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EngineeringPsychrometric Calculator
Compute dew point, wet bulb, humidity ratio, enthalpy and specific volume of moist air with altitude correction, plus an ASHRAE 55 comfort check.
The Psychrometric Calculator helps you determine air properties for HVAC design and analysis. Calculate relative humidity, dew point, wet bulb temperature, enthalpy, and more from dry bulb temperature and one humidity parameter.
Have feedback? Report bugs, suggest features, or share your thoughts — we read them allWhat is a Psychrometric Calculator?
A psychrometric calculator is an HVAC engineering tool that determines the thermodynamic properties of moist air. Given dry bulb temperature and one humidity parameter (such as relative humidity, wet bulb temperature, or dew point), it calculates all other psychrometric properties including enthalpy, humidity ratio, specific volume, and vapor pressure. These calculations are essential for HVAC system design, air conditioning load calculations, and indoor air quality management. The calculator implements the same principles as a psychrometric chart but provides precise numerical values.
Psychrometric Properties Explained
- Dry Bulb Temperature (DBT): The actual air temperature measured by a standard thermometer
- Wet Bulb Temperature (WBT): Temperature measured with a wet cloth covering the thermometer bulb, accounting for evaporative cooling
- Dew Point Temperature: Temperature at which air becomes saturated and water vapor begins to condense
- Relative Humidity (RH): Ratio of actual water vapor to maximum possible at given temperature (percentage)
- Humidity Ratio: Mass of water vapor per unit mass of dry air (g/kg or lb/lb)
- Specific Enthalpy: Total heat content of moist air per unit mass (kJ/kg or BTU/lb)
- Specific Volume: Volume of moist air per unit mass of dry air (m³/kg or ft³/lb)
- Vapor Pressure: Partial pressure of water vapor in the air mixture
Key Psychrometric Equations
- Saturation Pressure (kPa, T in Kelvin): Pws = exp(77.345 + 0.0057T - 7235/T) / T^8.2 / 1000
- Humidity Ratio: W = 0.622 × Pw / (P - Pw)
- Enthalpy: h = 1.006T + W(2501 + 1.86T) kJ/kg
- Relative Humidity: RH = Pw / Pws × 100%
- Dew Point: Inverse of saturation pressure equation at RH=100%
Common Applications
- HVAC system design: Sizing cooling and heating equipment
- Cooling load calculations: Determining sensible and latent heat loads
- Dehumidification: Designing moisture removal systems
- Humidification: Calculating water addition requirements
- Air mixing: Determining properties of mixed air streams
- Energy recovery: Analyzing heat and moisture transfer
- Industrial processes: Drying, conditioning, and climate control
- Indoor air quality: Managing comfort and preventing condensation
- Data center cooling: Precision environmental control
Usage Tips
- Standard conditions: 20-25°C dry bulb, 40-60% RH for comfort
- Cooling mode: Process moves air from warm/humid to cool/dry
- Heating mode: Air typically becomes drier (lower RH) when heated
- High altitude: Lower atmospheric pressure affects calculations
- Dew point below 10°C: Low risk of mold and condensation
- Enthalpy difference: Drives cooling/heating energy requirements
- Summer design: Typically 35°C DB, 24°C WB for outdoor air
- Winter design: Varies by climate, often -10 to 5°C with low humidity
Frequently Asked Questions
A psychrometric calculator analyzes moist air — a mixture of dry air and water vapor — to compute its complete thermodynamic state from two independent measurements. Given dry-bulb temperature plus relative humidity (or wet-bulb, or dew point, or humidity ratio), it derives all the other psychrometric properties: dew-point temperature, wet-bulb temperature, humidity ratio (kg water per kg dry air), specific enthalpy (kJ/kg), specific volume (m³/kg), and water-vapor partial pressure. These properties drive HVAC sizing, cooling-coil selection, dehumidification system design, and indoor-comfort analysis. The calculations follow the ideal-gas mixture model with corrections from ASHRAE Fundamentals Handbook chapter 1.
Most calculators accept dry-bulb temperature (°C in SI, °F in imperial), atmospheric pressure (101.325 kPa standard at sea level), and one second independent variable: relative humidity (0 to 100 percent), wet-bulb temperature, dew-point temperature, or humidity ratio (g water/kg dry air). For altitude work, you must specify the local pressure because saturation pressure changes with altitude — at 1500 m elevation, atmospheric pressure drops to about 84 kPa, and the dew point and wet bulb shift accordingly. Output includes all derived properties plus a point that can be plotted on a psychrometric chart for visual verification.
