Psychrometric Enthalpy Calculator and Formula

A psychrometric enthalpy calculator estimates the enthalpy of moist air from its dry-bulb temperature, relative humidity, and atmospheric pressure. Enter those three values to get specific enthalpy, usually reported in kilojoules per kilogram of dry air, along with humidity ratio and dew point for context.

Formula

The calculator uses a common engineering approximation for moist-air enthalpy:

h = 1.006t + w(2501 + 1.86t)

• h is moist-air enthalpy in kJ per kg of dry air.
• t is dry-bulb temperature in °C.
• w is humidity ratio, in kg of water vapour per kg of dry air.

Humidity ratio is calculated from water-vapour partial pressure: w = 0.621945 pw / (p − pw). Here, p is total atmospheric pressure in kPa and pw is water-vapour partial pressure in kPa. The calculator estimates saturation vapour pressure using the Buck-style approximation pws = 6.112 exp(17.67t / (t + 243.5)), with pressure in kPa. It then sets pw = (RH/100)pws. Relative humidity, RH, is entered as a percentage.

The dew-point estimate reverses the same saturation-pressure approximation: it finds the temperature at which the saturation vapour pressure equals the current vapour pressure. These equations are engineering approximations, not a full equation-of-state model.

Worked Example

Using the calculator defaults—25 °C, 50% relative humidity, and 101.325 kPa—first estimate saturation vapour pressure:

pws = 6.112 exp(17.67 × 25 / (25 + 243.5)) ≈ 3.1670 kPa

The vapour pressure is half of that at 50% RH:

pw = 0.50 × 3.1670 ≈ 1.5835 kPa

Then calculate humidity ratio:

w = 0.621945 × 1.5835 / (101.325 − 1.5835) ≈ 0.0098737 kg/kg dry air

Substitute into the enthalpy formula:

h = 1.006 × 25 + 0.0098737 × (2501 + 1.86 × 25) ≈ 50.32 kJ/kg dry air

The approximate dew point is about 13.9 °C. Small differences in displayed intermediate digits may change the last rounded digit.

Assumptions

• Air and water vapour are treated as an ideal moist-air mixture at the stated total pressure.
• The saturation-pressure approximation is intended for ordinary HVAC conditions; accuracy declines outside its useful temperature range.
• Enthalpy is expressed per kilogram of dry air, not per kilogram of the combined moist-air mixture.
• The enthalpy reference convention is embedded in the constants: dry air at 0 °C and liquid water at 0 °C are the usual reference states.

What Psychrometric Enthalpy Means

Psychrometrics describes mixtures of dry air and water vapour. Enthalpy is a measure of the mixture’s energy relative to a chosen reference state. In HVAC calculations it is useful because it accounts for both sensible energy associated with temperature and latent energy associated with moisture.

The first term, 1.006t, represents the approximate sensible enthalpy of dry air. The second term includes water vapour: 2501 kJ/kg is the approximate latent contribution near the reference temperature, while 1.86 kJ/(kg·°C) accounts for the vapour’s sensible heat. These coefficients are approximate specific heats and latent heat values used in conventional psychrometric calculations; they are not universal constants for every condition.

A common misconception is that enthalpy is simply temperature. Two air samples at the same dry-bulb temperature can have different enthalpies if their moisture contents differ. Conversely, a warmer, drier sample and a cooler, more humid sample can sometimes have similar enthalpy while having very different comfort and condensation characteristics.

When to Use the Result

Enthalpy helps compare air states and estimate the energy change when air is heated, cooled, humidified, or dehumidified. For a process with dry-air mass flow rate, the approximate heat-transfer rate is the dry-air mass flow multiplied by the enthalpy difference. If mass flow is in kg/s and enthalpy difference is in kJ/kg, the result is in kJ/s, equivalent to kW.

• Compare entering and leaving air conditions across a cooling or heating coil.
• Estimate the total energy change during ventilation or air mixing, provided the states and dry-air flow rates are known.
• Check whether a change in moisture content is important alongside a temperature change.

Enthalpy alone does not determine equipment capacity or performance. Real systems also involve airflow, heat gains, pressure losses, controls, coil characteristics, and possible condensation. Use a psychrometric chart or a more detailed model when design accuracy, unusual conditions, or compliance requirements matter.

Inputs, Limits, and Common Errors

Use dry-bulb temperature, not wet-bulb temperature, for the temperature field. Relative humidity must be entered as a percentage from 0 to 100; entering 0.50 to represent 50% would be interpreted as 0.50%. Atmospheric pressure means local total air pressure, not water-vapour pressure. Standard sea-level pressure is a convenient default, but elevation and weather can change it.

• Check the pressure unit: the saturation-pressure equation returns kPa, so total pressure must also be in kPa.
• Keep the basis clear: the output is per kg of dry air. It is not per kg of humid air.
• Avoid false precision: measured temperature and humidity have uncertainty, and the equations use rounded coefficients.
• Do not extrapolate casually: the saturation-pressure expression and constant heat capacities are approximations. Very low temperatures, high pressures, and conditions outside normal air-conditioning work may need validated property data.

At 0% relative humidity the calculated humidity ratio is zero and dew point is not physically defined as a finite value. A calculator may therefore be unable to return a meaningful dew-point result at exactly zero humidity. Saturation at 100% RH is mathematically possible, but measurement error can make an apparent value above saturation; such a state should be checked rather than treated as ordinary unsaturated air.

Frequently Asked Questions

What is a typical unit for psychrometric enthalpy?
It is commonly expressed as kJ/kg of dry air in SI calculations. Some charts and references use Btu per pound of dry air, so verify the basis and units before comparing figures.

Does higher relative humidity always mean higher enthalpy?
At a fixed temperature and pressure, increasing relative humidity increases humidity ratio and therefore enthalpy in this model. Across different temperatures or pressures, relative humidity alone is not enough to compare enthalpy.

Why does the calculator need atmospheric pressure?
Humidity ratio depends on the ratio of water-vapour pressure to the remaining dry-air pressure. At a different total pressure, the same temperature and relative humidity imply a different humidity ratio.

Is enthalpy the same as heat?
No. Enthalpy is a property of a state, reported here per unit mass of dry air. Heat is energy transferred during a process. A change in enthalpy can help estimate heat transfer under suitable process assumptions, but the terms are not interchangeable.

How can I verify the result?
Compare the temperature, humidity ratio, and enthalpy with a psychrometric chart or a trusted psychrometric property tool using the same pressure and reference convention. Expect small differences between charts and approximations.

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in t | Dry-bulb temperature | °C | 25 | -50 | 100
in rh | Relative humidity | % | 50 | 0 | 100
in p | Atmospheric pressure | kPa | 101.325 | 1 | 200
out h | Moist-air enthalpy | kJ/kg dry air | 1.006 * t + (rh / 100 * 6.112 * exp(17.67 * t / (t + 243.5)) * 0.621945 / (p - rh / 100 * 6.112 * exp(17.67 * t / (t + 243.5)))) * (2501 + 1.86 * t) | 2
out w | Humidity ratio | kg water/kg dry air | rh / 100 * 6.112 * exp(17.67 * t / (t + 243.5)) * 0.621945 / (p - rh / 100 * 6.112 * exp(17.67 * t / (t + 243.5))) | 5
out td | Approximate dew point | °C | 243.5 * ln((rh / 100 * 6.112 * exp(17.67 * t / (t + 243.5))) / 6.112) / (17.67 - ln((rh / 100 * 6.112 * exp(17.67 * t / (t + 243.5))) / 6.112)) | 2