Efficiency Calculator

Created by Bogna Szyk
Reviewed by Jack Bowater
Last updated: Jul 22, 2022

This energy efficiency calculator is a simple tool for calculating the ratio of useful energy output to the energy input. You can use it for determining the proportions of heat energy, electric power, mechanical work, or even chemical energy. Continue reading to learn how to calculate efficiency, and discover the real-life applications of the efficiency formula.

How to use the efficiency calculator

The efficiency calculator is simple to use:

  • Enter two of the three values.
  • The efficiency calculator will work out the remaining value and display it in its field.

What is efficiency?

Efficiency is defined as the ratio of energy output to energy input. Every time that you supply energy or heat to a machine (for example, to a car engine), a certain part of this energy is wasted, and only some is converted to output in the form of actual work. The more efficient the machine, the greater output it produces for a given input.

A special type of efficiency is the Carnot efficency. It is defined as the efficiency of the Carnot engine, which is an ideal engine that maximizes the energy output.

How to calculate efficiency?

In order to calculate efficiency, you need to apply the following formula:

η=Eout/Ein100%η = E_\text{out} / E_\text{in} \cdot 100\%


  • ηη is the efficiency (expressed as a percentage),
  • EoutE_\text{out} is the energy output (in Joules), and
  • EinE_\text{in} is the energy input (also in Joules).

Any efficiency calculated from real-world values will be between 0% and 100%.

  • An efficiency of 0% means that all the input energy is wasted, and the energy output is equal to zero.
  • On the other hand, an efficiency of 100% means that there is no waste of energy whatsoever.

The fundamental law of energy conservation states that you cannot create energy. Therefore, the efficiency of any machine can never exceed 100%. Nevertheless, you will probably come across articles saying that LED lights or heat pumps can have an efficiency of 300% or more.

How is it possible? The apparent efficiency of 300% results from the definition of efficiency that we use. The electrical power supplied to LED lights might be actually lower than the output, but it doesn't mean that energy was created in the process. It merely means that the lights have received some heat energy from the surroundings and converted it into the output energy. As we aren't able to measure this additional input, the apparent efficiency rises above 100%.

Real-life applications

Even though you probably don't notice it, we apply the definition of efficiency to other real-life phenomena. Some examples include:

  • Return On Investment (ROI). If you look at the ROI formula more closely, you will see that it's analogical to the efficiency equation. This value describes what the "efficiency" of your money on an investment is. Unlike energy efficiency, the ROI metric can (and in fact should) exceed 100% --- if it's lower than 100%, you're losing money!

  • Fuel efficiency. Even though the formula for MPG (miles per gallon) is not directly related to the efficiency equation, it describes how efficiently does your engine convert fuel to actual power. The less efficient the engine is, the more fuel it needs to cover the same distance.


How do I calculate the efficiency of a machine?

To calculate the efficiency of a machine, proceed as follows:

  1. Determine the energy supplied to the machine or work done on the machine.

  2. Find out the energy supplied by the machine or work done by the machine.

  3. Divide the value from Step 2 by the value from Step 1 and multiply the result by 100.

  4. Congratulations! You have calculated the efficiency of the given machine.

What is the unit of efficiency?

Efficiency is a unitless quantity. It is the ratio of the energy output to the energy input; hence it has no units.

What is meant by 60% efficiency of a machine?

An efficiency of 60% means that only 60% of the energy supplied to the machine can be converted into useful work and the rest is lost.

Can a real machine have an efficiency of 100%?

No, a real machine can't have an efficiency of 100%. An efficiency of 100% means that there is no loss and the output energy (or work) of the machine is equal to input energy (or work). In real machines, there is always some loss of energy to overcome friction and air resistance.

Bogna Szyk
Energy input
Energy output
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