Entropy of Air Calculator

Calculator Inputs

Calculation mode

Basis

Quantity

Enter the amount of air in kg.

Temperature unit

Pressure unit

Heat capacity Cp

Default dry-air value on mass basis: 1.005 kJ/kg·K.

Gas constant R

Default dry-air value on mass basis: 0.287 kJ/kg·K.

State 1 and State 2

Initial temperature

Initial pressure

Final temperature

Final pressure

Current State and Reference State

Current temperature

Current pressure

Reference temperature

Reference pressure

Model note: This calculator treats dry air as an ideal gas and uses a constant heat capacity. For wide temperature ranges or humid air, use more advanced property methods.

Formula Used

Entropy change between two states:

Δs = Cp ln(T₂ / T₁) − R ln(P₂ / P₁)

Entropy relative to a reference state:

s − s_ref = Cp ln(T / T_ref) − R ln(P / P_ref)

Where: T is absolute temperature in kelvin, P is absolute pressure, Cp is constant-pressure heat capacity, and R is the gas constant. Keep units consistent. Use mass basis for kJ/kg·K or molar basis for kJ/mol·K.

The total entropy result is the specific entropy value multiplied by the selected quantity of air.

How to Use This Calculator

Choose whether you want entropy change between two states or entropy relative to a reference state.
Select a basis. Use mass basis for kilograms or molar basis for moles.
Pick one temperature unit and one pressure unit for the whole calculation.
Enter your air quantity, then confirm or edit the default Cp and R values.
Fill the visible state fields and press Calculate Entropy of Air.
The result appears above the form, with a graph plus CSV and PDF download buttons.

Example Data Table

Mode	Inputs	Basis	Approximate Result
Entropy change	T₁ = 25°C, P₁ = 1 atm, T₂ = 125°C, P₂ = 2 atm, quantity = 2 kg	Mass basis	Specific Δs ≈ 0.091 kJ/kg·K, total ≈ 0.182 kJ/K
Reference entropy	T = 60°C, P = 1.5 bar, T_ref = 25°C, P_ref = 1 bar, quantity = 3 kg	Mass basis	Specific s ≈ -0.004 kJ/kg·K, total ≈ -0.013 kJ/K
Entropy change	T₁ = 150°C, P₁ = 5 bar, T₂ = 50°C, P₂ = 1 bar, quantity = 1.5 kg	Mass basis	Specific Δs ≈ 0.192 kJ/kg·K, total ≈ 0.288 kJ/K

FAQs

1) What equation does this entropy calculator use?

It uses the ideal-gas entropy relation for dry air with constant heat capacity: Δs = Cp ln(T₂/T₁) − R ln(P₂/P₁). The reference mode uses the same relation against a selected baseline state.

2) Does this give absolute entropy?

No. It gives entropy change between two states or entropy relative to a chosen reference state. Absolute thermodynamic entropy requires a more detailed property framework and reference convention.

3) Why can entropy rise when pressure falls?

For an ideal gas, lower pressure means more available volume and greater molecular dispersal. That increases entropy, which is why the pressure term carries a negative sign in the formula.

4) Can I enter Celsius, Fahrenheit, bar, or psi?

Yes. The calculator accepts Kelvin, Celsius, and Fahrenheit for temperature. It also accepts Pa, kPa, bar, atm, and psi for pressure, then converts them internally.

5) Should I keep the default Cp and R values?

Use the defaults for dry air in ordinary engineering work. Change them only when you have a different property model, special gas mixture, or a basis change between mass and molar units.

6) What does the quantity field change?

Quantity scales the specific entropy result into a total entropy result. On mass basis, use kilograms. On molar basis, use moles. The total output is shown in kJ/K.

7) Is this calculator valid for humid air?

Not exactly. Moist air contains water vapor, so its entropy depends on mixture composition and humidity. This page is best treated as a dry-air ideal-gas calculator.

8) When will this method lose accuracy?

Accuracy drops for large temperature spans, real-gas behavior, very high pressures, and humid conditions. In those cases, use temperature-dependent properties or a full air-property database.