Drawing No. EH–TH–034 // Thermal Engineering & HVAC
Psychrometric Calculator & Interactive Chart
Reviewed August 2026
Solve a complete moist-air state from dry-bulb temperature plus one independent humidity property. Pressure can be entered directly or estimated from elevation. The interactive chart is recalculated for the selected pressure and can be clicked to choose a state.
1. Define the moist-air state
2. Calculated state
What this calculator is solving
Psychrometrics treats moist air as a mixture of dry air and water vapor. At a known total pressure, two independent state properties normally determine the remaining properties. The calculator keeps one internal SI state so SI and US customary results are exact unit conversions of the same thermodynamic solution.
Humidity ratio
Mass of water vapor per mass of dry air. ASHRAE 2025 gives 0.621945 from the ratio of water and dry-air molar masses.
Relative humidity
The calculator uses the perfect-gas form consistent with the implemented ASHRAE chart-compatible equations.
Moist-air enthalpy
SI result in kJ per kg of dry air, using the standard ASHRAE perfect-gas enthalpy reference.
Specific volume
Volume of moist air per kg of dry air. Pressure has a strong effect, which is why altitude is part of the calculation.
Thermodynamic wet bulb
The state humidity ratio is matched to the ASHRAE adiabatic-saturation relationship. Wet bulb is solved numerically between dew point and dry bulb.
Dew point
The saturation-pressure equation is inverted numerically rather than using a short dew-point approximation. This avoids the loss of accuracy associated with narrow-range regressions.
Precision, limitations & interpretation
The numerical solver is intentionally tighter than the physical model requires. Model accuracy is governed primarily by the psychrometric formulation and by the quality of temperature, humidity and pressure measurements.
ASHRAE 2025 Fundamentals describes both perfect-gas relationships and more detailed real-moist-air formulations. It notes that perfect-gas calculations for saturated humidity ratio, enthalpy and specific volume at standard atmospheric pressure are within about 0.7% over approximately −50 to 50°C, with smaller errors at lower pressure. This calculator therefore targets high-quality HVAC/design calculations and psychrometric-chart work, but it is not a substitute for ASHRAE RP-1485 real-gas property software or calibrated humidity metrology.
Enthalpy reference note: ASHRAE SI and I-P psychrometric equations use different conventional enthalpy reference zeros. The calculator therefore evaluates the native SI or I-P enthalpy equation for display and enthalpy-input mode instead of applying a simple kJ/kg ↔ Btu/lb multiplier.
ASHRAE distinguishes thermodynamic wet-bulb temperature from the temperature measured by a physical wetted thermometer. Radiation, wick-water temperature, air velocity and heat/mass-transfer effects can create a difference. Treat a measured wet bulb as an experimental input with instrument and setup uncertainty.
The elevation option uses the ASHRAE standard-atmosphere expression p = 101325(1 − 2.25577×10⁻⁵Z)^5.2559 for station pressure. Actual weather pressure can differ from the standard atmosphere. For precision work, enter measured local station pressure rather than sea-level-corrected barometric pressure.
The implemented saturation-pressure equations are used from −100 to 200°C, switching at the 0.01°C triple point to maintain numerical continuity around freezing. The state solver is intentionally limited to the normal single-phase psychrometric range where the pure-water saturation pressure at dry-bulb temperature remains below total pressure. ASHRAE's extended relative-humidity definition for conditions such as atmospheric-pressure drying kilns where pws(T) ≥ p is outside this calculator's present scope. Inputs implying supersaturated single-phase air are rejected rather than silently clipped.
Validation benchmark
ASHRAE 2025 provides a published psychrometric-chart example at 40°C dry bulb, 20°C thermodynamic wet bulb and 101.325 kPa. The calculator includes this case as a one-click example so you can reproduce it.
| Property | ASHRAE chart reading | Expected calculator behavior |
|---|---|---|
| Humidity ratio | ≈ 6.5 g/kg dry air | Close to chart value; numerical equation result is more granular than chart reading |
| Enthalpy | ≈ 56.7 kJ/kg dry air | Close agreement |
| Dew point | ≈ 7°C | Close agreement |
| Relative humidity | ≈ 14% | Close agreement |
| Specific volume | ≈ 0.896 m³/kg dry air | Close agreement |
Primary references
The page deliberately links to the underlying technical sources rather than relying on anonymous web formulas.