How to work out percentage

There are various ways to work out percentages – but the main starting point should always be, what sort of percentage are you looking to calculate?  The main ones are:

  • Calculating a certain percentage of a value
  • Calculating what percentage one number is of another number
  • Calculating a percentage increase
  • Calculating a percentage decrease

This site has a calculator specifically for each of these types of percentage and the page will detail the calculation to show you how to work out the percentage – but here’s a quick guide on each.

Work out a set percentage of a value

To work out what a certain percentage of a number is I find it best to work out what one percent would be and then just multiply the number of percent you want to find by that number.  Once you know what one percent is, the rest is easy! 

To find what one percent of a number all you need to remember is that percentages work around the fact that a whole is 100%.  One percent is one hundredth of the full amount – so all you need to do is to move the decimal point of the whole two places to the right.

If you want to find 15% of 1,367 start by working out 1%, move the decimal places one point to the right:

1,367 shifted = 136.7

The do it again!

136.7 shifted = 13.67

If 1% of 1,367 is 13.67 you can multiply it by 15 to find 15%.

13.67 x 15 = 205.5

Calculating what percentage one number is of another number

For this calculation you want to know what proportion the number that you want to express as a percentage is of the whole amount.  From there you can use the principle of a whole being 100% to show it as a percentage.

If you wanted to work out what percentage 36 is of 240 you just divide the percentage amount by the total amount and then multiply it by 100.

36 / 240 = 0.15 x 100 = 15

So 36 is 15% of 240.  Let’s think about this a bit more and show you another way to remember it – fractions!

A fraction is a portion – one fifth, expressed as a fraction is one over five.  To show this as a decimal you just divide the top number by the bottom number.

1 / 5 = 0.20

To express this as a percentage you multiply it by 100 – 0.20 x 100 = 20%

Again, think of a whole and how that is 100%.  If you have a pie and slice it into five portions, each portion is a fifth of the pie and the whole pie is five fifths.

Five fifths as a fraction is five over five.  If you divide five by five the answer is 1 – a whole – multiply this by 100 and you have the percentage amount – 100%.

Work out a percentage increase

To work out a percentage increase is slightly more complex as you need to make two calculations.  The first calculation you need to make is to work out what the increase is – once you have the value of the increase you can calculate what percentage that amount is of the initial amount – this is the percentage increase.

If you grow potatoes in your garden and last year you harvested 25KG but this year you harvest 30KG we can work this out as a percentage increase.

Start by finding the difference – subtract the lower value from the higher value.

30 – 25 = 5

You had an increase of 5KG of potatoes.  Now we can work out what percentage this was of the original amount.  As a fraction this is five over twenty-five, we can then calculate it is a decimal.

5 / 25 = 0.2

Now show the decimal as a percentage by multiplying it by 100.

0.2 x 100 = 20%

You had a 20% increase in your potato crop!

Calculating a percentage decrease

This is the same as how you work out a percentage increase – you find the difference between your original number and the new number and then work out what percentage this is of the original number.

If you harvested 12KG of peas last year but only 9KG of peas this year you can work out the decrease by subtracting the new value from the old value.

12 – 9 = 3

Now we can express the result as a fraction of the old amount – the fraction is three over twelve – from here we can calculate it as a decimal figure.

3 / 12 = 0.25

Multiply this by 100 to show it as a percentage.

0.25 x 100 = 25%

You had a 25% decrease in your pea harvest – bad news!

I hope the examples above give you an insight into how to remember percentage calculations – a whole is 100% – an amount of that can be expressed as a fraction – calculate the fraction as a decimal and multiply by 100 to show the percentage amount.