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HVAC

Psychrometric Calculator

Enter a dry-bulb temperature and one more property. Get wet-bulb, dew point, humidity ratio, grains, enthalpy, and density for the air state — with the math shown.

°F

The plain air temperature.

Second property

Give any one more property; the rest are derived.

%
Advanced assumptions
ft

Sets barometric pressure. 0 = sea level.

Air state
50% RH
62.6°F wet bulb
Dew point
55.1°F
Grains
64.6gr/lb
Enthalpy
28.11BTU/lb
Humidity ratio
0.00924lb/lb
Density0.0738lb/ft³
Specific volume13.679ft³/lb
Vapor pressure Pv0.215psia
Saturation pressure Pws0.4301psia
Barometric pressure14.696psia
ƒShow the math
  1. 1.Barometric pressure at altitudeper ASHRAE Fundamentals 2017, Ch. 1
    14.696 × (1 − 6.8754e-6 × alt)^5.2559
    14.696 × (1 − 6.8754e-6 × 0)^5.2559
    = 14.696 psia
  2. 2.Humidity ratio from RHper ASHRAE Fundamentals 2017, Ch. 1
    W = 0.621945 × (RH × Pws) / (P − RH × Pws)
    Pv = 0.5 × 0.4301 = 0.215 psia
    = 0.00924 lb/lb
  3. 3.Wet-bulb temperatureper ASHRAE Fundamentals 2017, Ch. 1

    Solved from W by bisection on the wet-bulb relation (Twb between dew point and dry-bulb).

    = 62.6 °F
  4. 4.Dew pointper ASHRAE Fundamentals 2017, Ch. 1

    Temperature whose saturation pressure equals the actual vapor pressure Pv.

    Pv = 0.215 psia
    = 55.1 °F
  5. 5.Enthalpyper ASHRAE Fundamentals 2017, Ch. 1
    h = 0.240·Tdb + W·(1061 + 0.444·Tdb)
    0.240 × 75 + 0.00924 × (1061 + 0.444 × 75)
    = 28.11 BTU/lb
  6. 6.Densityper ASHRAE Fundamentals 2017, Ch. 1
    ρ = P·144 / (53.35·(Tdb+459.67)) × (1+W)/(1+1.6078·W)
    = 0.0738 lb/ft³

How it computes the air state

Moist air is fixed by three things: dry-bulb temperature, one moisture property, and total (barometric) pressure. From those, every other property follows from the standard ASHRAE relations.

  • Saturation pressure at any temperature comes from the Hyland–Wexler correlation (over water above 32 °F, over ice below).
  • Humidity ratio W is 0.621945 × Pv / (P − Pv). RH sets Pv directly; wet-bulb and dew point set it through their own relations.
  • Wet-bulb is solved by iteration — there is no algebraic formula — and dew point by inverting the saturation curve.
  • Enthalpy = 0.240·Tdb + W·(1061 + 0.444·Tdb), and density from the ideal-gas relation with the moisture correction.

Results track a published psychrometric chart to within about 1% across the comfort range. Feed the grains and enthalpy differences into the CFM · BTU · ΔT calculator for latent and total capacity.

Frequently asked

How do you calculate wet-bulb from dry-bulb and relative humidity?
There is no closed-form equation. You first get the humidity ratio from the dry-bulb and RH, then solve the ASHRAE wet-bulb relation for the temperature whose saturated air, cooled adiabatically, reaches that humidity ratio. This tool does that solve by iteration, so 75 °F at 50% RH returns about 62.5 °F wet-bulb.
What are grains of moisture?
Grains are a unit of the humidity ratio: 7,000 grains equal one pound of water, so grains per pound of dry air = humidity ratio × 7,000. Techs use grains for the latent side of a load — 75 °F / 50% RH air holds about 65 grains per pound. The difference in grains across a coil times 0.68 × CFM gives latent BTU/h.
What is enthalpy of air and why does it matter?
Enthalpy is the total heat content per pound of dry air, sensible plus latent, in BTU/lb. It is what the total-heat equation Q = 4.5 × CFM × Δh uses. Measuring enthalpy in and out of a coil is the most complete way to read total capacity, because it captures both temperature drop and moisture removal.
Why does altitude change the numbers?
Every property that depends on total pressure — humidity ratio, wet-bulb, dew point, density — shifts with barometric pressure, and pressure falls with altitude. At 5,000 ft the air is about 17% thinner, so the same dry-bulb and RH give a different wet-bulb and a lower density. Set the altitude field to your site elevation.
Is this the same as an ASHRAE psychrometric chart?
It plots the same point. The chart is a graphical solution of the moist-air equations; this calculator solves those same equations (Hyland–Wexler saturation pressure and the standard humidity-ratio and wet-bulb relations) numerically, which is more precise than reading gridlines by eye.

Related tools

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Return wet-bulb also drives the target superheat.

Sources & references

Estimates for planning purposes only. Verify all results against the code edition adopted in your jurisdiction and with your Authority Having Jurisdiction (AHJ). This tool is not a substitute for a licensed HVAC contractor. See our methodology, sources, and code editions.