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Psychrometric calculator

Enter air temperature and a second property to get humidity ratio, dew point, wet bulb and enthalpy; add the leaving state and airflow to see how much water the coil removes and the heat load.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

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State 1 (entering air)
Process and state 2

Air leaving a cooling coil is usually at 90% to 95% relative humidity.

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Process between states 1 and 2

Loads computed with the dry-air mass flow at the inlet.

Psychrometric chart

Relative humidity curves every 10% at site pressure, dashed constant-enthalpy lines and the process between the states.

Tap the chart to see the values.

Relative humidity when heating or cooling this air

Same humidity ratio as state 1: relative humidity rises on cooling and reaches 100% at the dew point.

Tap the chart to see the values.

The report gathers this page's inputs, results, equations, assumptions and references, with fields for project, engineer and revision. In the print dialog, choose Save as PDF.

Method, assumptions and references

Equations used

Saturation pressure (Hyland-Wexler): ln p_ws = C8/T + C9 + C10·T + C11·T² + C12·T³ + C13·ln T over water (32 to 392 °F) and the C1 to C7 form over ice (−148 to 32 °F); T in K, p_ws in Pa
p = 101.325 · (1 − 2.25577·10⁻⁵ · Z)^5.2559 kPa; W = 0.621945 · p_w ÷ (p − p_w); h = 1.006 · t + W · (2501 + 1.86 · t) kJ/kg (IP: 0.240 · t + W · (1061 + 0.444 · t) Btu/lb); v = 0.287042 · (t + 273.15) · (1 + 1.607858 · W) ÷ p
Wet bulb (iteration): W = [(2501 − 2.326 · t*) · W*s − 1.006 · (t − t*)] ÷ (2501 + 1.86 · t − 4.186 · t*); dew point: p_ws(t_d) = p_w
Process: ṁ_a = Q ÷ v1; water = ṁ_a · (W1 − W2); Q_total = ṁ_a · (h1 − h2); Q_sensible = ṁ_a · (1.006 + 1.86 · W2) · (t1 − t2); Q_latent = Q_total − Q_sensible; SHR = Q_s ÷ Q_t

Assumptions and limits

  • Dry air and vapor as perfect gases (equations of ASHRAE Fundamentals chapter 1). Compared with ASHRAE Table 2 moist air properties, which use real-gas data, saturation humidity ratio differs by less than 0.7% up to 122 °F.
  • Pressure from the standard atmosphere at the elevation. For cooling, state 2 is the air leaving the coil; moisture drops only if leaving air holds less vapor than entering air. Condensate enthalpy is shown separately.
  • In IP units enthalpy follows the ASHRAE IP datum (dry air at 0 °F), so it is about 7.7 Btu/lb above a straight conversion of the SI value.

Standards and technical documents

  • ASHRAE Handbook — Fundamentals (2021), chapter 1: Psychrometrics (equations 3, 5, 6, 20, 22, 28, 32 and 35; Tables 2 and 3)
  • Hyland, R. W.; Wexler, A.: Formulations for the thermodynamic properties of the saturated phases of H2O from 173.15 K to 473.15 K, ASHRAE Transactions 89(2A), 1983
  • ISO 2533:1975: Standard Atmosphere
  • WMO-No. 8: Guide to Instruments and Methods of Observation, chapter 4 (measurement of humidity)

U.S. regulations

  • ANSI/ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy
  • ANSI/ASHRAE Standard 62.1: Ventilation and Acceptable Indoor Air Quality
  • ANSI/ASHRAE/IES Standard 90.1: Energy Standard for Sites and Buildings Except Low-Rise Residential Buildings

Frequently asked questions

How do I calculate humidity ratio?

Find saturation pressure at the dry-bulb temperature, multiply by relative humidity to get vapor pressure and use W = 0.622 · p_w ÷ (p − p_w). At 77 °F, 50% RH and sea level, W is about 69 gr/lb (0.0099 lb/lb), enthalpy 29.3 Btu/lb and dew point 57 °F, matching the ASHRAE table.

How much water does a cooling coil remove?

Dry-air mass flow times the drop in humidity ratio. Cooling 6,000 cfm of 86 °F, 60% RH air to 57 °F at 95% RH drops humidity ratio from about 112 to 66 gr/lb and condenses about 170 lb/h of water, for a total load near 31 tons.

Why does elevation change the result?

At lower pressure the same vapor gives a higher humidity ratio and the air is less dense. At 3,300 ft pressure falls to about 13.0 psia and, at 77 °F and 50% RH, humidity ratio rises from about 69 to 78 gr/lb.

What is the difference between relative and absolute humidity?

Relative humidity compares the vapor present with the most the air can hold at that temperature, and it changes when air is heated or cooled. Absolute humidity is vapor mass per volume of air; humidity ratio, per pound of dry air, is the measure that does not change without condensation.