Enthalpy Calculator From Dry-Bulb Temperature

An enthalpy calculator cannot determine moist-air enthalpy from dry-bulb temperature alone: it also needs moisture information, usually relative humidity, plus air pressure. Enter those conditions and the calculator below estimates the specific enthalpy of moist air in kJ/kg of dry air using a standard psychrometric approximation.

Formula

The calculator estimates moist-air enthalpy per kilogram of dry air with the commonly used HVAC approximation:

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

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

Humidity ratio is derived from relative humidity and saturation vapor pressure:

w = 0.621945pᵥ / (p − pᵥ), where pᵥ = φpₛ.

Here, φ is relative humidity as a fraction (the entered percentage divided by 100), pₛ is saturation vapor pressure, and p is total air pressure. The calculator approximates saturation vapor pressure in kPa as pₛ = 0.6112 exp(17.67t / (t + 243.5)). In the calculator expression, 611.2 is used in Pa, and pressure entered in kPa is multiplied by 1,000 to convert it to Pa. The coefficient 0.621945 is the ratio of the molecular masses of water vapor and dry air. The enthalpy coefficients are conventional engineering approximations, not universal constants.

Worked Example

Using the calculator’s default inputs: dry-bulb temperature 25 °C, relative humidity 50%, and pressure 101.325 kPa.

1. Estimate saturation vapor pressure: pₛ = 0.6112 × exp(17.67 × 25 / (25 + 243.5)) ≈ 3.1685 kPa.
2. Find water-vapor partial pressure: pᵥ = 0.50 × 3.1685 ≈ 1.5843 kPa.
3. Calculate humidity ratio: w = 0.621945 × 1.5843 / (101.325 − 1.5843) ≈ 0.009879 kg/kg.
4. Calculate enthalpy: h = 1.006 × 25 + 0.009879 × (2501 + 1.86 × 25) ≈ 50.32 kJ/kg dry air.

The headline result rounds to 50.32 kJ/kg dry air. Intermediate figures are rounded for display; using full precision in the formula gives the calculator result.

Assumptions

• Air is treated as a mixture of dry air and water vapor, with no liquid water or ice included in the enthalpy value.
• The saturation-pressure relationship and enthalpy coefficients are engineering approximations, most suitable for ordinary HVAC conditions near atmospheric pressure.
• Pressure is absolute total pressure, not gauge pressure. The formula assumes the entered relative humidity describes the air at the entered dry-bulb temperature.
• The result is per kilogram of dry air, not per kilogram of the complete moist-air mixture.

Why Dry-Bulb Temperature Is Not Enough

Dry-bulb temperature is the ordinary air temperature measured by a thermometer shielded from direct radiant heating. It describes sensible thermal state, but not how much water vapor is present. At the same dry-bulb temperature, dry air and humid air have different total enthalpies because water vapor carries latent energy as well as sensible energy.

For example, air at 25 °C and 50% relative humidity has a different humidity ratio and enthalpy from air at 25 °C and 90% relative humidity. A dry-bulb-only calculation must silently assume a moisture condition; that assumption can lead to a misleading answer. Relative humidity is a convenient extra input, though dew point or humidity ratio can also define the moisture content if a calculator is designed for those inputs.

Using and Interpreting the Result

Choose the measured or specified dry-bulb temperature, relative humidity, and local absolute pressure. Relative humidity must be between 0 and 100%. For pressure, use local atmospheric pressure where known; standard sea-level pressure is approximately 101.325 kPa, but elevation and weather can change it. Do not enter a gauge-pressure reading as though it were absolute pressure.

The displayed humidity ratio and vapor partial pressure help explain the result. Enthalpy increases with temperature and, at a fixed temperature, generally increases as moisture content rises. In heating, ventilation, and air-conditioning calculations, enthalpy differences can help estimate energy changes when air is heated, cooled, humidified, or dehumidified. A total energy rate still requires a mass-flow rate and consistent units; this calculator returns a specific property, not a heating or cooling load.

Accuracy, Range, and Common Mistakes

This simplified equation is a practical psychrometric estimate, not a substitute for a full moist-air property model where high accuracy is needed. The saturation-pressure fit is intended for common air-conditioning conditions and can lose accuracy at extreme temperatures. Near saturation, rounding, sensor error, and condensation can have a noticeable effect. If the air temperature is below freezing, liquid-water and ice saturation conventions matter; this calculator does not distinguish them.

• Confusing enthalpy and temperature: enthalpy includes moisture effects; it is not another way to report dry-bulb temperature.
• Mixing pressure units: this calculator takes kPa, while its internal vapor-pressure relationship uses Pa and converts explicitly.
• Reading the denominator incorrectly: humidity ratio uses total pressure minus vapor partial pressure, p − pᵥ.
• Misreading the basis: kg dry air is the denominator. It is not kg of moist air.
• Assuming a default humidity: 50% is only the calculator’s example setting, not a general property of air.

For design decisions, check the input measurements and use an appropriate psychrometric chart, validated property software, or engineering standard for the temperature and pressure range. Small differences between references may arise from different saturation-pressure equations and reference conventions.

Frequently Asked Questions

Can I calculate air enthalpy from dry-bulb temperature alone?
No. You also need the air’s moisture content, such as relative humidity, dew point, or humidity ratio, and generally total pressure. Without it, multiple enthalpy values are possible at the same temperature.

What does kJ/kg dry air mean?
It is energy per unit mass of dry air in the moist-air mixture. Water vapor contributes to the enthalpy, but the denominator is the mass of dry air alone, a common HVAC convention.

Can I use dew point instead of relative humidity?
Yes, if you first use dew point and pressure to determine water-vapor partial pressure and humidity ratio. This calculator specifically accepts relative humidity, so do not enter a dew-point value in that field.

Is 101.325 kPa always the right pressure?
No. It is a standard sea-level reference pressure. Use local absolute pressure when available, especially at higher elevations or when accurate psychrometric calculations matter.

Why do two enthalpy calculators give slightly different answers?
They may use different saturation-vapor-pressure equations, constants, reference states, or rounding. Compare the formula, input pressure, and whether enthalpy is reported per kilogram of dry air before judging the difference.

Does this calculate an HVAC heating or cooling load?
No. It calculates specific moist-air enthalpy. A load estimate also needs air mass flow and a change in enthalpy, along with appropriate system and design assumptions.

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in t | Dry-bulb temperature | °C | 25 | -273.15
in rh | Relative humidity | % | 50 | 0 | 100
in p | Air pressure | kPa | 101.325 | 0.001
out h | Moist-air enthalpy | kJ/kg dry air | 1.006 * t + (rh / 100 * 0.621945 * (611.2 * exp(17.67 * t / (t + 243.5)) / (p * 1000 - rh / 100 * 611.2 * exp(17.67 * t / (t + 243.5)))) * (2501 + 1.86 * t)) | 2
out w | Humidity ratio | kg water/kg dry air | rh / 100 * 0.621945 * (611.2 * exp(17.67 * t / (t + 243.5)) / (p * 1000 - rh / 100 * 611.2 * exp(17.67 * t / (t + 243.5)))) | 5
out pv | Water-vapor partial pressure | kPa | rh / 100 * 611.2 * exp(17.67 * t / (t + 243.5)) / 1000 | 3