Psychrometric Calculator

Use this psychrometric calculator to determine the properties of moist air from dry-bulb temperature and either wet-bulb temperature or relative humidity. The calculator supports both IP and SI units and adjusts atmospheric pressure based on the entered altitude.

Enter dry-bulb and wet-bulb temperatures or relative humidity. The calculator will determine enthalpy, dew point, humidity ratio, moisture content, specific volume, vapor pressures, atmospheric pressure, and dry-air density. Set the altitude as necessary before you calculate.

How to Use the Psychrometric Calculator

Start by selecting either IP or SI units.

Then enter the dry-bulb temperature and one of the following:

• Wet-bulb temperature
• Relative humidity

You do not need to enter both wet-bulb temperature and relative humidity. Entering a value in one field automatically clears the other field.

Enter the site altitude to account for the effect of atmospheric pressure. At zero altitude, the calculator uses standard sea-level conditions.

Press Calculate to display the air properties.

IP Units

The IP version uses:

• Temperature in °F
• Altitude in ft
• Enthalpy in BTU/lb dry air
• Humidity ratio in lb water/lb dry air
• Moisture content in grains/lb dry air
• Specific volume in ft³/lb dry air
• Vapor pressure in in. Hg
• Dry-air density in lb dry air/ft³

SI Units

The SI version uses:

• Temperature in °C
• Altitude in m
• Enthalpy in kJ/kg dry air
• Humidity ratio in kg water/kg dry air
• Moisture content in g/kg dry air
• Specific volume in m³/kg dry air
• Vapor pressure in kPa
• Dry-air density in kg dry air/m³

What Is Psychrometrics?

Psychrometrics is the study of the thermodynamic properties of moist air.

In HVAC engineering, air is normally treated as a mixture of:

• Dry air
• Water vapor

The amount of water vapor in the air affects comfort, cooling load, dehumidification, humidification, ventilation, and equipment performance.

Psychrometric calculations are commonly used for:

• Air-conditioning design
• Cooling-coil analysis
• Dehumidification
• Humidification
• Ventilation calculations
• Air mixing
• Heating and cooling processes
• Indoor humidity control
• Drying applications

A psychrometric chart displays these properties graphically. This calculator determines the same basic air properties numerically.

Required Air Properties

A moist-air condition is generally defined by atmospheric pressure and two independent psychrometric properties.

This calculator uses dry-bulb temperature as the first property and allows either wet-bulb temperature or relative humidity as the second property.

Dry-Bulb Temperature

Dry-bulb temperature is the ordinary air temperature measured by a standard thermometer without considering the effect of moisture evaporation.

It is the temperature normally shown by:

• Room thermostats
• Temperature sensors
• Weather reports
• HVAC control systems

Dry-bulb temperature is represented by:

• °F in IP units
• °C in SI units

Wet-Bulb Temperature

Wet-bulb temperature is the temperature measured when water evaporates from a wetted sensor exposed to moving air.

Evaporation removes heat from the sensor. The amount of cooling depends on the moisture content of the air.

Dry air produces more evaporation and a lower wet-bulb temperature. Humid air produces less evaporation, so the wet-bulb temperature approaches the dry-bulb temperature.

The wet-bulb temperature cannot be higher than the dry-bulb temperature under normal unsaturated conditions.

Relative Humidity

Relative humidity compares the actual vapor pressure of water in the air with the saturated vapor pressure at the same dry-bulb temperature.

A relative humidity of 50% means that the actual vapor pressure is approximately half of the saturated vapor pressure at that temperature.

Relative humidity does not directly indicate the total mass of water vapor in the air because the saturation pressure changes strongly with temperature.

Warm air can therefore have a lower relative humidity than cool air even when both contain the same humidity ratio.

Altitude and Atmospheric Pressure

Atmospheric pressure decreases as altitude increases.

This affects psychrometric calculations because humidity ratio, specific volume, density, and wet-bulb relationships depend on total atmospheric pressure.

