While shopping, you may need to calculate how much to pay, for example, during 20 percent off sale. A percentage is a ratio of a part of something over the whole, converted to units of 100. What happens, however, if you don't know how to calculate 20 per cent off without a calculator? There are a number of maths tricks you can employ to figure out percentages while shopping -- even if you left your calculator at home.

Understand how to calculate percentage off with a pencil and paper. To calculate 20 percent off £30, multiply £30 by 20 and divide by 100, giving you 6. This means that 20 percent of £30 is an £6 decrease in price, so you will pay only £24. After calculating the amount of the price discounted, subtract this from the original price to get the sale price.

Memorise the fractional equivalents of common percentages. Twenty percent is one-fifth, meaning that you divide the original price by 5 to get the price reduction. Dividing £30 by five is an easier way of getting to £6 than using the traditional calculation. Other easy fractional equivalents to remember are:

50 percent off -- the original price divided by two.

25 percent off -- the original price divided by four.

10 percent off -- the original price divided by ten.

Use the fractional equivalents that you memorised to calculate other percentages. If you know that 10 percent off is the original price divided by 10, and you have 30 percent off, calculate 10 percent off and triple the result. If the old price was £130, then 10 percent off is £13; tripling this amount gives you the 30 percent reduction of £39.

Learn how to divide by one or 10 by moving the decimal in the price to the left. To calculate 1 percent off, take the pound figure and move the decimal point two spaces to the left. For example, 1 percent of £260 is £2.60. Ten percent of £260 is £26, because you would move the decimal one space to the left.

Piece together the answer by using what you know. If the sale is 33 per cent off £260, take 10 per cent off £260, which you can get by moving the decimal spot one space to the left, arriving at the answer of £26. Triple this to get 30 per cent, or £78. You now have 3 per cent left to figure out. You know that 1 per cent of £260 is calculated by moving the decimal spot to the left twice, arriving at the answer of £2.60 -- tripling this gives you the remaining 3 per cent, or £7.80. Add the parts together -- £78 plus £7.80 is £85.80. The original price of £260 minus £85.80 is £174.20, which is the amount of money you need to bring to the till to purchase this item.