Psychrometric Calculator

How It Works

1. Enter Altitude or Station Pressure

   Set site elevation for a standard-atmosphere prompt or enter actual station pressure. Weather barometers are often sea-level corrected and may need conversion before field use.

2. Select a Bounded Input Pair

   Choose dry-bulb plus RH, wet bulb, dew point, or humidity ratio. The app screens a local point-property state; it does not validate instrument quality or a project psychrometric chart.

3. Enter Measured Values

   Use calibrated instruments and stable conditions. Thermometer placement, RH sensor drift, wet-wick airflow, and barometric pressure uncertainty can dominate the result.

4. Review Source Warnings

   Outputs include dry bulb, wet bulb, dew point, RH, humidity ratio, enthalpy, specific volume, vapor pressure, assumptions, and source pointers. Treat them as preliminary prompts.

5. Carry Results to Qualified Review

   Use the values to frame HVAC, moisture, comfort, coil, ventilation, or commissioning questions before current standards, manufacturer data, field data, code/AHJ, and qualified review.

Built For

• HVAC technicians organizing wet-bulb, RH, dew-point, and humidity-ratio questions before diagnostics
• Building scientists comparing dew-point prompts with measured surface-temperature and envelope data
• Engineers preparing enthalpy and humidity-ratio inputs before formal load or coil calculations
• IAQ and moisture reviewers documenting source gaps before dehumidification or humidification decisions
• Commissioning teams screening supply and return air point properties before acceptance criteria review
• HVAC instructors demonstrating psychrometric relationships with explicit source boundaries
• High-altitude teams checking whether actual station pressure or elevation assumptions need review

Features & Capabilities

Pressure Prompt

Enter elevation or direct station pressure. Altitude mode uses a standard-atmosphere relationship and is not a substitute for measured pressure when field accuracy matters.

Four Input Pairs

Screen dry-bulb plus RH, wet bulb, dew point, or humidity ratio with bounded state hydration and live numeric guards.

Point-Property Output

Returns Tdb, Twb, Tdp, RH, W, h, v, Pv, degree of saturation, source warnings, assumptions, and residual gaps for review.

Unit Toggle

Switch between IP (Fahrenheit, BTU/lb, grains/lb) and SI (Celsius, kJ/kg, g/kg) units. Input in either system and the calculator converts all outputs to your preferred unit system.

Comfort Prompt

Shows a coarse local comfort band while stating that ASHRAE 55 compliance is not determined.

Report Export

Export source warnings, assumptions, source pointers, and local point-property results for planning records.

Assumptions

• Standard atmospheric pressure of 14.696 psia (29.92 in Hg) at sea level
• ASHRAE-style psychrometric equations for local moist-air point-property screening
• Dry-bulb and one independent moisture property as bounded user inputs
• Air treated as an ideal gas mixture of dry air and water vapor
• Enthalpy reference convention follows the local app formula and requires source review before formal use

Limitations

• Does not reproduce licensed ASHRAE Handbook tables or chart artwork
• Does not calculate process lines, mixing, heating, cooling, humidification, or dehumidification equipment size
• Does not render a graphical psychrometric chart - numeric output only
• Does not validate field instruments, station pressure, surface temperatures, envelope conditions, or commissioning criteria
• Does not determine ASHRAE 55 comfort compliance, mold risk, condensation safety, code/AHJ approval, or final HVAC design

References

• ASHRAE 2025 Handbook Fundamentals - Chapter 1 Psychrometrics source pointer
• ASHRAE Standard 55-2023 thermal comfort source pointer
• ISO 2533:1975 Standard Atmosphere source pointer
• NIST SP 811 Appendix B.8 unit conversion source pointer

Frequently Asked Questions