Calculator · Psychrometrics
Psychrometric calculator - full air state from two readings.
Enter dry-bulb and one more reading: wet-bulb from a sling psychrometer, RH from a meter, or dew point. You get wet-bulb, dew point, RH, humidity ratio, enthalpy, specific volume and density. Use it for return and supply checks at startup, coil leaving-air readings and commissioning notes.
Example result for the default inputs
75 °F dry-bulb · 62 °F wet-bulb · sea level (14.696 psia)
Dry-bulb
75.0°F
Wet-bulb
62.0°F
Dew point
54.0°F
Relative humidity
48.0%
| Humidity ratio | 0.00884 lb water / lb dry air |
|---|---|
| Enthalpy | 27.68 BTU/lb dry air |
| Specific volume | 13.668 ft³/lb dry air |
| Density | 0.0738 lb/ft³ |
For estimation purposes only. Consult a licensed engineer for final designs.
Free without signup, 3 questions a day
How the air state is calculated.
The calculator runs the same deterministic code the BuildSolver chat uses for a psychrometric state. It takes dry-bulb plus one of wet-bulb, relative humidity or dew point, and returns the full state, from wet-bulb and dew point through enthalpy, specific volume and density. Equations are from ASHRAE Handbook - Fundamentals, Chapter 1. Pressure is fixed at sea level, 14.696 psia. Temperatures are in °F; saturation pressure is computed in °C and converted.
- Saturation pressure
p_ws(t) = 6.112 × exp(17.62·t / (243.12 + t)) hPa × 0.0145038 psi/hPaMagnus-Tetens with Sonntag (1990) coefficients over water. The formula takes t in °C, converted from the °F temperature you enter. Within ±0.5°F of Hyland-Wexler in the HVAC range. The dew point calculator uses the same function, so the two pages agree.- From RH or dew point
p_w = RH/100 × p_ws(T_db) or p_w = p_ws(T_dp); RH = 100 × p_w / p_ws(T_db)With an RH input, dew point comes from the inverted Magnus-Tetens equation. With a dew point input, RH is the ratio of the two saturation pressures.- Humidity ratio (Eq. 22)
W = 0.621945 × p_w / (14.696 − p_w)Pounds of water vapor per pound of dry air. 0.621945 is the molecular weight ratio of water to dry air; 14.696 psia is sea-level pressure.- From wet-bulb (Eq. 35)
W = ((1093 − 0.556·T_wb)·W_s,wb − 0.240·(T_db − T_wb)) / (1093 + 0.444·T_db − T_wb)W_s,wb is Eq. 22 at saturation at the wet-bulb temperature. Over-water form only, so wet-bulb must be 32°F or higher. RH then follows from p_w = W × 14.696 / (0.621945 + W), and dew point from Magnus-Tetens.- Wet-bulb from RH or dew point
solve Eq. 35 for T_wb by bisection on [T_dp, T_db]No closed form exists. The solver halves the bracket until it is narrower than 0.001°F.- Enthalpy (Eq. 32)
h = 0.240·T_db + W·(1061 + 0.444·T_db) BTU/lb dry airSensible heat of the dry air plus the heat carried by the water vapor. Reference is 0°F dry air.- Specific volume (Eq. 26)
v = 0.3704·(T_db + 459.67)·(1 + 1.6078·W) / 14.696 ft³/lb dry airCubic feet of moist air per pound of dry air at sea-level pressure.- Density
ρ = (1 + W) / v lb/ft³Mass of the dry air and the water vapor together, per cubic foot.
A worked example with a sling psychrometer at a return grille.
A tech spins a sling psychrometer at the return grille during a summer startup. Dry-bulb reads 75°F, wet-bulb reads 62°F. Because the input is wet-bulb, humidity ratio comes straight from Eq. 35; RH and dew point follow from it, then enthalpy and volume.
75 °F dry-bulb · 62 °F wet-bulb · sea level (14.696 psia)
- Dry-bulb
- 75.0°F
- Wet-bulb
- 62.0°F
- Dew point
- 54.0°F
- Relative humidity
- 48.0%
| Humidity ratio | 0.00884 lb water / lb dry air |
|---|---|
| Enthalpy | 27.68 BTU/lb dry air |
| Specific volume | 13.668 ft³/lb dry air |
| Density | 0.0738 lb/ft³ |
For estimation only · ASHRAE Fundamentals Ch. 1 · computed by the calculator above
Take the same reading in the supply duct and run it as a second point. The drop in humidity ratio between the two is the moisture the coil removed per pound of dry air; the drop in enthalpy is the total heat removed per pound.
