Entropy Change Calculator
Find entropy change from heat and temperature.
Results update as you type.
How it works
Entropy is a measure of disorder or, more precisely, the number of microscopic ways a system's energy can be arranged, and this calculator finds the entropy change during a reversible heat transfer at constant temperature. You enter the heat transferred in joules and the absolute temperature in kelvin, and it returns the entropy change in joules per kelvin. The formula, entropy change equals heat divided by temperature, comes from classical thermodynamics and is one of the foundations of the second law, which states that the total entropy of an isolated system never decreases. Adding heat to a system increases its entropy because the added energy can be distributed among more possible states, while extracting heat decreases it. The temperature in the denominator has an important consequence: transferring a given amount of heat changes the entropy more when the temperature is low than when it is high, which is why heat naturally flows from hot to cold and why engines cannot be perfectly efficient. Entropy underlies the direction of spontaneous processes, the efficiency limits of heat engines and refrigerators, and concepts across chemistry, physics, and even information theory. Enter the heat and temperature to compute the change, remembering that heat added is positive and heat removed is negative.
ΔS = Q / T
For a reversible process at constant temperature, the change in entropy equals the heat transferred divided by the absolute temperature in kelvin. Adding heat to a system raises its entropy because energy spreads out among more microscopic arrangements, while removing heat lowers it. The temperature in the denominator means the same amount of heat produces a larger entropy change at low temperature than at high temperature.
Source:Thermodynamics (Clausius definition of entropy)
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Worked example
How to use this calculator
- 1
Enter your values
Enter heat transferred (q) and temperature (t).
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Entropy change (ΔS) updates instantly as you type — no button to press.
Frequently asked questions
What is entropy?
Entropy is a thermodynamic quantity that measures the number of microscopic arrangements consistent with a system's overall state, often described loosely as the amount of disorder. Higher entropy means energy and matter are more spread out and less ordered. It is central to the second law of thermodynamics, which says the total entropy of an isolated system tends to increase over time, setting the direction of natural processes.
How do I calculate entropy change?
For a reversible process at constant temperature, divide the heat transferred by the absolute temperature in kelvin. Heat added to the system counts as positive and raises entropy, while heat removed counts as negative and lowers it. For processes where temperature changes, you would integrate over the process, but the simple ratio applies well to isothermal steps and is the basis of the calculation here.
Why must temperature be in kelvin?
Entropy is defined using absolute temperature, which starts at absolute zero, and only the kelvin scale measures this correctly. Using Celsius or Fahrenheit, which have arbitrary zero points, would give meaningless results and could even involve dividing by zero or negative numbers. Convert any Celsius temperature to kelvin by adding 273.15 before calculating the entropy change.
What does the second law of thermodynamics say?
The second law states that the total entropy of an isolated system never decreases; it either stays the same in a reversible process or increases in any real, irreversible one. This is why heat flows spontaneously from hot to cold, why perpetual motion machines are impossible, and why no heat engine can convert all its input heat into work. Entropy sets the arrow of time for physical processes.
Can entropy decrease?
The entropy of a particular system can decrease locally, for example when water freezes into ordered ice, but only if entropy increases by at least as much somewhere else, such as in the surroundings that absorb the released heat. The total entropy of the system plus its surroundings never decreases. A refrigerator lowers the entropy inside it only by dumping more entropy into the room.