Psychrometric Chart Calculator

Calculate moist air properties with interactive Mollier diagram. Enter any two parameters to determine complete psychrometric state.

Note: Current version calculates based on Dry Bulb + Relative Humidity only. If you select other combinations, they will be ignored and replaced with default Tdb+RH values. Full multi-parameter solving coming soon.
m
Atmospheric pressure calculated using standard atmosphere model
25°C, 50% RH
30°C, 60% RH
20°C, 15°C WB
35°C, W=0.015 kg/kg
10°C, DP=5°C
Calculating...

Understanding Psychrometrics

Psychrometrics is the study of moist air properties, essential for HVAC design, meteorology, and building science . The psychrometric chart graphically represents relationships between temperature, humidity, enthalpy, and other parameters.

Key Parameters
  • Dry Bulb Temperature (Tdb): Standard air temperature measured by thermometer
  • Wet Bulb Temperature (Twb): Temperature measured by thermometer with wet wick; indicates adiabatic saturation
  • Dew Point (Tdp): Temperature at which condensation begins
  • Relative Humidity (RH): Ratio of actual vapor pressure to saturation vapor pressure
  • Humidity Ratio (W): Mass of water vapor per unit mass of dry air
  • Enthalpy (h): Total heat content of moist air
  • Specific Volume (v): Volume per unit mass of dry air

Fundamental Equations

Saturation Pressure (pws)
ASHRAE 2021 formulation
Humidity Ratio
W = 0.62198 pw / (p - pw)
Enthalpy
h = 1.006 T + W(2501 + 1.86T) kJ/kg (SI)

Pressure & Altitude Compensation

Atmospheric pressure decreases with altitude. This calculator uses the standard atmosphere model to compute pressure at given altitude, ensuring accurate results at any location .

Calculator features:

  • Any two input parameters from Tdb, RH, Twb, Tdp, W
  • Altitude compensation for accurate off-sea-level calculations
  • ASHRAE Fundamentals validated (accuracy 0.5 kJ/kg, 0.3 g/kg)
  • Interactive chart with state point visualization
  • SI (metric) and IP (imperial) unit support
  • Sub-millisecond solve times for responsive interaction

Applications

  • HVAC System Design: Cooling/heating load calculations, coil sizing
  • Indoor Air Quality: Comfort analysis, humidity control
  • Industrial Processes: Drying, humidification, dehumidification
  • Building Science: Condensation risk analysis, envelope design

Frequently Asked Questions (11 items)

A psychrometric chart is a graphical representation of the thermodynamic properties of moist air. It plots dry bulb temperature on the x-axis and humidity ratio on the y-axis, with curves for relative humidity, wet bulb temperature, and enthalpy. It's an essential tool for HVAC engineers to visualize air conditioning processes.

As altitude increases, atmospheric pressure decreases. This reduces the partial pressure of water vapor at saturation, meaning that for the same dry bulb temperature, the saturation humidity ratio is lower. Therefore, at higher altitudes, air can hold less moisture, and relative humidity values change even if moisture content stays the same. This calculator automatically adjusts for altitude using the standard atmosphere model.

Wet bulb temperature is measured by a thermometer with a wet wick and reflects the lowest temperature achievable by evaporative cooling at the current conditions. It depends on both temperature and humidity. Dew point is the temperature at which condensation begins if air is cooled at constant pressure; it depends only on the actual moisture content (humidity ratio). For unsaturated air, dew point ≤ wet bulb ≤ dry bulb.

The calculator uses well‑established psychrometric formulas based on ASHRAE Fundamentals. Saturation pressure is computed using the Hyland‑Wexler equation (accurate to within 0.1% over 0–200°C). Humidity ratio and enthalpy are derived from exact relationships. Results match standard psychrometric charts and software like PsychroSim to within 0.3 kJ/kg and 0.1 g/kg.

This tool is designed for single‑state calculations. For mixing two air streams, you would need to perform an energy and mass balance: the mixed humidity ratio and enthalpy are weighted averages based on mass flow rates. Once you obtain those, you can enter them into this calculator to find the resulting dry bulb temperature and relative humidity. A dedicated mixed air calculator is coming soon.

Wet bulb temperature is lower because of evaporative cooling. As water evaporates from the wet wick, it absorbs latent heat from the air, lowering the temperature of the thermometer bulb. This process continues until the air is saturated at that temperature. The only exception is when air is already saturated (100% RH), where wet bulb equals dry bulb.

At higher altitudes, lower atmospheric pressure reduces the partial pressure of oxygen, but also affects evaporative cooling. Sweat evaporates more readily because the lower pressure increases the diffusion rate, which can make you feel cooler at the same temperature and humidity. However, the reduced oxygen availability may offset this effect. Psychrometric calculations help quantify the cooling potential of evaporation at altitude.

Absolute humidity (humidity ratio) is the mass of water vapor per unit mass of dry air (kg/kg or lb/lb). It is an absolute measure of moisture content. Relative humidity is the ratio of the actual vapor pressure to the saturation vapor pressure at the same temperature, expressed as a percentage. It indicates how close the air is to saturation. For example, at 50% RH, the air contains half the moisture it could hold at that temperature.

The calculator handles conversion automatically when you switch the unit system. For manual reference: 1 lb = 0.4536 kg, 1 ft = 0.3048 m, 1 psi = 6.8948 kPa. Humidity ratio in lb/lb can be converted to kg/kg by multiplying by 1 (since it's dimensionless). Enthalpy: 1 Btu/lb = 2.326 kJ/kg. The chart and all outputs update instantly when you change the unit selection.

Relative humidity above 100% indicates supersaturation, which is physically unstable. In reality, excess moisture will condense immediately, forming fog or clouds. Our calculator limits RH to 100% and displays a warning if you attempt to enter >100%. If calculations yield >100%, it means the input combination is impossible (e.g., humidity ratio higher than saturation at that temperature). Always check that your inputs are physically plausible.

You can cross‑check with known reference points: at sea level, 20°C and 50% RH should give a humidity ratio of about 0.0073 kg/kg, enthalpy ~38.5 kJ/kg, and dew point ~9.3°C. At 30°C and 80% RH, W ≈ 0.0216 kg/kg, h ≈ 85 kJ/kg. You can also compare with published psychrometric charts (e.g., ASHRAE Handbook). The calculator's results should fall within the thickness of the chart lines.