How to: find a percentage of a quantity

17 05 2010

Find 27% of 250. Need some help with how to do this? If yes, read on!

The secret to understanding what percentages are is hidden in the word itself. The word ‘percent’ originated in the mid-16th century from Latin in which ‘centum’ means one hundred. Percent can thus be thought of as meaning ‘per one hundred’. When you see a percentage, the first thing that should go through your mind is ‘this means out of one hundred’.

Think about a glass that has some water in it. If we are told that the glass is 25% full of water what does that mean? Start off by imagining the glass were completely full of water. Then imagine that we split all the water in the glass into one hundred different groups that were all the same size. For example, if the glass had 200ml of water in when it is full then when we’d divided it into one hundred groups, each group would have 2ml of water in. Each one of the one hundred groups is 1%. So 1% of 200ml is 2ml. If 1% is 2ml then 25% must be 25 X 2 = 50ml.

This leads to the golden rule I always teach people about finding percentages of quantities:

To find 1% of something just divide what you’ve got by 100. Then multiply the answer by how many percent you need.

For example:

1% of 300 = 300 / 100 = 3

1% of 1000 = 1000 / 100 = 10

1% of 250 = 250 / 100 = 2.5

Once you know what 1% of something is it’s easy to work out any other percentage by multiplying. In the question above it asked what is 27% of 250.

Well 1% of 250 is 250 / 100 = 2.5

Now 27% of 250 must be 2.5 multiplied by 27 which is 67.5

Easy hey?!

Lets do one more example:

Find 34 % of 200

1% of 200 = 200 / 100 = 2

So 34% of 200 is 2 X 34 which is 68

There are lots of other rules that you can use to find percentages like halving to find 50% or dividing by 10 to find 10% but I find that lots of people get confused with these and end up doing things like dividing by 20 to find 20% which is totally wrong. If you stick to the golden rule you can’t go wrong with finding a percentage of a quantity!

To find 1% of something just divide what you’ve got by 100. Then multiply the answer by how many percent you need.

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