Free · Thermodynamics and heat calculators 🌡️

Ideal Heat Engine Carnot Efficiency

Find the reversible heat-engine efficiency limit between two reservoir temperatures, using absolute temperature internally.

Calculator inputs

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Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
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Reservoirs at 26.85°C and 326.85°C are 300 K and 600 K. Their ideal Carnot heat-engine efficiency is 50%.

Efficiency = 1 − cold absolute temperature / hot absolute temperature

Enter these known values and leave the other values blank.

The hot reservoir is above 0 K, the cold reservoir is at or above 0 K, and hot temperature is at least cold temperature. The result lies between 0% and 100%.
Celsius and Fahrenheit selections are converted to kelvins for the ratio. A ratio of Celsius values is not valid for this formula.
This is a reversible upper bound between fixed reservoirs, not the performance of a real engine or a heat pump’s coefficient of performance. The 0 K cold-reservoir boundary yields a formal 100% limit and is physically unattainable; it does not uniquely determine hot temperature when supplied with 100% efficiency.

Yes. Without a temperature difference, the ideal heat-engine efficiency is zero.

No. The temperature ratio must use an absolute scale. The calculator handles that conversion internally.

Bookify requires a hot-reservoir temperature above absolute zero and caps ideal heat-engine efficiency at 100%.