How to Calculate Percentages with Basic Mathematics

Using basic mathematics to calculate percentages doesn't have to be hard or require the use of an online percentage calculator. Figuring percentages is a simple matter of dividing one number by another, then multiplying the answer by 100.

Step 1

Just like in math classes, first understand what decimals are and what they mean when calculating percentages. The decimal point in a number separates the amounts that are greater than 1 from the amounts that are less than 1. For example in the number 33.76, the digits to the left of the decimal point stand for 33, an amount greater than 1. The digits to the right of the decimal point stand for a fractional amount less than 1, in this case 76/100ths. If a number is written without a decimal point, it is understood that the decimal point is to the right of the last digit on the right. So 136 is understood to be 136.0 with the 0 after the decimal only there to show where the decimal is, and not changing the value of the number. Cash amounts are written the same way. The figure $26.45 means 26 dollars and 45/100th of a dollar or 45 cents. In order to figure the percent yield on savings accounts, certificates of deposit, interest bearing bonds, and annuities it is necessary to work with percentages.

Step 2

The word "Percentage" means per 100 or parts out of 100. First it is necessary to change the percentage to a decimal. To do this simply move the decimal point two places to the left. To change 76 percent to a decimal, move the decimal point, that is accepted to be after the six, two places to the left to give .76 as a decimal. If there are not two or more digits to the left of the decimal, then zeroes are placed to the left of the given digits to mark the places the decimal point will be moved over. So in the case of 5 percent, it is equal to 0.05 when changed to a decimal. Likewise, 8.09 percent becomes 0.0809 as a decimal. The zeroes do not change the value of the number when annexed on to the left side of the number.

Step 3

After changing the percentage to a decimal, simply multiply using the decimal amount in place of the percentage. For example, 45 percent of 15 is equal to .45 times 15, which gives 6.75 for an answer. When multiplying by numbers that have digits to the right of a decimal point, move the decimal in the resulting answer over to the left. It is moved over as many places to the left as there are total digits to the right of decimals in the numbers being multiplied. In the example, .45 has two digits to the right of the decimal and 15 has none, so the answer 675 had the decimal moved over two places to give 6.75 for the correct answer. To find 0.9 percent of $840.54 first change the percent to 0.009 and multiply it by 840.54 to get 756486 which becomes 7.56486 when the decimal is moved over five places to the left. 0.009 has three digits to the right of the decimal and 840.54 has two, so in the answer the decimal is moved over three plus two or five places to the left to give 7.56486. Because it was a cash amount that was multiplied, round it to $7.56 for the answer.

Step 4

By knowing that 50 percent = 1/2, 25 percent = 1/4, 75 percent = 3/4, and 33 percent equals approximately 1/3, it is possible to figure many percentages easily. Thus, 50 percent of 12 equals 6 because 1/2 of 12 is 6, and 25 percent of 12 is 3 because 1/4 of 12 is 3. To find the final cost of an item with a percentage off sale, first multiply by the percent to find the amount off, and then subtract that amount from the starting cost. A $24 item on sale for 25 percent off will cost: $24 times 0.25 = $6 then $24 - $6 = $18 for a final cost of $18. With a CD from paying 3.5 percent interest annually, with $2,000 put into it for a year, the yield would be 0.035 times $2,000 or $75 paid at the end of the year.


Calculators are only useful if you know how to use them. Double-check answers on paper to be sure they are right.


Watch those decimal points. One place off and the answer is wrong by a factor of 10.

Things You'll Need

  • Pencil and paper

  • Calculator