You can often calculate percentages quickly without a calculator by using these "building block" methods:

Percentage (%)=(PartWhole)×100Percentage (%) equals open paren the fraction with numerator Part and denominator Whole end-fraction close paren cross 100

To find what percentage one number is of another (for example, finding your percentage of marks or a download's progress), use this formula:

If you scored 420 marks out of 500 , your percentage is:

(420÷500)×100=84%open paren 420 divided by 500 close paren cross 100 equals 84 %

The total possible amount (e.g., total marks or total file size).

To find 1% of a number, move the decimal point two places to the left . Example: 1% of 150 is 1.5. Building Up: Once you know 10% or 1%, you can find others: 20%: Find 10% and multiply by 2. 5%: Find 10% and divide by 2. 15%: Find 10%, find 5%, and add them together. The "Swap" Trick Percentages are reversible ( ), which can make hard problems much easier: Problem: What is 18% of 50? (Difficult) Swap: What is 50% of 18? (Easy! It is just half) Answer: 9. Common Conversions