Psychrometric Calculator — Dew Point, Wet Bulb, Humidity Ratio & Enthalpy
Every point on a psychrometric chart can be calculated. Enter the dry-bulb temperature and any one moisture property — relative humidity, dew point or wet bulb — and get dew point, humidity ratio, enthalpy, wet bulb, specific volume and air density. Add a leaving-air state and an airflow to get the coil's total, sensible and latent load and condensate.
No signup · Works on mobile · Formulas shown below · Updated 2026-11-02
Moist Air Properties
Saturation pressure from the ASHRAE Hyland-Wexler equations (over ice below 32 °F / 0 °C); barometric pressure corrected for altitude.
Results
Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.
How a psychrometric calculator works
Moist air is a mixture of dry air and water vapor. Once you know the barometric pressure and two independent properties — for example dry bulb and relative humidity — every other property follows from a few equations. A psychrometric chart is simply these equations plotted for sea-level pressure:
Saturation pressure pws(t) — ASHRAE Hyland-Wexler (over water above 32 °F, over ice below) Vapor pressure pw = φ × pws(t) // φ = relative humidity Humidity ratio W = 0.621945 × pw / (p − pw) // lb water per lb dry air; × 7,000 = grains/lb Enthalpy (IP) h = 0.240·t + W·(1061 + 0.444·t) // BTU/lb dry air, t in °F Enthalpy (SI) h = 1.006·t + W·(2501 + 1.86·t) // kJ/kg dry air, t in °C Dew point the temperature where pws(td) = pw // Barometric pressure p = 101.325 × (1 − 2.25577×10⁻⁵ × altitude in m)^5.2559 kPa
Enthalpy and humidity ratio are both per pound (or kg) of dry air, so for loads convert CFM to dry-air mass flow with the specific volume rather than assuming 0.075 lb/ft³. The familiar shortcuts — sensible BTU/h ≈ 1.08 × CFM × ΔT and total BTU/h ≈ 4.5 × CFM × Δh — use standard sea-level air and drift at altitude or high temperature.
Psychrometric properties at common conditions (sea level)
Typical condition
Dry bulb °F
RH
Dew / frost point °F
Humidity ratio gr/lb
Enthalpy BTU/lb
Wet bulb °F
Specific volume ft³/lb
Winter outdoor, cold climate
14
80 %
9.5
9.0
4.7
12.8
11.97
Winter outdoor
32
80 %
27.2
21.1
10.9
29.9
12.46
Cold, damp day / cold-room anteroom
41
90 %
38.3
34.0
15.1
39.7
12.72
Winter indoor (heated)
70
30 %
37.2
32.5
21.9
53.0
13.45
Office / data hall
75
50 %
55.1
64.6
28.1
62.6
13.68
Summer indoor, humid
78
60 %
63.0
86.1
32.2
67.9
13.82
Dry summer outdoor
95
25 %
53.7
61.3
32.5
68.4
14.18
Humid summer outdoor
95
60 %
78.9
150.1
46.5
82.7
14.47
Very humid outdoor
90
75 %
81.0
161.0
46.9
83.1
14.37
Plant room / hot process area
104
40 %
74.9
130.7
45.6
82.1
14.64
Values are calculated with the same equations as the calculator above and can be used to check hand calculations or a printed chart.
Worked example: outdoor-air coil load and condensate
Given: a dedicated outdoor-air unit moves 4,000 CFM of outdoor air at 95 °F / 60 % RH; the cooling coil leaves the air at 55 °F / 95 % RH; sea level.
Entering air: humidity ratio 150.1 gr/lb, enthalpy 46.45 BTU/lb, specific volume 14.47 ft³/lb → dry air = 4,000 × 60 ÷ 14.47 ≈ 16,590 lb/h
Leaving air: humidity ratio 61.1 gr/lb, enthalpy 22.67 BTU/lb
Sensible ≈ 162,000 BTU/h, latent ≈ 233,000 BTU/h — a sensible heat ratio of 0.41, so this outdoor-air load is mostly dehumidification
Condensate = 16,590 × (150.1 − 61.1) ÷ 7,000 ≈ 211 lb/h (about 25 US gal/h) — size the drain pan and trap for it.
The shortcut 4.5 × CFM × Δh gives 428,000 BTU/h, about 8 % high, because hot humid air is lighter than standard air. Load the preset “Outdoor air 95 °F / 60 % → coil leaving 55 °F / 95 %” to see every value.
Why psychrometrics matters for fan selection
Density: hot, humid or high-altitude air is lighter, so a fan develops less static pressure and draws less power at the same speed. Correct catalog data with the air density & altitude calculator.
Condensation: in cold rooms, cold-chain equipment and air handlers, fan housings and motors often sit below the dew point. Condensation degrades insulation and corrodes bearings — check the IP rating, insulation class and whether defrost is needed.
How do you calculate dew point from temperature and relative humidity?
Find the vapor pressure pw = RH × saturation pressure at the dry-bulb temperature, then find the temperature at which saturation pressure equals pw. At 75 °F and 50 % RH, pw ≈ 0.43 in. Hg and the dew point is about 55 °F.
What is the difference between humidity ratio and relative humidity?
Humidity ratio (grains or pounds of water per pound of dry air) is the actual amount of moisture and stays constant unless you add or remove water. Relative humidity is vapor pressure as a percentage of saturation pressure at the current temperature, so it falls when the same air is heated and rises when it is cooled.
Does altitude change psychrometric values?
Yes. Lower barometric pressure means the same vapor pressure gives a higher humidity ratio and a larger specific volume. At 5,280 ft (Denver) barometric pressure is about 12.1 psia, roughly 18 % below sea level; a sea-level chart underestimates humidity ratio and the airflow needed for a given load.
How do I get relative humidity from wet-bulb and dry-bulb readings?
Select “Dry bulb + wet bulb” and enter both readings from a sling or aspirated psychrometer. The calculator solves the ASHRAE wet-bulb energy balance for the humidity ratio, then returns relative humidity, dew point and enthalpy.
Why are my ducts or fan housing dripping water?
Their surface is colder than the dew point of the surrounding air. Enter the measured surface temperature in this calculator to confirm. The fixes are insulation (so the jacket stays above the dew point), lower room humidity or a warmer surface.
Link to or Cite This Tool
This calculator is free to reference in articles, forums, specifications and course material. Please credit it with a link:
<a href="https://www.longwellfans.com/resources/psychrometric-calculator/">Psychrometric Calculator (Dew Point, Wet Bulb, Enthalpy)</a> by LONGWELL Fans
