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Machine Efficiency Calculator

Find the efficiency of a machine.

How it is calculated

Efficiency compares the useful work a machine delivers with the work put into it: η = (Output work / Input work) × 100. Both values must be in the same unit (joules, kilojoules, kWh), and the same applies if you use power in watts for both. Input must be above zero and output cannot be negative. The result is a percentage with two decimals. For an ordinary machine the result is at or below 100%, because some energy always leaves as heat, friction or sound; a figure above 100% means the numbers were swapped or measured inconsistently. The tool does not stop you, so check the output yourself.

Worked example

An electric winch takes 1,000 J of electrical energy and lifts a load, gaining 800 J of useful mechanical energy. Divide output by input: 800 / 1,000 = 0.8. Multiply by 100 to get an efficiency of 80.00%. The other 200 J (20%) is lost, mostly as heat in the motor and friction in the cable drum.

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Common mistakes

  • Swapping input and output, which gives a value above 100% that looks like a perpetual motion machine.
  • Mixing units, such as output in kJ and input in J, which is off by a factor of 1000.
  • Comparing power to energy, like input in kW and output in kWh.
  • Counting only part of the input, such as fuel energy but not the electricity for pumps and fans.
  • Treating efficiency as a constant; it changes with load, speed and temperature.

About this calculator

This calculator computes mechanical efficiency, η = (output work or energy / input work or energy) × 100, the percentage of energy put into a machine that actually shows up as useful output rather than being lost — mostly to friction, heat, and vibration. Because no real machine is perfectly efficient, η is always below 100%, and the gap between input and output is a direct measure of how much energy the machine wastes.

Mechanical engineers use this figure to compare motors, engines, gearboxes, and pulley systems, and to decide when a maintenance issue (like a worn bearing) is dragging performance down. It's also a standard homework calculation in physics and engineering courses covering work, energy, and simple machines, where students measure input and output work directly to find how a real system compares to an ideal one.

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