There are many occasions when having a ream of paper full of different values, such as a classroom's worth of test results or a report on individual sales for the month, makes those values almost meaningless. On these occasions, it can be useful to see how those values relate to a set target or maximum attainable value. Knowing how to calculate percentages helps achieve this clarity.
Note down the number you wish to calculate the percentage for. For example, if you have scored 15 points in a test and want to know what percentage you achieved, note down this value of 15 on your paper. This will be the numerator of the fraction you need to use to calculate percentages.
Note down the total value on your paper; ie, the maximum value for which your number in Step 1 is a part. This will be be the total number of points you could have scored in the test. This value represents the denominator of the fraction you will use to calculate percentages.
Divide your numerator value by your denominator value to find your fraction as a decimal. Multiply this decimal by 100 to determine your percentage. For example, if you scored 15 points in a test and the total number of points you could have scored was 40, you would divide 15 by 40, giving you 0.375. Multiplying this by 100 gives you 37.5 percent.
If you are comfortable reading decimals, you do not need to multiply by 100. All this multiplication does is shift the decimal point two units to the right, making the fraction easier to understand. For a decimal value like 0.375, it is clear that the percentage is 37.5 even before you multiply by 100. This saves a little time.
Knowing a percentage is not always useful. Where large values are involved, it is possible for two or more numerators to achieve the same percentage value after rounding-off has taken place. In situations such as these, calculating percentages only serves to mask true values and loose accuracy.