At standard sea level, the calculator uses approximately:

• 29.921 in. Hg
• 101.325 kPa

For other altitudes, atmospheric pressure is estimated using a standard-atmosphere relationship.

Actual local barometric pressure may differ because of weather conditions. For highly precise engineering calculations, measured barometric pressure is preferable.

Psychrometric Properties Calculated

Moist-Air Enthalpy

Moist-air enthalpy represents the total heat content of the air-water vapor mixture relative to a selected reference condition.

It includes:

• Sensible heat associated with dry-air temperature
• Sensible and latent heat associated with water vapor

In IP units, enthalpy is expressed as:

BTU/lb dry air

A commonly used IP relationship is:

h = 0.24T + W(1061 + 0.444T)

Where:

• h = moist-air enthalpy, BTU/lb dry air
• T = dry-bulb temperature, °F
• W = humidity ratio, lb water/lb dry air

In SI units:

h = 1.006T + W(2501 + 1.86T)

Where:

• h = moist-air enthalpy, kJ/kg dry air
• T = dry-bulb temperature, °C
• W = humidity ratio, kg water/kg dry air

Enthalpy is especially useful for analyzing total heating and cooling processes.

Dew-Point Temperature

Dew-point temperature is the temperature at which water vapor begins to condense when the air is cooled at constant pressure without changing its moisture content.

When a surface temperature falls below the dew point, condensation can form.

Dew point is important for:

• Cooling-coil design
• Condensation control
• Insulation design
• Duct sweating analysis
• Mold-risk assessment
• Building-envelope analysis

Unlike relative humidity, dew point directly reflects the actual water-vapor content of the air.

Humidity Ratio

Humidity ratio is the mass of water vapor per unit mass of dry air.

It is also called:

• Specific humidity in some HVAC references
• Mixing ratio
• Moisture ratio

The calculator reports humidity ratio as:

• lb water/lb dry air in IP units
• kg water/kg dry air in SI units

Humidity ratio is one of the most useful psychrometric properties because it remains constant during sensible heating or cooling when no moisture is added or removed.

The basic relationship is:

W = 0.621945Pv / (P − Pv)

Where:

• W = humidity ratio
• Pv = partial vapor pressure
• P = total atmospheric pressure

Moisture Content

The calculator also displays humidity ratio in more convenient moisture-content units.

In IP mode:

Moisture content = W × 7000

because one pound contains 7000 grains.

The result is expressed as:

grains/lb dry air

In SI mode:

Moisture content = W × 1000

The result is expressed as:

g/kg dry air

This form is often easier to read than a small decimal humidity-ratio value.

Specific Volume

Specific volume is the volume occupied by moist air per unit mass of dry air.

It is expressed as:

• ft³/lb dry air in IP units
• m³/kg dry air in SI units

Specific volume increases when:

• Air temperature increases
• Altitude increases
• Atmospheric pressure decreases
• Moisture content increases

Specific volume is useful when converting between air mass flow and volumetric airflow.

For example:

Dry-air mass flow = Volumetric airflow ÷ Specific volume

Dry-Air Density

Dry-air density is the mass of dry air contained in a unit volume of moist air.

It is calculated as the reciprocal of specific volume:

Dry-air density = 1 ÷ Specific volume

The calculator reports:

• lb dry air/ft³ in IP units
• kg dry air/m³ in SI units

Dry-air density is not an independent input to the psychrometric calculation. It is a resulting property derived after specific volume has been calculated.

It is useful when converting volumetric airflow into dry-air mass flow.

Partial Vapor Pressure

Partial vapor pressure is the portion of total atmospheric pressure contributed by water vapor.

The remaining pressure is primarily contributed by dry air.

Partial vapor pressure can be calculated from humidity ratio:

Pv = PW / (0.621945 + W)

Where:

• Pv = partial vapor pressure
• P = total atmospheric pressure
• W = humidity ratio

Partial vapor pressure determines dew point and is also used in relative-humidity calculations.

Saturated Vapor Pressure

Saturated vapor pressure is the maximum water-vapor pressure possible at a particular temperature.

It depends mainly on dry-bulb temperature.