Relative humidity (RH) is the ratio of actual water-vapor partial pressure to saturation pressure at the same temperature, expressed as a percentage. RH is temperature-dependent: the same absolute moisture content can show 60 percent RH at 25 °C but jump to 100 percent RH (saturation) when cooled to 17 °C. Humidity ratio (also called moisture content, omega) is mass of water vapor per mass of dry air (g/kg), and it stays constant when air is heated or cooled sensibly (no moisture added or removed). Dew point is the temperature at which the air becomes saturated if cooled at constant pressure. For comfort and condensation prediction, dew point is the most actionable: any surface below the dew point will collect liquid water.
Dry-bulb temperature is what an ordinary thermometer reads in shaded, still air. Wet-bulb temperature is what a thermometer reads when its bulb is wrapped in a wet wick exposed to airflow — evaporation cools the bulb until the moisture loss to the air balances the heat gain from the air. Wet bulb depends on both temperature and humidity: at 100 percent RH the wet bulb equals the dry bulb (no evaporation possible); at lower RH the wet bulb is lower. Wet bulb is the theoretical minimum temperature achievable by evaporative cooling — desert swamp coolers exploit this. ASHRAE 55 thermal-comfort calculations and cooling-tower sizing both pivot on the wet-bulb depression.
Sensible heat changes air temperature without changing moisture content — heating or cooling air at constant humidity ratio moves horizontally on the chart. Latent heat changes moisture content without changing temperature — humidifying or dehumidifying moves vertically. Most real HVAC processes combine both: a cooling coil simultaneously lowers temperature and (if surface temperature is below dew point) condenses moisture, tracing a diagonal path. The sensible heat ratio (SHR) is sensible load divided by total load (sensible + latent). Comfort cooling typically has SHR 0.7 to 0.8 (mostly sensible); a swimming-pool dehumidifier has SHR around 0.4 (mostly latent). The chart instantly shows whether equipment can deliver the required SHR for a given space.
Saturation vapor pressure depends only on temperature, but the relationship between humidity ratio, partial pressure, and total pressure depends on atmospheric pressure. At 1500 m altitude the air pressure is about 84 kPa instead of 101.325 kPa, so the same humidity ratio gives a higher partial-pressure fraction and the air feels relatively more humid even though the moisture mass is identical. Wet-bulb depression is also smaller at altitude because less water can evaporate before saturation. Cooling-tower performance, evaporative cooler efficiency, and dehumidifier capacity all scale with altitude. Always check that the calculator either uses local pressure or applies an altitude correction; ignoring this can produce 5 to 10 percent error in cooling-load and humidification sizing.
The authoritative reference is ASHRAE Handbook — Fundamentals, chapter 1 (Psychrometrics), which gives the equations for saturation pressure (Hyland-Wexler), humidity ratio, enthalpy, and specific volume valid from −100 °C to +200 °C. NIST publishes the underlying steam-table data through the IAPWS-IF97 formulation. ASHRAE 55 sets the thermal-comfort acceptability zone (typically dry-bulb 20 to 27 °C with humidity ratio between 0.004 and 0.012 kg/kg). ISO 7730 (PMV/PPD) gives the international comfort standard. AHRI 210/240 prescribes the test conditions for residential AC equipment ratings — 80 °F DB, 67 °F WB indoors. EN 16798 covers indoor environmental input parameters for the European building energy performance framework.
A psychrometric chart plots dry-bulb temperature on the x-axis and humidity ratio on the y-axis, with curved lines for constant relative humidity, sloping lines for wet-bulb temperature and specific enthalpy, and another set for specific volume. To analyze a process — say cooling and dehumidifying from 26 °C/60 percent RH to 12 °C/95 percent RH — plot the two state points, draw the line between them, and read off the change in enthalpy (in kJ/kg dry air) and moisture content (in g/kg dry air). Multiply by air mass flow rate to get sensible, latent, and total cooling capacity. For mixing fresh and recirculated air, draw a straight line between the two state points and interpolate based on mass-flow ratio. The chart is the single most-used design tool in HVAC engineering — this calculator replaces tedious chart reading with numerical precision.
After computing the state point, this calculator evaluates it against the ASHRAE Standard 55 / EN 16798 occupied-zone comfort envelope and returns a colored PASS, MARGINAL, or FAIL verdict with the reason. PASS means the dry-bulb temperature (about 20–27 °C), the humidity ratio (about 4–12 g/kg) and the relative humidity (under roughly 70 percent) all fall inside the comfort band. MARGINAL means one value is just outside the strict band but within a small tolerance — worth noting but usually acceptable. FAIL flags a specific problem: too warm, too cold, too humid (dehumidification needed), too dry (humidification needed), or condensation risk when the dew point is high enough to wet cool surfaces. This turns raw psychrometric numbers into a design decision: whether the state point is code-compliant and whether you need to add cooling, heating, humidification, dehumidification, or surface insulation. The verdict uses the same dry-bulb, humidity ratio, dew point and RH already shown, so it needs no extra input.
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