Psychrometric Calculator — Free Online Calculator
Psychrometrics is the thermodynamics of moist air — the mixture of dry air and water vapour. Every coil selection, dehumidifier sizing, and ventilation energy-recovery calculation depends on at least two psychrometric properties. Enter dry-bulb temperature and relative humidity below to instantly calculate all eight key properties of the air sample: vapour pressure, dew point, wet-bulb temperature, humidity ratio, specific enthalpy, specific volume, and air density.
Enter air conditions
Standard sea level = 14.696 psia. Reduce for altitude: 12.2 psia at 5,000 ft, 11.1 psia at 7,500 ft.
Warning
Specific Enthalpy
Humidity Ratio
—
gr/lb
Dew Point
—
Tdp
Wet-Bulb
—
Twb
Vapour Pressure
—
in. Hg
Specific Volume
—
ft³/lb
See the breakdown
Calculated using ASHRAE Fundamentals (2021) correlations. Valid for −40°F to 120°F at pressures 10–16 psia. Psychrometric relationships assume ideal gas behaviour for dry air and water vapour.
ASHRAE psychrometric state reference
Common design conditions from ASHRAE Fundamentals. All values at standard sea-level pressure.
| Condition | Tdb | RH | W (gr/lb) | h (BTU/lb) |
|---|---|---|---|---|
| Winter indoor design | 70°F | 30% | 32 | 25.5 |
| Summer indoor design | 75°F | 50% | 65 | 28.1 |
| Summer outdoor design (hot-humid) | 95°F | 60% | 123 | 46.4 |
| Summer outdoor design (hot-dry) | 105°F | 15% | 41 | 32.0 |
| Cooling coil leaving air | 55°F | 95% | 64 | 23.2 |
| Chilled beam supply air | 60°F | 75% | 61 | 24.2 |
Psychrometric formulas used in this calculator
The psychrometric equations are derived from the ideal gas law applied to the mixture of dry air and water vapour. The saturation pressure is calculated using the ASHRAE Fundamentals Hyland-Wexler equation — a polynomial fit that is accurate to within 0.1% from −40°F to 200°F. All other properties follow directly from saturation pressure, relative humidity, and atmospheric pressure.
Humidity ratio vs. relative humidity
Relative humidity describes saturation percentage and changes with temperature (same moisture content, different RH as temperature changes). Humidity ratio (grains/lb or g/kg) is an absolute measure — it does not change when air is heated or cooled without adding or removing moisture. Engineering calculations use humidity ratio.
Specific enthalpy and coil sizing
The change in specific enthalpy across a coil, multiplied by airflow (in lb/h), gives total coil load. This includes both sensible (temperature change) and latent (moisture change) components. Total load = ṁ × Δh = (CFM × ρ × 60) × (hentering − hleaving).
Dew point and condensation
When a surface temperature falls below the air's dew point, moisture condenses. This determines whether cooling coils, duct walls, or radiant panels will see condensation. Use the dew point value here with measured surface temperatures to assess condensation risk.
Altitude correction
At higher elevations, lower atmospheric pressure allows the same mass of water vapour to occupy more volume, reducing the air's density. Psychrometric calculations at 5,000 ft (patm ≈ 12.2 psia) will show significantly higher specific volume and lower air density — relevant for airflow-based coil sizing.
Worked examples
Three scenarios showing outdoor summer air, cooling coil leaving conditions, and a validation check.
Summer outdoor air — 95°F, 60% RH, sea level
Result: High enthalpy outdoor air — significant latent load on the cooling coil.
Cooling coil leaving air — 55°F, 95% RH, sea level
Result: Near-saturated leaving conditions — the coil has removed 19.2 BTU/lb of total heat (sensible + latent) from the outdoor air stream.
Wet-bulb violation check — physical impossibility
Result: Triggers warning: "Physical limit violation: Computed wet-bulb temperature exceeds dry-bulb. Check your inputs." Wet-bulb temperature cannot exceed dry-bulb temperature under stable atmospheric conditions.
Frequently asked questions
Common questions about psychrometrics, humidity ratio, enthalpy, and moist air properties.
What is psychrometrics?
Psychrometrics is the branch of engineering thermodynamics that studies the physical and thermal properties of moist air — the mixture of dry air and water vapour. HVAC engineers use psychrometric relationships to calculate coil loads, dehumidification capacity, ventilation energy recovery, and indoor comfort conditions.
What is the humidity ratio (W)?
The humidity ratio — also called specific humidity or mixing ratio — is the mass of water vapour per unit mass of dry air. It is expressed in grams per kilogram (g/kg) in SI units or grains per pound (gr/lb) in US units. One pound contains 7,000 grains, so a humidity ratio of 70 gr/lb equals 10 g/kg. Unlike relative humidity, it does not change as air is heated or cooled.
What is specific enthalpy of moist air?
Specific enthalpy (h) is the total heat content of one pound (or kg) of dry air including both sensible heat (temperature) and latent heat (moisture). It equals approximately: h = 0.240 × T_db + (W/7000) × (1061 + 0.444 × T_db) in BTU/lb. The change in enthalpy multiplied by airflow rate gives the total load on a coil — including both sensible and latent cooling.
What is the dew point temperature?
The dew point is the temperature at which a given air sample reaches saturation (100% RH) when cooled at constant pressure without adding or removing moisture. When a surface falls below the dew point, condensation forms. It is a fixed property of the moisture content — lowering dew point requires removing water vapour through dehumidification or cold-surface condensation.
What is wet-bulb temperature and how is it different from dew point?
Wet-bulb temperature is measured by a thermometer whose bulb is covered with a water-saturated wick — evaporation cools the bulb to an equilibrium temperature between dry-bulb and dew point. It reflects both temperature and humidity but is not the same as dew point: wet-bulb temperature equals dry-bulb at saturation (100% RH) and approaches dew point as RH approaches 100%. Wet-bulb is used for cooling tower sizing and evaporative cooler performance.
Why does altitude affect HVAC calculations?
At higher elevations, lower atmospheric pressure allows the same moisture content to produce a higher relative humidity and lower air density. Fan airflow must be de-rated for altitude because CFM is a volumetric measure — the same CFM delivers less mass (and less latent cooling capacity) at altitude. Cooling coil capacity, humidifier output, and dehumidifier capacity all decrease at altitude and must be corrected using the actual atmospheric pressure.
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