As temperature increases, saturated vapor pressure rises rapidly. This is why warm air can support a much higher moisture content before reaching saturation.

Relative humidity is calculated from:

RH = Pv / Pws × 100%

Where:

• RH = relative humidity
• Pv = actual partial vapor pressure
• Pws = saturated vapor pressure at the dry-bulb temperature

Why Altitude Changes Psychrometric Properties

At higher altitude, atmospheric pressure is lower.

For the same dry-bulb temperature and relative humidity:

• Saturated vapor pressure remains primarily temperature-dependent
• Actual vapor pressure remains tied to relative humidity
• Humidity ratio increases as total pressure decreases
• Specific volume increases
• Dry-air density decreases
• Air mass flow for a given CFM or m³/s decreases

This is important for HVAC systems installed at elevated locations.

A fan may deliver the same volumetric airflow while moving less air mass than it would at sea level. Heating and cooling calculations based on standard air density may therefore require altitude correction.

Example Psychrometric Calculation

Consider air at:

• Dry-bulb temperature: 75°F
• Wet-bulb temperature: 63°F
• Altitude: Standard sea level

The calculator first determines atmospheric pressure from altitude.

It then uses the dry-bulb and wet-bulb temperatures to calculate humidity ratio.

Once humidity ratio is known, the remaining properties can be calculated, including:

• Relative humidity
• Dew point
• Enthalpy
• Specific volume
• Partial vapor pressure
• Saturated vapor pressure
• Dry-air density

The exact result may vary slightly depending on the equations, constants, rounding method, and psychrometric reference used.

Dry-Bulb and Relative Humidity Example

Suppose the air condition is:

• Dry-bulb temperature: 24°C
• Relative humidity: 50%
• Altitude: Standard sea level

The saturated vapor pressure is first calculated at 24°C.

The partial vapor pressure is then:

Partial vapor pressure = 50% × Saturated vapor pressure

Humidity ratio is calculated using the partial vapor pressure and atmospheric pressure.

From this state, the calculator determines wet-bulb temperature, dew point, enthalpy, specific volume, and the other displayed properties.

Using Psychrometric Properties in HVAC

Sensible Heating

During sensible heating:

• Dry-bulb temperature increases
• Humidity ratio remains constant
• Relative humidity decreases
• Enthalpy increases
• Specific volume increases

No moisture is added or removed.

A typical example is air passing across an electric heater or hot-water heating coil.

Sensible Cooling

During sensible cooling above the dew point:

• Dry-bulb temperature decreases
• Humidity ratio remains constant
• Relative humidity increases
• Enthalpy decreases

If the air is cooled below its dew point, condensation begins and the process becomes cooling with dehumidification.

Cooling and Dehumidification

When moist air passes over a cooling coil below its dew point:

• Dry-bulb temperature decreases
• Humidity ratio decreases
• Dew point decreases
• Enthalpy decreases
• Water condenses from the air

This process is common in air-conditioning systems.

The difference in humidity ratio between entering and leaving air can be used to calculate moisture removal when dry-air mass flow is known.

Humidification

Humidification adds water vapor to the air.

Depending on the humidification method:

• Steam humidification increases humidity ratio and enthalpy
• Evaporative humidification increases humidity ratio while reducing dry-bulb temperature
• Relative humidity generally increases

Psychrometric properties help determine the amount of moisture that must be added.

Air Mixing

When two air streams mix, the resulting condition depends on:

• Dry-air mass flow of each stream
• Enthalpy of each stream
• Humidity ratio of each stream

Typical HVAC examples include:

• Outdoor air mixed with return air
• Bypass air mixed with coil air
• Two supply-air streams combined
• Exhaust-air energy-recovery systems

A single-state calculator does not directly calculate the final mixing condition, but the enthalpy and humidity-ratio results can be used in the mixing equations.

Accuracy and Limitations

This calculator is suitable for general HVAC analysis, education, preliminary calculations, and engineering checks.

