Psychrometric Calculator & Chart
Calculate moist air thermodynamic properties (Dry Bulb, Wet Bulb WBT, Relative Humidity RH, Dew Point, Enthalpy, Humidity Ratio) with live interactive psychrometric chart and ASHRAE equations.
Thermodynamic State Results
Within standard ASHRAE 55 indoor thermal comfort zone.
Online Interactive Psychrometric Chart
Click or hover anywhere on the chart to inspect thermodynamic coordinates and set state points.
Adiabatic Air Stream Mixing Calculator
Calculate the resulting mixed air properties when two airstreams (e.g. Return Air + Outdoor Fresh Air) combine in an Air Handling Unit (AHU).
Airstream 1 (e.g. Return Air)
Airstream 2 (e.g. Outdoor Fresh Air)
Mixed Air Stream Outcome (Condition C)
ASHRAE Mathematical Formulations & Live Step-by-Step Substitution
Complete breakdown of thermodynamic equations used to solve your currently active state point.
What is a Psychrometric Calculator?
A psychrometric calculator is an essential engineering instrument used by HVAC designers, mechanical engineers, meteorologists, and building scientists to analyze the physical and thermodynamic properties of moist air (gas-vapor mixtures).
Under Gibbs' phase rule, moist air at a fixed barometric pressure has two degrees of freedom. This means that if you measure any two independent psychrometric properties—such as Dry Bulb Temperature ($T_{db}$) and Relative Humidity ($RH$), or Wet Bulb Temperature ($T_{wb}$) and Dew Point ($T_{dp}$)—all other thermodynamic properties can be calculated with high precision.
Core Moist Air Properties & Definitions
1. Dry Bulb Temperature (Tdb)
The actual ambient air temperature measured by a standard thermometer shielded from direct radiation and moisture.
2. Wet Bulb Temperature (Twb / WBT)
The lowest temperature reachable by evaporative water cooling alone. Critical for sizing cooling towers, evaporative coolers, and heat stress limits.
3. Relative Humidity (% RH)
The ratio of the partial pressure of water vapor in the air to the saturation vapor pressure at the same dry bulb temperature.
4. Dew Point Temperature (Tdp)
The temperature to which moist air must be cooled at constant barometric pressure for water vapor to condense into fog, dew, or rain.
5. Humidity Ratio / Moisture Content (W)
The exact mass of water vapor per unit mass of dry air (expressed in $g/kg$, $kg/kg$, or $grains/lb$). Does not change with temperature unless moisture is added or removed.
6. Specific Enthalpy (h)
The total thermal energy contained in moist air per unit mass, combining sensible heat (air temperature) and latent heat (evaporated moisture).
How to Read an Online Psychrometric Chart
A psychrometric chart visually represents moist air states on a 2-dimensional plane. Here is how each line is oriented on the chart:
- Dry Bulb Temperature ($T_{db}$): Marked on the horizontal bottom X-axis. Constant dry bulb lines extend vertically upwards.
- Humidity Ratio ($W$): Marked on the right vertical Y-axis. Constant moisture content lines extend horizontally to the left.
- Relative Humidity ($RH$): Represented as curved isopleths spanning from bottom-left to top-right. The uppermost boundary curve is the 100% Saturation Line.
- Wet Bulb Temperature ($T_{wb}$): Marked along the 100% saturation boundary curve. Constant wet bulb lines slope diagonally down and to the right.
- Enthalpy ($h$): Follows nearly parallel diagonal slopes to wet bulb lines, measured along the outer scale.
Dew Point & Human Comfort Index Table
Unlike relative humidity (which fluctuates wildly with temperature changes), dew point is the most reliable scientific measure of how muggy or comfortable the air feels on human skin:
| Dew Point Range (°C) | Dew Point Range (°F) | Human Perception | HVAC & Indoor Environment |
|---|---|---|---|
| < 10 °C | < 50 °F | Dry / Crisp | May cause skin dryness and static electricity. Humidification recommended in winter. |
| 10 °C to 16 °C | 50 °F to 60 °F | Comfortable / Ideal | Optimal ASHRAE 55 indoor zone. Low mold risk and excellent thermal comfort. |
| 16 °C to 18 °C | 60 °F to 65 °F | Pleasant / Slightly Humid | Noticeable moisture in air. Standard summer condition in temperate climates. |
| 18 °C to 21 °C | 65 °F to 70 °F | Humid / Muggy | Sweat evaporates slowly. Requires active dehumidification in HVAC cooling coils. |
| > 21 °C | > 70 °F | Oppressive / Dangerous | Tropical conditions. High risk of heat exhaustion during physical exertion. |
Psychrometric Quick Reference Table (Sea Level 101.3 kPa)
| Dry Bulb (°C) | RH (%) | Wet Bulb (°C) | Dew Point (°C) | Humidity Ratio (g/kg) | Enthalpy (kJ/kg) |
|---|---|---|---|---|---|
| 20.0 | 50% | 13.8 | 9.3 | 7.26 | 38.5 |
| 22.0 | 50% | 15.3 | 11.1 | 8.22 | 43.0 |
| 24.0 | 50% | 16.8 | 12.9 | 9.30 | 47.8 |
| 26.0 | 50% | 18.4 | 14.8 | 10.51 | 52.9 |
| 30.0 | 50% | 22.0 | 18.4 | 13.34 | 64.3 |
| 35.0 | 40% | 23.8 | 19.4 | 14.23 | 71.7 |
Frequently Asked Questions (FAQ)
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