Psychrometric Calculator — Air Properties from Any Two Known Values

How It Works

1. Enter Altitude or Barometric Pressure

   Set your site elevation or barometric pressure. Psychrometric calculations depend on atmospheric pressure. At Denver (5,280 ft), atmospheric pressure is about 12.2 psia vs 14.696 at sea level. Using sea-level defaults at altitude introduces 5-15% error in humidity ratio and enthalpy.

2. Select Two Known Properties

   Choose any two of: dry-bulb temperature, wet-bulb temperature, dew point temperature, relative humidity, or humidity ratio. The calculator solves the psychrometric equations for all remaining properties. The most common field pair is dry-bulb and relative humidity, but dry-bulb and wet-bulb is preferred for HVAC calculations.

3. Enter Known Values

   Type in the values for your two selected properties. Use accurate instruments: a calibrated thermometer for dry-bulb, a sling psychrometer or digital hygrometer for wet-bulb or RH. The quality of results depends entirely on the accuracy of your inputs.

4. Review All Calculated Properties

   The calculator outputs dry-bulb, wet-bulb, dew point, relative humidity, humidity ratio (grains/lb and lb/lb), enthalpy (BTU/lb), specific volume (ft3/lb), and vapor pressure. All values are altitude-corrected for your site.

5. Apply Results to Your HVAC Problem

   Use dew point to check for condensation on cold surfaces. Use enthalpy difference across a coil to calculate cooling load. Use humidity ratio to size dehumidification. Use specific volume to convert between mass flow and volumetric flow (CFM).

Built For

• HVAC technicians calculating wet-bulb temperature from dry-bulb and RH for superheat charging charts
• Building scientists assessing condensation risk on windows, walls, and cold piping by comparing surface temperature to dew point
• Engineers calculating total cooling load (sensible + latent) using enthalpy difference across air handler coils
• Indoor air quality specialists sizing dehumidification equipment based on humidity ratio and target RH
• Commissioning agents verifying supply and return air conditions match design specifications during system startup
• HVAC instructors teaching psychrometric relationships with an interactive calculation tool
• High-altitude HVAC contractors in Denver, Salt Lake City, and mountain resort towns who need altitude-corrected air properties

Features & Capabilities

Altitude Correction

Enter elevation in feet or barometric pressure in inHg/psia. All psychrometric calculations adjust for local atmospheric pressure. Critical for installations above 3,000 feet where sea-level assumptions introduce significant errors.

Any Two Input Properties

Solve from any combination of dry-bulb, wet-bulb, dew point, relative humidity, or humidity ratio. The calculator selects the correct psychrometric equations based on your input pair and solves iteratively where needed.

Complete Property Output

Returns all psychrometric properties: Tdb, Twb, Tdp, RH, W (humidity ratio), h (enthalpy), v (specific volume), and Pw (vapor pressure). Everything you need for coil calculations, duct sizing, and IAQ analysis in one output.

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.

Condensation Risk Flag

Compares your dew point to common surface temperatures and flags conditions where condensation is likely. Useful for assessing cold window frames, chilled water piping, and concrete slabs in contact with humid air.

PDF Export

Export your psychrometric calculation results as a branded PDF for project documentation, commissioning reports, or training records.

Frequently Asked Questions