Calculate dry air enthalpy or temperature from the other using h ≈ 1.005·T for dry air, with results in kJ/kg or BTU/lb and °C, °F, or K.

Dry Air Enthalpy Calculator

Uses the approximation h ≈ 1.005·T (h in kJ/kg, T in °C relative to 0°C). For moist air, enthalpy depends on humidity.

Enter any 1 value to calculate the missing variable

Dry Air Enthalpy Formula

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

h = c_p T
h \approx 1.005T

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

T = h/1.005
  • h = specific enthalpy of dry air, in kJ/kg
  • cp = specific heat of dry air at constant pressure, approximately 1.005 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 = 1.005T. If you enter enthalpy, it converts it to kJ/kg first, then calculates temperature using T = h / 1.005.

For unit handling, the calculator uses these conversions:

T_{\degree C} = (T_{\degree F} - 32)\cdot 5/9
T_{\degree C} = T_K - 273.15
1\ \text{BTU/lb} \approx 2.326\ \text{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 10.050 kJ/kg 4.321 BTU/lb
20°C 20.100 kJ/kg 8.641 BTU/lb
25°C 25.125 kJ/kg 10.802 BTU/lb
30°C 30.150 kJ/kg 12.962 BTU/lb
40°C 40.200 kJ/kg 17.283 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 = 1.005T
h = 1.005 \times 25 = 25.125\ \text{kJ/kg}

The dry air enthalpy is 25.125 kJ/kg.

Example 2: Find temperature from dry air enthalpy

If the dry air enthalpy is 20.1 kJ/kg, calculate the air temperature in °C.

T = h/1.005
T = 20.1/1.005 = 20\degree 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 1.005?

The value 1.005 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?

No. Moist air enthalpy also includes the effect of water vapor and depends on humidity or humidity ratio. This calculator is for dry air only, so it is best used when humidity is ignored or treated separately.