Standards and data sources.
- ASHRAE Handbook - Fundamentals (2021), Ch. 1 Psychrometrics - humidity ratio (Eq. 22), specific volume (Eq. 26), moist-air enthalpy (Eq. 32) and the wet-bulb energy balance (Eq. 35).
- Sonntag (1990), Magnus-Tetens coefficients (WMO/CIMO Guide) - saturation vapor pressure of water and the dew point from RH.
- Standard sea-level atmosphere, 14.696 psia - the only pressure the calculator uses.
Limitations of this calculator.
- Sea level only. Pressure is fixed at 14.696 psia and there is no altitude correction. At high-elevation jobs the humidity ratio, enthalpy and volume shown will not match local conditions.
- Wet-bulb below 32°F is rejected, and so is an RH or dew point input that puts the dew point below 32°F. See the FAQ below for dry indoor air.
- A chart or program built on Hyland-Wexler can differ from this page by up to about 0.5°F on dew point and wet-bulb.
- One state point per run. Mixed air, coil process lines and SHR need several points plus airflow; run each point separately or use the chat.
- A dry wick or a probe in stratified air skews every property on this page. Check the wick and the probe location before you record the point.
- Dry-bulb range is −40 to 150°F. The on-page calculators share 10 calculations per day per visitor.
Common mistakes with psychrometric readings.
- Reading the wet-bulb too early. A sling psychrometer needs a clean wick wetted with clean water, and it needs to spin until the wet-bulb stops dropping. A reading taken early or with a dry wick reads high, which overstates humidity.
- Entering the wrong kind of reading. Wet-bulb and dew point are not interchangeable. Wet-bulb always sits between dew point and dry-bulb. If a meter shows both, check which one you typed and match the selector.
- Comparing RH across a coil. RH depends on temperature, so supply air near 55°F can show a high RH even when the coil removed moisture. Compare humidity ratio or dew point between return and supply to see latent removal.
- Measuring in unmixed air. Right at the coil face or next to a strip heater, the air is not uniform. Take supply readings where the air has mixed, and return readings at the return grille or in the return duct, away from leaks.
- Judging capacity from enthalpy alone. An enthalpy drop tells you heat removed per pound of air. Without measured airflow it does not give BTU/h, so a large drop across a coil with low airflow can still mean low capacity.
Common questions about psychrometrics.
+How do I enter a sling psychrometer reading?
Type the dry-bulb temperature, choose Wet-bulb as the second reading and type the wet-bulb temperature. The calculator returns RH, dew point, humidity ratio, enthalpy, specific volume and density for that air.
+My meter shows wet-bulb and dew point. Which one do I enter?
Either one works with dry-bulb. Pick the matching option in the selector. Wet-bulb always reads between dew point and dry-bulb, so the lower of your two readings is the dew point. All three are equal only at 100% RH.
+Why can't I enter a wet-bulb below 32°F?
Below freezing the wick can ice over and the energy balance uses different coefficients over ice. The calculator only implements the over-water form, so it rejects wet-bulb below 32°F and RH or dew point inputs that put the dew point below 32°F. A wet-bulb of 32°F or more is calculated even when the air is dry enough for the dew point to fall below 32°F.
+Does the calculator correct for altitude?
No. It assumes sea-level pressure of 14.696 psia for every result. On high-elevation jobs, treat humidity ratio, enthalpy and specific volume as sea-level values.
+How do I check moisture removal across a cooling coil?
Run the return air reading and the supply air reading as two separate calculations. The drop in humidity ratio or dew point shows moisture removed. The drop in enthalpy covers sensible and latent heat together. Turning that into BTU/h or SHR also needs measured airflow.
+How accurate are the results?
Saturation pressure from Magnus-Tetens is within about 0.5°F of Hyland-Wexler in the HVAC range, and the wet-bulb solver converges to 0.001°F. In the field, check the wick, the water and the probe location first. Those move the result the most.
+What is specific volume used for?
It converts between cubic feet of air and pounds of dry air. Fans move air in CFM, while humidity ratio and enthalpy are per pound of dry air, so specific volume links airflow to heat and moisture.
Run return and supply readings through chat.
In the BuildSolver chat, describe the return and supply readings and the airflow. The chat runs the same psychrometric tool for each point and explains the numbers with the ASHRAE references.
Open the chat