Why does a glass of iced water "sweat," why does a 30 °C day in Chennai feel unbearable while 30 °C in Jaipur feels fine, and how big a cooling coil do you actually need? Every one of these questions is answered by psychrometrics — the science of moist air. Air isn't just "hot" or "cold"; it carries water vapor, and that moisture controls comfort, condensation and energy use. This free psychrometric calculator lets you enter any two properties — dry-bulb, wet-bulb, relative humidity or dew point — and instantly get all the rest, using validated ASHRAE equations.
Table of Contents
The Psychrometric Calculator
Choose your input pair (the two properties you know), enter the values and the atmospheric pressure, and get the full moist-air state: humidity ratio, enthalpy, dew point, wet-bulb, relative humidity, vapor pressure and specific volume.
💧 Psychrometric Calculator (Moist Air Properties)
What Is Psychrometrics?
Psychrometrics is the study of the thermodynamic properties of moist air — the mixture of dry air and water vapor that surrounds us. It relates temperature, moisture content and pressure to the air's energy, and it's the foundation of virtually all air conditioning, heating, ventilation, drying and humidity-control engineering.
Traditionally these properties are read from a psychrometric chart — the dense, curve-filled diagram every HVAC engineer knows. This calculator does the same job numerically, using the ASHRAE psychrometric equations, so you get exact values instead of eyeballing a chart.
The 7 Key Properties
| Property | What it is | Why it matters |
|---|---|---|
| Dry-bulb temp (Tdb) | Ordinary thermometer reading | The air's sensible heat |
| Wet-bulb temp (Twb) | Temperature with evaporative cooling | Reflects heat + moisture together |
| Relative humidity (RH) | Pw / Psat × 100% | How "full" the air is with vapor |
| Dew point (Tdp) | Temp where vapor condenses | Condensation & comfort |
| Humidity ratio (W) | Mass of water per kg dry air | Actual moisture content |
| Enthalpy (h) | Total energy per kg dry air | Coil & load sizing |
| Specific volume (v) | Volume per kg dry air | Airflow & density |
The Core Equations
The whole calculation starts from the saturation vapor pressure (Magnus form), then cascades:
Here Pw is the actual vapor pressure, P the total (atmospheric) pressure, W the humidity ratio and h the enthalpy. Wet-bulb temperature is found by iterating the psychrometric equation — which the calculator does automatically.
Why You Need Two Properties
At a fixed atmospheric pressure, moist air has two independent degrees of freedom. That means any two independent properties pin down the complete state — and everything else follows.
Real-World Applications
- HVAC load & coil sizing — enthalpy difference across a cooling coil gives the total (sensible + latent) load. This pairs directly with our HVAC AC load calculator.
- Airflow & duct design — specific volume converts mass flow to the volumetric airflow you size ducts and diffusers for; see the CFM calculator.
- Condensation control — the dew point tells you which surfaces will "sweat," critical for avoiding mold and building-envelope damage.
- Comfort & ventilation — RH and dew point drive thermal comfort; balancing them is central to natural vs mechanical ventilation.
- Drying, cooling towers & dehumidification — every moisture-transfer process is designed on the psychrometric chart.
- Data centers & cleanrooms — tight humidity windows protect equipment and processes.
Worked Example
Room air at 22 °C dry-bulb and 50% RH at sea level:
- Psat(22 °C) = 0.61078·exp(17.27×22/259.3) = 2.64 kPa
- Pw = 0.50 × 2.64 = 1.32 kPa
- W = 0.622 × 1.32 / (101.325 − 1.32) = 8.2 g/kg
- Dew point (solve Psat = 1.32 kPa) = 11.1 °C
- Wet-bulb (iterated) = 15.4 °C
- Enthalpy = 1.006×22 + 0.0082×(2501 + 1.86×22) = 43 kJ/kg
So any window or pipe surface below 11.1 °C in this room will start to condense — a practical, useful result you get in one click.
Common Mistakes
- Confusing relative humidity with moisture content. RH changes when you simply heat or cool air; the humidity ratio does not. Heating dry air lowers its RH without adding any water.
- Ignoring altitude. Atmospheric pressure drops with elevation, shifting every property. Use the local pressure, not always 101.325 kPa.
- Mixing SI and IP units. kJ/kg vs BTU/lb, °C vs °F — keep one system throughout.
- Assuming wet-bulb = dew point. They're equal only at 100% RH; otherwise wet-bulb sits between dry-bulb and dew point.
- Using dry-bulb alone for comfort. Two rooms at the same temperature can feel completely different depending on humidity.
- Forgetting enthalpy for cooling loads. Sensible temperature change alone misses the latent (moisture) load, which can dominate in humid climates.
- Reading charts imprecisely. Chart-reading introduces error; a calculator gives exact, repeatable values.
Frequently Asked Questions
What is psychrometrics?
The study of the thermodynamic properties of moist air — the mix of dry air and water vapor. It relates temperature, moisture and pressure to the air's energy, and underpins HVAC, drying, cooling-tower and humidity-control design.
What is the difference between dry-bulb and wet-bulb temperature?
Dry-bulb is the ordinary air temperature (sensible heat). Wet-bulb is measured with a wet wick and airflow, so evaporation cools it — the drier the air, the lower the wet-bulb. They're equal only when the air is saturated.
What is the humidity ratio?
The mass of water vapor per unit mass of dry air (g/kg), W = 0.622·Pw/(P−Pw). Unlike RH, it doesn't change when air is merely heated or cooled without adding/removing moisture.
What is the dew point temperature?
The temperature to which air must be cooled (at constant pressure and moisture) for vapor to start condensing. At the dew point RH = 100%. Surfaces cooled below it form condensation.
What is enthalpy in psychrometrics?
The total energy per kg of dry air, combining sensible heat and the latent heat of its vapor: h = 1.006·T + W·(2501 + 1.86·T) kJ/kg. It's essential for sizing cooling and heating coils.
Why do you need two properties to define moist air?
At fixed pressure, moist air has two independent degrees of freedom, so any two independent properties fix the state and determine all others — which is why a psychrometric chart locates a point from any two.
Conclusion
Psychrometrics turns "it's hot and sticky" into precise, actionable engineering numbers. Once you know any two properties of moist air, the ASHRAE equations give you the humidity ratio, enthalpy, dew point and everything else you need to size a coil, prevent condensation, or design for comfort. This calculator replaces squinting at a psychrometric chart with exact values in a single click.
Bookmark it for any HVAC, drying, or humidity problem — enter two properties, read the complete state of the air, and design with confidence.
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