Entropy Change Calculator

Entropy is the measure of disorder, of how widely energy is spread out in a system, and entropy change tells you which way that disorder shifts during a process. This calculator works it out from the heat involved and the temperature. Enter the heat transferred reversibly and the temperature at which it happens, choose kelvin or Celsius, and it returns the entropy change in joules per kelvin, along with whether entropy increased or decreased, updating as you type. The relationship is one of the cleanest in thermodynamics: the entropy change equals the heat divided by the absolute temperature. Add heat to a system and its entropy rises; remove heat and it falls; and the same amount of heat changes the entropy more when the temperature is low than when it is high, which is a subtle but important feature of the formula. The result connects directly to the second law of thermodynamics, the deep principle that the total entropy of the universe always tends to increase, which is why heat flows from hot to cold, why engines can never be perfectly efficient, and why time seems to run one way. A crucial detail is that the temperature must be absolute, in kelvin, so the calculator converts any Celsius value for you by adding 273.15 before dividing. The same formula, using the enthalpy of a phase change at its transition temperature, gives the entropy of melting or boiling. That makes the tool genuinely useful for chemistry and physics students learning thermodynamics, entropy and the second law and checking homework, and for anyone needing a quick entropy change for a heat-transfer or phase-change problem. Using joules and kelvin gives the answer in joules per kelvin. The formula and a worked example are explained clearly below.

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Use positive heat for energy added to the system, negative for energy removed.