How to Calculate Percentages (The Easy Way)
Master the three percentage questions that cover almost every real-life situation — percent of a number, one number as a percent of another, and percent change.
Percentages are everywhere — tips, discounts, taxes, test scores, price changes — yet they cause a surprising amount of hesitation. The trick is realising that almost every real-world percentage problem is one of just three questions. Learn those and you are set.
The three questions
1. What is X% of a number? Multiply the number by the percent and divide by 100. 15% of 200 is 200 × 15 ÷ 100 = 30. This is your tip-and-discount workhorse.
2. A is what percent of B? Divide the part by the whole and multiply by 100. 30 out of 200 is (30 ÷ 200) × 100 = 15%. Use this for scores and shares.
3. What is the percent change? Subtract old from new, divide by old, multiply by 100. From 200 to 250 is ((250 − 200) ÷ 200) × 100 = 25% increase. A negative answer means a decrease.
Everyday shortcuts
- 10% — just move the decimal one place left. 10% of 84 is 8.4.
- A tip of 20% — take 10% and double it.
- Percent off — a 25% discount means you pay 75% of the price.
The trap that catches everyone
A percentage decrease and the increase that undoes it are not the same size. If a $100 item drops 20% to $80, going back from $80 to $100 is a 25% increase — because the base changed. This is why “prices fell 20% then rose 20%” never gets you back to where you started.
Percentage points vs. percent
If an interest rate rises “from 4% to 5%,” that is a one percentage-point increase — but a 25% relative increase (1 is 25% of 4). News headlines often blur the two; knowing the difference makes you harder to mislead.
Run any of these instantly with the percentage calculator. It keeps all three on screen at once, alongside three more that come up almost as often: what number a percentage is of, the percentage difference between two values, and increasing or decreasing a value by a percentage.