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Psychrometric calculator

Use this psychrometric calculator to find humidity ratio, dew point, wet-bulb temperature and air enthalpy from measured conditions.

Method version1.0.0 · Reviewed November1,2026 · Calculated on your device

Your calculation

Choose a method, then enter your figures. Examples are illustrative, not recommended settings or rates.

Measurement system

Changing units changes how entries are interpreted; existing numbers are not converted. Review every dimension.

Available moisture measurement

Choose the second measurement you actually have.

Dry-bulb temperature (°F for US;°C for metric)

Supported dry-bulb range is -4°F through140°F (-20°C through60°C).

Relative humidity (%) OR selected moisture temperature

For RH enter a percent. For dew/frost point or wet bulb enter°F in US mode or°C in metric mode. A moisture temperature cannot exceed dry bulb.

Local ABSOLUTE atmospheric pressure (psi US; kPa metric)

Use measured local station pressure. This is not gauge pressure or weather pressure corrected to sea level.

How this calculation works

Humidity ratio W=0.621945×vaporPressure/(absolutePressure−vaporPressure); h=1.006T+W(2501+1.86T), with T in°C.

What information do I need?

Only fields for your selected method are required. Use consistent units and scope. Changing an entry method or unit system does not convert the numbers already entered. Previously calculated results are cleared when inputs change.

Input definitions and where to find them

Measurement system: Changing units changes how entries are interpreted; existing numbers are not converted. Review every dimension.
Available moisture measurement: Choose the second measurement you actually have.
Dry-bulb temperature (°F for US;°C for metric): Supported dry-bulb range is -4°F through140°F (-20°C through60°C).
Relative humidity (%) OR selected moisture temperature: For RH enter a percent. For dew/frost point or wet bulb enter°F in US mode or°C in metric mode. A moisture temperature cannot exceed dry bulb.
Local ABSOLUTE atmospheric pressure (psi US; kPa metric): Use measured local station pressure. This is not gauge pressure or weather pressure corrected to sea level.

Worked example

This example uses the initial method shown in the selector. Load an example in the form to try it.

Example inputs

Measurement system: us
Available moisture measurement: rh
Dry-bulb temperature (°F for US;°C for metric): 77
Relative humidity (%) OR selected moisture temperature: 50
Local ABSOLUTE atmospheric pressure (psi US; kPa metric): 14.6959487755

Example answer: Relative humidity = 50 %

Use the result well

Uses ASHRAE saturation and moist-air relations, with an ice reference below freezing. Below freezing the reported saturation point is a frost point. Use measured local absolute atmospheric pressure, not gauge pressure or corrected sea-level pressure. Enthalpy retains its SI reference even when temperatures are shown in°F. This is a single air-state worksheet, not a load calculation, equipment design or condensation guarantee.

Review the limits shown with each result. Browser arithmetic has finite precision; small differences may reflect rounding. Print or download important results before closing the tab. This workspace does not store projects online or replace an approved examination calculator, engineering design or professional judgment.

Method and sources

ASHRAE Fundamentals: psychrometrics

References support the stated mathematical concepts; the interface and implementation are original. Publishers do not endorse this site.