Math
Percentage Calculator
Enter a percentage and a number to see the amount, plus the totals with it added and removed.
The formula
amount = percentage ÷ 100 × value
increased = value + amount
decreased = value − amount
Worked example
A 17% deposit on a $248 order
- 17 ÷ 100 = 0.17
- 0.17 × 248 = 42.16
- Balance to pay: 248 − 42.16 = 205.84
Where this goes wrong
Taking the percentage of the wrong number
A 20% rise followed by a 20% fall does not return you to where you started. The rise is 20% of the original, the fall is 20% of the larger figure, so $100 becomes $120 and then $96. Percentages always apply to whatever number they sit next to, not to the one you had in mind.
Questions
- How do I calculate a percentage of a number by hand?
- Divide the percentage by 100, then multiply by the number. For 17% of 248: 17 ÷ 100 = 0.17, and 0.17 × 248 = 42.16.
- Why does adding 10% then removing 10% not return the original number?
- Because the second percentage applies to the larger amount. 100 plus 10% is 110, and 10% of 110 is 11, so you land on 99. To reverse a 10% increase, divide by 1.1 instead of subtracting 10%.
- What is the difference between percent of and percentage change?
- Percent of takes a share of one number. Percentage change compares two numbers and tells you how much one grew or shrank relative to the first.