Efficiency Calculator: Efficiency = Output / Input

How much of the input becomes useful output.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose which quantity you want to find.

Enter the other values in the units shown.

The calculator gives the answer with the formula, the substituted numbers, and the answer in other units.

Formulas used

  • \eta = \frac{E_{out}}{E_{in}}\times100
  • E_{out} = \frac{\eta\,E_{in}}{100}

Worked example

With the default numbers in the calculator above:

  1. The formula, arranged to find the efficiency: efficiency = (useful output) / (total input) x 100
  2. Put the values in (every value in its SI unit): efficiency = (750) / (1000) x 100
  3. The answer: efficiency: 75 %
  4. Energy or power lost (input minus useful output): 250 (25 % of the input)

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: What do you want to find? Total input; Efficiency (leave it as it is if this is what you are finding) 75 %; Useful output (leave it as it is if this is what you are finding) 750 units.

  1. The formula, arranged to find the total input: total input = (100 x useful output) / (efficiency)
  2. Put the values in (every value in its SI unit): total input = (100 x 750) / (75)
  3. The answer: total input: 1000 units
  4. Energy or power lost (input minus useful output): 250 (25 % of the input)

Common questions

What is the formula for efficiency?

Efficiency equals the useful output divided by the total input, times 100 percent. 750 J out for 1000 J in is 75 percent.

Where does the lost energy go?

Mostly to heat, through friction, electrical resistance and air resistance.

Can the efficiency be more than 100 percent?

Not for a machine. A heat pump can deliver more heat than the electrical energy it uses, but that is called a coefficient of performance, and it takes extra heat from outside.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any school physics text (efficiency).

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