Percentages show up in tips, taxes, discounts, grades, and interest. Once you know the three core formulas, every "what's the percent" question becomes quick mental math. Here they are with clear examples.
Key takeaways
- Percent of a number: (percent ÷ 100) × number.
- What percent is X of Y: (X ÷ Y) × 100.
- Percent change: ((new − old) ÷ old) × 100.
- "Percent" just means "per hundred."
1. Find a percentage of a number
(percent ÷ 100) × number
20% of 80 = (20 ÷ 100) × 80 = 162. What percent is one number of another?
(part ÷ whole) × 100
18 is what % of 60 → (18 ÷ 60) × 100 = 30%3. Percentage increase or decrease
((new − old) ÷ old) × 100
Price 50 → 60 = ((60 − 50) ÷ 50) × 100 = +20%
Price 80 → 60 = ((60 − 80) ÷ 80) × 100 = −25%4. Reverse percentage (find the original)
If a value already includes a change, divide it out:
After +20%: original = value ÷ 1.20
After −20%: original = value ÷ 0.80
120 after a 20% rise → 120 ÷ 1.2 = 100Let the calculator do it
Percent of a number, percent change, and more — pick the mode and type your values.
Open the Percentage Calculator →Mental-math shortcuts
| To find | Trick |
|---|---|
| 10% | Move the decimal one place left (10% of 240 = 24) |
| 5% | Half of 10% |
| 1% | Move the decimal two places left |
| 15% tip | 10% + half of that |
| Swap trick | X% of Y = Y% of X (18% of 50 = 50% of 18 = 9) |
Frequently asked questions
How do I find a percentage of a number?
Divide the percent by 100 and multiply by the number. 20% of 80 = 16.
How do I calculate percentage change?
(new − old) ÷ old × 100. From 50 to 60 is +20%.
What is a reverse percentage?
Finding the original before a change. 120 after +20% means the original was 120 ÷ 1.2 = 100.
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