How To Calculate Percentages

How To Calculate Percentages

Everyone's forgotten most of their maths from school, algebra, geometry, trigonometry, but most of that is useless right? Well the one thing that is still useful to us in everyday life is the ability to calculate percentages, so let us refresh you memory in all the ways that you may need to work with percentages.

Step 1: How do I work out the percentage of something?

This is Kenny. He spends a lot of his income on food every month, £450 in fact, on all sorts of delicious delicacies. This comes out of his total monthly income of £1500. So, what percentage of his monthly income does Fred spend on food?

Firstly, you need to take the partial amount, in this case £450, and divide it by the total amount, again, in this case that's £1500. This gives you a figure of 0.3. Now a percentage is simply a way of expressing a number as a fraction of 100, so take the figure of 0.3 & multiply by 100, which should leave you with an answer of 30, showing that Kenny spends 30% of his monthly income on food. At that rate Kenny may have to apportion some of his budget to a pair of trousers with a larger waistband!

Step 2: When I have a percentage, how do I work out the total amount?

This is Terrance. He loves to play computer games, and one of his favourites is a puzzle game on the PSP. The game tells him he's completed 25% of the levels and he's currently on level 15. So how many levels are there for Terrance to play through in total?

First things first you need to convert the percentage that you've been given, in this case that's 25%, into a decimal - so divide 25 by 100 to give a value of 0.25. Now take the partial figure of 15, which is the level Terrance is currently on, and divide it by 0.25 to get the total number of levels in the game which is 60. Quite a long way to go still then Terrance!

Step 3: How would I work out what a tip would be?

This is Stan and Wendy. They are out for a meal together and the bill has come to £90. I know, a fancy meal at that price... or Stan's just eaten more than his share of Pizza. But how do they work out how much there 12.5% tip should be?

Similarly to the previous task, divide the percentage by 100 to convert to a decimal. So 12.5% becomes 0.125. Now take this figure and multiply it by £90, which is the total of their bill. This should give you an answer of 11.25. So a 12.5% tip on a £90 bill is £11.25. And money well spent too!

Step 4: How do I work out a percentage increase?

This is Phillip, he works for a large financial institution and was earning £25,000 per year, but recently had an increase to £28,000 per year. He wants to know what sort of a percentage increase this is in his salary, but how does he do it?

First, take the 'After' amount, in this case that's 28,000, and divide it by the 'Original amount, 25,000. This gives you a figure of 1.12. Multiply this by 100 to turn this decimal into a percentage value of 112%. Now compare this number to the value of 100, to see if it is larger or smaller. In this case 112 is 12 units greater than 100 meaning Phillip got an increase on his previous salary of 12%. An inflation busting boost for someone!

Step 5: How do I work out a percentage decrease?

This is Sheila, she's a property tycoon who has just had her housing stock revalued. When she last had her portfolio valued it was worth £3,000,000, but is now worth £2,700,000, a loss of £300,000. But Sheila wants to know what sort of percentage decrease this is?

Similar to working out a percentage increase, take the 'after' value of 2,700,000 and divide by the before value of 3,000,000 to get a figure of 0.9. Multiply this by 100 to change this decimal to a percent and you should end up with a value of 90. Compare this to the figure of 100, to see if it is larger or smaller, and in this case it is 10 units smaller, meaning Sheila's property portfolio has decreased in value by 10%.

Step 6: How do I work out the original amount of something after a percentage increase or decrease?

This is Kyle, he loves his music and is always buying CD's & other paraphernalia. He sees a CD he wants that's priced at £12 after a 25% discount, but what Kyle doesn't know is what the original price was and