88 g/kg. Enthalpy: 1.006 × 25 + 0.00988 × (2501 + 46.5) = 50.3 kJ/kg. Specific volume: 287.042 × 298.15 × 1.0159 / 101 325 = 0.858 m³/kg. The dew point, where pws = 1585 Pa, is 13.9 °C, and the wet bulb 17.9 °C.

A field check: the U.S. EPA's moisture guide reads 85 °F at 60 % RH off a chart as a 70 °F dew point [6]; enter it here with IP units and the dew point is 69.6 °F. Nebraska Extension's simplified chart gives 67 °F wet bulb and 59 °F dew point for 80 °F at 50 % [7]; the calculator gives 66.7 °F and 59.7 °F.

Assumptions and valid range

  • Accepted inputs: dry bulb −40 to 60 °C (−40 to 140 °F); barometric pressure 50 to 110 kPa (7.25 to 15.95 psi); altitude −500 to 5000 m (−1640 to 16 400 ft). Outside those, the calculator says why instead of computing.
  • Impossible pairs are refused in words: relative humidity above 100 %, a dew point or wet bulb above the dry bulb, a humidity ratio above saturation (all supersaturated air, which in a building condenses until it is at 100 %), and a wet bulb lower than even perfectly dry air would give.
  • Ideal-gas mixture; the enhancement factor, a correction of about 0.4 to 0.5 % to the saturation pressure of water in air, is taken as 1. Below 0.01 °C saturation is over ice, so the dew point there is a frost point and relative humidity is relative to ice.
  • The wet bulb is the thermodynamic wet bulb. A sling or aspirated psychrometer reads close to it; an unventilated wet bulb reads higher [8].
  • Pressure from altitude is the standard atmosphere's, not today's barometer. For the most precise work enter the measured station pressure (not the sea-level-corrected value from a weather report).
  • Results are rounded for display; the copied result and shared links keep your exact inputs. This is a calculation of air properties, not an equipment sizing or code determination.

Sources

  1. Meyer, D. & Thevenard, D. (2019). PsychroLib: a library of psychrometric functions to calculate thermodynamic properties of air. Journal of Open Source Software 4(33), 1137. doi:10.21105/joss.01137. peer-reviewed software paper Open-source implementation (C, Fortran, JavaScript, Python, VBA) of the moist-air equations of the ASHRAE Handbook—Fundamentals (2017), chapter 1.
  2. Meyer, D. & Thevenard, D. (2019). PsychroLib source code, JavaScript implementation (src/js/psychrolib.js). GitHub, psychrometrics/psychrolib (MIT licence). source code Comments give the ASHRAE (2017) ch. 1 equation numbers: saturation pressure eqs 5 and 6 (Hyland–Wexler; ice at or below the 0.01 °C triple point; valid −100 to 200 °C); humidity ratio eq 20 (0.621945·pw/(p − pw)); specific volume eq 26 (R_da = 287.042 J/kg·K, factor 1.607858); enthalpy eq 30 (1.006·t + W·(2501 + 1.86·t) kJ/kg; IP 0.240·t + W·(1061 + 0.444·t) Btu/lb); wet bulb eqs 33 and 35; standard atmosphere eq 3 (101 325·(1 − 2.25577×10⁻⁵·Z)^5.2559 Pa).
  3. NOAA, NASA and U.S. Air Force (1976). U.S. Standard Atmosphere, 1976 (NOAA-S/T-76-1562, NASA-TM-X-74335). U.S. Government Printing Office, Washington, DC. government standard Sea-level pressure 101 325 Pa, temperature 288.15 K, tropospheric lapse rate 6.5 K/km; the pressure–altitude relation behind ASHRAE eq 3.
  4. Thompson, A. & Taylor, B. N. (2008). Guide for the Use of the International System of Units (SI), NIST SP 811 (2008 edition), Appendix B. National Institute of Standards and Technology. standard 1 Btu_IT/lb = 2326 J/kg; 1 grain = 6.479891×10⁻⁵ kg (7000 grains per pound); 1 psi = 6894.757 Pa; standard atmosphere = 101 325 Pa; conventional inch of mercury = 3386.389 Pa.
  5. Lemmon, E. W., McLinden, M. O. & Friend, D. G. (NIST Thermophysical Properties of Fluid Systems) (2025). Saturation Properties for Water: Temperature Increments, NIST Chemistry WebBook, SRD 69 (IAPWS-95 formulation of Wagner & Pruss 2002). National Institute of Standards and Technology. reference data Saturation pressure of water: 0.611654 kPa at 0.01 °C, 1.22902 at 10.01, 2.34077 at 20.01, 4.24941 at 30.01, 7.38888 at 40.01, 12.3581 at 50.01, 101.454 kPa at 100.01 °C.
  6. U.S. Environmental Protection Agency, Indoor Environments Division (2013). Moisture Control Guidance for Building Design, Construction and Maintenance, EPA 402-F-13053. U.S. EPA, Washington, DC. government guidance pp. 14–17: dew point vs relative humidity; 85 °F at 60 % RH has a dew point of 70 °F (read along the chart to the saturation curve); at 90 % RH a surface 3 °F colder than the air condenses, at 50 % about 20 °F; a 55 °F duct sweats in air with a 70 °F dew point; ventilation dries a building only when the outdoor dew point is below the indoor one; windows are usually the first cold-weather condensation sites; thermal bridges and air leaks chill indoor surfaces.
  7. Shelton, D. P. (2008). Air Properties: Temperature and Relative Humidity, NebGuide G1849. University of Nebraska–Lincoln Extension. university extension Dry-bulb lines vertical, wet-bulb lines sloping, dew-point lines horizontal, RH curves sweep from lower left to upper right; any two properties fix the rest. Example 1: 80 °F and 50 % RH → wet bulb 67 °F, dew point 59 °F (read from a simplified chart).
  8. World Meteorological Organization, Commission for Instruments and Methods of Observation (2018). Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I, Chapter 4: Measurement of humidity (2018 edition). World Meteorological Organization, Geneva. standards body guide Annex 4.B eq 4.B.5: enhancement factor of moist air f(p) = 1.0016 + 3.15×10⁻⁶·p − 0.074/p (p in hPa), 1.0047 at 1013 hPa; its pressure dependence 0 to +0.6 %. Section 4.3: psychrometers need ventilation of 3 to 5 m/s past the wet bulb; insufficient ventilation can lead to overestimation of humidity (the wet bulb reads too high).