What formula does this calculator use?

For a reversible process, DS = Q_rev / T, where Q is heat transferred in joules and T is absolute temperature in Kelvin. For an irreversible process, DS > Q / T, so the calculator shows the lower-bound value and labels it accordingly.

Why must temperature be in Kelvin?

Entropy calculations require absolute temperature. The Kelvin scale starts at absolute zero (0 K = -273.15 °C), ensuring T is never zero or negative. Entering Celsius is allowed and the tool adds 273.15 automatically.

What does a positive or negative DS mean?

A positive DS means the system gains entropy (e.g., heat absorbed, disorder increases). A negative DS means the system loses entropy (e.g., heat released). The total entropy of the universe always increases or stays the same — the second law of thermodynamics.

What is the difference between reversible and irreversible processes?

A reversible process proceeds through equilibrium states and produces no entropy in the universe. An irreversible process (friction, free expansion, mixing) always generates additional entropy. For irreversible processes, DS_system > Q / T.

Yes. An adiabatic reversible (isentropic) process has Q = 0, giving DS = 0. This is the case in ideal compressors and turbines.

What units does the result use?

Results are shown in J/K (the SI unit), kJ/K, and cal/K simultaneously so you can pick the most convenient unit for your work.

Entropy Change Calculator

Calculate entropy change (DS = Q/T) for reversible and irreversible thermodynamic processes. Supports multiple temperature and energy units.

Entropy Change — ΔS = Q / T

Enter heat transferred and temperature to compute ΔS.

Process Type

Reversible: ΔS = Q / T (exact)

Heat Transferred (Q) — positive = absorbed, negative = released

Absolute Temperature (T)

Temperature must be above absolute zero (0 K / −273.15 °C).

Result

Enter values and click Calculate

Reference: Phase-Change Entropy

Standard molar entropy of fusion/vaporization at 1 atm. Click a row to auto-fill Q and T.

Substance	Process	T (K)	Q (kJ/mol)

Summary

The Entropy Change Calculator computes the change in entropy (DS) for thermodynamic processes based on the Clausius definition DS = Q_rev / T. For reversible processes the formula is exact; for irreversible processes the tool shows the lower bound (DS > Q/T). Enter heat transferred and the absolute temperature at which the transfer occurs to get the entropy change in J/K, kJ/K, or cal/K.

How it works

Select whether the process is reversible or irreversible.
Enter the heat transferred (Q) — positive for heat absorbed, negative for heat released.
Choose the heat unit (J, kJ, or cal).
Enter the absolute temperature (T) in Kelvin, or switch to Celsius and the tool converts automatically.
Click Calculate to see DS with a step-by-step breakdown and process interpretation.

Use cases

Verify the second law of thermodynamics for a given process.
Calculate entropy change during isothermal expansion of an ideal gas.
Determine DS for phase transitions such as melting or vaporization.
Solve thermodynamics homework problems involving the Clausius inequality.
Compare entropy production between reversible and irreversible paths.
Analyze heat engine cycles and refrigeration efficiency.

Frequently Asked Questions

Last updated: 2026-09-30 · Reviewed by Nham Vu