Results may differ slightly from printed psychrometric charts or other software because of:

• Equation selection
• Numerical precision
• Rounding
• Atmospheric-pressure assumptions
• Wet-bulb formulations
• Reference-state conventions

The calculator estimates pressure from altitude using standard-atmosphere conditions.

Actual site pressure can vary because of weather. For high-accuracy work, use measured local barometric pressure and project-specific design data.

The calculator also assumes that:

• The air-water vapor mixture behaves approximately as an ideal gas
• Air properties are evaluated at steady conditions
• The entered air state is physically valid
• No additional contaminants materially affect air properties

Psychrometric Calculator Versus Psychrometric Chart

A psychrometric chart is useful for visualizing HVAC processes, such as:

• Sensible heating
• Sensible cooling
• Dehumidification
• Humidification
• Evaporative cooling
• Air mixing

A calculator is more convenient when exact numerical values are required.

The calculator provides a direct result without requiring the user to interpolate between chart lines.

For HVAC design, both tools are useful:

• Use the calculator for precise numerical properties
• Use the chart to visualize the direction and relationship of air-treatment processes

Frequently Asked Questions

What two values are needed for a psychrometric calculation?

At a known atmospheric pressure, two independent moist-air properties are generally required.

This calculator uses dry-bulb temperature together with either wet-bulb temperature or relative humidity.

Can wet-bulb temperature be higher than dry-bulb temperature?

No. For a normal unsaturated moist-air condition, wet-bulb temperature cannot exceed dry-bulb temperature.

At saturation, wet-bulb temperature, dry-bulb temperature, and dew-point temperature are equal.

Is humidity ratio the same as relative humidity?

No.

Humidity ratio represents the mass of water vapor per unit mass of dry air.

Relative humidity compares actual vapor pressure with the saturated vapor pressure at the same temperature.

Relative humidity changes when air temperature changes, even when humidity ratio remains constant.

What is the difference between humidity ratio and moisture content?

In this calculator, moisture content is humidity ratio displayed in more convenient units.

In IP mode, it is shown in grains/lb dry air.

In SI mode, it is shown in g/kg dry air.

Why does altitude affect humidity ratio?

Humidity ratio depends on both vapor pressure and total atmospheric pressure.

At higher altitude, total atmospheric pressure is lower. For the same temperature and relative humidity, this generally produces a higher humidity ratio.

Why does altitude affect air density?

Air density decreases as atmospheric pressure decreases.

At high altitude, the same volumetric airflow contains less air mass than at sea level.

Is saturated vapor pressure affected by altitude?

Saturated vapor pressure is primarily determined by temperature.

Altitude changes total atmospheric pressure, but the saturated vapor pressure at a given temperature remains approximately the same.

What is the difference between dew point and wet bulb?

Dew point is the temperature at which condensation begins when air is cooled without changing moisture content.

Wet-bulb temperature reflects the cooling effect of water evaporation from a wetted surface.

Dew point is normally lower than or equal to wet-bulb temperature.

Can I use this calculator for cooling-coil selection?

The calculator can determine entering and leaving air properties, but full coil selection also requires information such as:

• Airflow
• Entering and leaving conditions
• Sensible and latent loads
• Coil apparatus dew point
• Bypass factor
• Water or refrigerant conditions
• Manufacturer performance data

What atmospheric pressure is used at sea level?

The calculator uses standard sea-level atmospheric pressure of approximately:

• 29.921 in. Hg
• 101.325 kPa

Why might the result differ from a psychrometric chart?

Printed charts involve graphical interpolation and may use different atmospheric pressures or equation sets.

Small differences are normal, especially after rounding.

Final Notes

Psychrometric calculations are fundamental to HVAC design because temperature alone does not fully describe an air condition.

By combining dry-bulb temperature with wet-bulb temperature or relative humidity, this calculator determines the moisture and energy properties needed for practical HVAC analysis.

Use the results for preliminary design, technical checks, education, troubleshooting, and comparison with psychrometric charts. For final equipment selection or critical engineering work, confirm the calculation basis, local atmospheric pressure, and manufacturer performance data.