Psychrometric Calculator
Get the full set of moist air properties from any two readings: dry bulb, wet bulb, relative humidity or dew point. Results use the ASHRAE Handbook equations, account for altitude, and plot your air on a psychrometric chart.
Last updated:
°F or °C, following the units choice
feet (US) or meters (SI) above sea level
psia (US) or kPa (SI), station pressure, not sea-level corrected
How to use the Psychrometric Calculator
- Choose which two values you know, for example dry bulb and wet bulb from a sling psychrometer.
- Pick US or SI units and enter the readings.
- Set the altitude (or a barometric pressure) because air properties depend on pressure.
- Read the results: RH, wet bulb, dew point, humidity ratio, enthalpy, specific volume and more.
- Check the chart and table to see where the air sits and how it changes with humidity.
Psychrometric formulas used
- Pressure at altitude: p = 101.325 × (1 − 2.25577×10⁻⁵ × Z)5.2559 kPa, with Z in meters.
- Humidity ratio: W = 0.621945 × pw ÷ (p − pw), with pw the partial pressure of water vapor.
- Relative humidity: RH = pw ÷ pws(t), where pws is the saturation pressure at the dry bulb (Hyland–Wexler, over ice below 0 °C).
- Enthalpy: h = 1.006·t + W·(2501 + 1.86·t) kJ/kg (SI) or h = 0.240·t + W·(1061 + 0.444·t) Btu/lb (US).
- Specific volume: v = 0.287042 × (t + 273.15) × (1 + 1.607858·W) ÷ p m³/kg dry air.
- Wet bulb: W = [(2501 − 2.326·t*)·Ws* − 1.006·(t − t*)] ÷ (2501 + 1.86·t − 4.186·t*), solved for the wet-bulb temperature t*.
Common conditions at sea level
| Condition | Dry bulb | RH | Wet bulb | Dew point | W (gr/lb) | h (Btu/lb) |
|---|
Values computed with the same equations as the calculator.
Using psychrometrics in HVAC
- Coil load: total cooling = 4.5 × CFM × (hentering − hleaving) Btu/h at standard air density (the 4.5 factor is 60 min/h × 0.075 lb/ft³).
- Latent vs sensible: moisture removal shows up as a drop in humidity ratio; latent load ≈ 0.68 × CFM × Δgrains Btu/h.
- Condensation risk: any surface colder than the dew point will sweat, such as ducts, cold-water pipes and windows.
- Measuring: a sling or digital psychrometer gives dry and wet bulb; move it in the airstream until the wet-bulb reading stops falling.
Frequently asked questions
What is a psychrometric calculator?
It computes the thermodynamic properties of moist air, the same values you would read from a psychrometric chart: relative humidity, wet bulb, dew point, humidity ratio, enthalpy and specific volume. HVAC designers use them to size coils, humidifiers and dehumidifiers and to check comfort conditions.
How is the enthalpy of moist air calculated?
In SI units h = 1.006·t + W·(2501 + 1.86·t) kJ per kg of dry air, with t in °C and W the humidity ratio in kg/kg. In US units h = 0.240·t + W·(1061 + 0.444·t) Btu per lb of dry air with t in °F. For 75 °F and 50% RH at sea level, h is about 28.1 Btu/lb.
What is humidity ratio?
Humidity ratio (also called moisture content or mixing ratio) is the mass of water vapor per unit mass of dry air. It is often given in grains per pound (7,000 grains = 1 lb) or grams per kilogram. W = 0.621945 × pw ÷ (p − pw), where pw is the vapor pressure and p is barometric pressure.
Why does altitude change the results?
Lower barometric pressure at altitude means the same vapor pressure gives a higher humidity ratio, and air is less dense, so specific volume rises. At 5,000 ft the pressure is about 12.2 psia instead of 14.7, so ignoring altitude can make humidity ratio and enthalpy errors of several percent.
How do I get relative humidity from wet bulb and dry bulb?
Choose Dry bulb + wet bulb. The calculator uses the ASHRAE wet-bulb equation to find the humidity ratio, then the vapor pressure and finally RH = pw ÷ pws. For example 95 °F dry bulb with 75 °F wet bulb at sea level is about 40% RH.
What is the difference between dew point and wet bulb?
Dew point is the temperature where moisture starts to condense when air is cooled without changing its moisture content. Wet bulb is the temperature a wetted thermometer reaches through evaporative cooling. Wet bulb always lies between dew point and dry bulb, and all three are equal at 100% RH.
What equations does this calculator use?
It follows the ASHRAE Handbook—Fundamentals psychrometrics chapter: Hyland–Wexler saturation vapor pressure over water and ice, the ideal-gas humidity ratio, enthalpy and specific volume equations, and the psychrometric wet-bulb equation. Pressure at altitude uses the standard atmosphere.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.