Calculate moist or dry air enthalpy from temperature, relative humidity, and pressure, or solve dry-air temperature in SI and HVAC imperial units.

Relative humidity uses water saturation at/above 0°C and ice below 0°C.

Approximate enthalpy per kg of dry air, referenced to 0°C. Converted Btu/lb retains this datum; it is not the usual 0°F HVAC chart datum.

Atmospheric pressure

Blank = 101.325 kPa, regardless of selected unit. Supported: 20–200 kPa.

Dry Air Enthalpy Formula

The dry air enthalpy calculator uses a constant-pressure specific heat approximation for dry air:

h = câ‚š T
h ≈ 1.006T

To solve for temperature from enthalpy, the calculator rearranges the same formula:

T = h / 1.006
  • h = specific enthalpy of dry air, in kJ/kg
  • cp = specific heat of dry air at constant pressure, approximately 1.006 kJ/(kg·K)
  • T = air temperature in °C, measured relative to 0°C

If you enter temperature, the calculator converts it to °C first, then calculates dry air enthalpy using h = 1.006T. If you enter enthalpy, it converts it to kJ/kg first, then calculates temperature using T = h / 1.006.

For unit handling, the calculator uses these conversions:

Tdegree C = (Tdegree F - 32) × 5 / 9
Tdegree C = TK - 273.15
1 BTU / lb ≈ 2.326 kJ / kg

Common Dry Air Enthalpy Values

These values use the same dry air approximation, with enthalpy referenced to 0°C.

Air temperature Dry air enthalpy Approximate value in BTU/lb
0°C 0.000 kJ/kg 0.000 BTU/lb
10°C 10.060 kJ/kg 4.325 BTU/lb
20°C 20.120 kJ/kg 8.650 BTU/lb
25°C 25.150 kJ/kg 10.813 BTU/lb
30°C 30.180 kJ/kg 12.975 BTU/lb
40°C 40.240 kJ/kg 17.300 BTU/lb

Temperature and Enthalpy Unit Conversions

Conversion Formula or factor
°F to °C (°F − 32) × 5/9
K to °C K − 273.15
kJ/kg to BTU/lb kJ/kg × 0.429922614
BTU/lb to kJ/kg BTU/lb × 2.326

Example Problems

Example 1: Find dry air enthalpy from temperature

If the dry air temperature is 25°C, calculate the specific enthalpy.

h = 1.006T
h = 1.006 × 25 = 25.150 kJ / kg

The dry air enthalpy is 25.150 kJ/kg.

Example 2: Find temperature from dry air enthalpy

If the dry air enthalpy is 20.12 kJ/kg, calculate the air temperature in °C.

T = h / 1.006
T = 20.12 / 1.006 = 20degree C

The air temperature is 20°C.

FAQ

What does dry air enthalpy mean?

Dry air enthalpy is the sensible heat content of air without including water vapor. In this calculator, it is measured relative to 0°C, so air at 0°C has an enthalpy of 0 kJ/kg using this approximation.

Why does the calculator use 1.006?

The value 1.006 kJ/(kg·K) is the approximate specific heat of dry air at constant pressure near normal atmospheric conditions. Multiplying this value by the temperature change from 0°C gives the approximate specific enthalpy of dry air.

Is this the same as moist air enthalpy?

The moist-air mode includes water vapor using h = 1.006T + W(2501 + 1.86T), with humidity ratio W = 0.621945pv/(p − pv). Saturation pressure uses the ASHRAE 2017 ice/water equations; relative humidity is referenced to ice below freezing and water above. Pressure is absolute and defaults to 101.325 kPa. At 0% RH there is no finite dew/frost point; roots below −100°C are reported outside range. Dry-air modes ignore water vapor. All enthalpy units retain the 0°C datum, so converted Btu/lb is not the usual 0°F HVAC chart reference.