A psychrometric calculator based on the ASHRAE equations
This psychrometric calculator finds every property of moist air from two measurements: the dry-bulb temperature and one of relative humidity, wet-bulb temperature, dew point or humidity ratio. It returns the humidity ratio, dew point, wet bulb, enthalpy, specific volume, density, vapour and saturation pressures and the degree of saturation, in SI and IP units side by side, and plots the state on a small psychrometric chart.
Pressure matters: air at altitude holds more water per kilogram at the same relative humidity, so you can enter the site altitude (standard atmosphere) or a measured barometric pressure. It is built for HVAC engineers, students working through a cooling-coil problem, greenhouse growers and anyone who needs a psychrometric calculator online without a chart and a ruler.
How to use the Psychrometric Calculator
- Pick SI or IP units; values you typed convert when you switch.
- Enter the dry-bulb temperature.
- Choose the second property — relative humidity, wet bulb, dew point or humidity ratio — and type its value.
- Enter the altitude, or switch to barometric pressure if you have a reading.
- Read the humidity ratio and the other properties; the dot on the chart shows the state. Copy the full table for a report.
Psychrometric Calculator features
ASHRAE equations
Hyland–Wexler saturation pressure over water and ice, and the chapter 1 formulas for humidity ratio, enthalpy, volume and wet bulb.
Four ways in
Relative humidity, wet bulb, dew point or humidity ratio, with the other three worked out.
Any pressure
Standard atmosphere from altitude, or barometric pressure in kPa or psia.
SI and IP together
g/kg and grains per pound, kJ/kg and Btu/lb, m³/kg and ft³/lb in one table.
Live chart
Relative humidity curves at your pressure, with the state point gliding to each new value.
Psychrometric equations used (ASHRAE Fundamentals 2017, chapter 1)
- Saturation pressure pws: Hyland–Wexler, ln pws = C₈/T + C₉ + C₁₀T + C₁₁T² + C₁₂T³ + C₁₃ ln T over liquid water (with a separate set of constants over ice below 0.01 °C), T in kelvin.
- Humidity ratio: W = 0.621945 pw ÷ (p − pw), in kg of water per kg of dry air (× 1,000 for g/kg, × 7,000 for grains per pound).
- Wet bulb: W = [(2501 − 2.326 t*) Ws* − 1.006 (t − t*)] ÷ (2501 + 1.86 t − 4.186 t*), solved for t* by bisection; an ice-bulb form applies below freezing.
- Enthalpy: h = 1.006 t + W (2501 + 1.86 t) kJ/kg dry air; in IP, h = 0.240 t + W (1061 + 0.444 t) Btu/lb. The two scales use different zero points, so they do not convert by a simple factor.
- Specific volume: v = 0.287042 (t + 273.15)(1 + 1.607858 W) ÷ p, with p in kPa.
- Standard atmosphere: p = 101.325 (1 − 2.25577 × 10⁻⁵ Z)5.2559 kPa for altitude Z in metres; dew point is the temperature at which pws equals the vapour pressure.
Checked against ASHRAE values
The self-test reproduces the handbook’s tables and its worked Example 1: for air at 40 °C dry bulb and 20 °C wet bulb at sea level the handbook gives a humidity ratio of 0.0065 kg/kg, enthalpy 56.7 kJ/kg, specific volume 0.896 m³/kg, relative humidity 14% and a dew point near 7 °C; this page gives 0.00640 kg/kg, 56.7 kJ/kg, 0.896 m³/kg, 14.0% and 7.4 °C. The small gap in W comes from the enhancement factor built into the handbook’s tables, which the ideal-gas equations leave out. Saturation pressure at 25 °C comes out at 3.1692 kPa (table: 3.1697 kPa), and the standard-atmosphere pressure at 1,000 m at 89.875 kPa. The same equations are used by the open-source PsychroLib library, whose test values are also checked.
| Condition (sea level) | Humidity ratio | Dew point | Wet bulb | Enthalpy |
|---|---|---|---|---|
| 25 °C, 50% RH | 9.88 g/kg | 13.9 °C | 17.9 °C | 50.3 kJ/kg |
| 40 °C dry, 20 °C wet bulb | 6.40 g/kg | 7.4 °C | 20 °C | 56.7 kJ/kg |
| 35 °C, 60% RH | 21.4 g/kg | 26.1 °C | 28.2 °C | 90.2 kJ/kg |
Limits
The equations treat moist air as an ideal-gas mixture, which is accurate to well under 1% at normal pressures; they are valid from −60 to 100 °C here. Wet bulb means the thermodynamic wet-bulb temperature, which an aspirated psychrometer approximates; a sling or unventilated sensor can read slightly high. For quick weather comfort figures such as heat index use the weather calculator; for steam and water above 100 °C, the steam table calculator.
Psychrometric Calculator FAQ
How do I find the humidity ratio from dry bulb and relative humidity?
Multiply the RH by the saturation pressure at the dry bulb to get the vapour pressure, then W = 0.621945 × pw ÷ (p − pw). At 25 °C and 50% at sea level that is 9.88 g/kg.
Is this psychrometric calculator based on ASHRAE?
Yes. It uses the equations of the ASHRAE Handbook — Fundamentals (2017), chapter 1, and is checked against that chapter’s tables and Example 1.
How is wet-bulb temperature calculated?
There is no closed formula; the calculator finds the temperature t* at which the ASHRAE wet-bulb equation gives your humidity ratio, by repeated halving between the dew point and the dry bulb.
Why does altitude change the result?
At lower pressure the same vapour pressure is a larger share of the total, so the humidity ratio rises and the specific volume grows. At 1,500 m, 25 °C and 50% RH holds about 11.9 g/kg instead of 9.9.
What units is enthalpy in?
kJ per kg of dry air (SI) or Btu per pound of dry air (IP). Values can be negative below 0 °C in SI because the zero is dry air at 0 °C.
Can I use it for a cooling coil?
Yes: calculate the entering and leaving states and subtract their enthalpies and humidity ratios to get the sensible and latent load per kg of dry air.