To calculate mechanical efficiency, divide work output by work input and multiple by 100. For example, you do 3,000 joules of work pushing a box up a ramp. The work done by the ramp is 2,400 joules. The mechanical efficiency of the ramp is 2,400 divided by 3,000, times 100. This equals 80%. That means the other 20% of the work input was used to overcome friction.