Calculation sequence
Step-by-step method
- Calculate saturation vapor pressure at dry-bulb temperature.
- Apply relative humidity for water-vapor pressure and humidity ratio.
- Calculate moist-air enthalpy and invert vapor pressure for dew point.
MechanicalAdvancedPsychrometrics
Psychrometric Properties Calculator
Calculate humidity ratio, moist-air enthalpy, dew point, and vapor pressure from dry-bulb temperature, relative humidity, and pressure.
Why it matters: Moist-air properties govern latent loads, condensation risk, economizer decisions, and airside total heat.
Calculated result
Engineering evaluation
Humidity ratio (W)0.009 lb/lb dry air
- Moist-air enthalpy (h)
- 28.1 Btu/lb
- Dew-point temperature (Tdp)
- 55.05 °F
- Water-vapor pressure (Pv)
- 0.215 psia
Engineering interpretation
The entered state has humidity ratio 0.00923367 lb/lb dry air, enthalpy 28.1 Btu/lb, and dew point 55.05 °F.
Important limitations
- Equations are suitable for HVAC screening within ordinary atmospheric conditions. Use authoritative psychrometric software for extreme temperatures, high pressures, or precision design.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Pv = RH·Pws(Tdb)/100
W = 0.621945Pv/(P−Pv)
h = f(Tdb,W)
Tdp = f(Pv)
Current substitution
At Tdb = 75, RH = 50%, and P = 14.696, Pv = 0.215, W = 0.00923367, h = 28.1, and Tdp = 55.05.
Equation legend
Inputs and calculated quantities
- Tdb
- Dry-bulb temperature — °F or °C
- RH
- Relative humidity — %
- P
- Barometric pressure — psia or kPa
- W
- Humidity ratio — lb/lb dry air
- h
- Moist-air enthalpy — Btu/lb
- Tdp
- Dew-point temperature — °F
- Pv
- Water-vapor pressure — psia
Applications
How this calculation is used
Building design
Screen moist-air properties for load and process calculations.
Construction
Check sensor and control-point relationships.
Commissioning
Convert field dry-bulb, relative-humidity, and pressure readings into engineering properties.
Quality controls
Assumptions and limitations
Assumptions
- Entered values represent one stable operating condition and use consistent measurement boundaries.
- Fluid and air properties are representative of the evaluated condition.
Limitations
- This is a preliminary evaluation and field-verification tool, not a final sizing or code-compliance determination.
- Manufacturer data, adopted codes, project criteria, and professional engineering review remain controlling.
Field use
Verification procedure
- Confirm measurement boundaries, operating mode, and instrument calibration.
- Stabilize the system and record simultaneous readings.
- Compare the calculation with project requirements and manufacturer data before disposition.
Common engineering mistakes
- Mixing values from different operating conditions.
- Using nominal rather than measured flow, temperature, power, or fluid properties.
Technical basis
References and source standards
- ASHRAE Handbook—FundamentalsPsychrometrics, heat transfer, load relationships, and HVAC calculation context; verify the licensed current edition during professional review.
- NIST Guide for the Use of the International System of UnitsFederal unit-system and conversion reference used for SI/I-P presentation.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement