To calculate X% of a number, multiply the number by X and divide by 100. For example, 15% of 200 is 30 because (15 × 200) ÷ 100 = 30.
This is the most basic percentage skill — and once you know it, every other percentage problem becomes easier. This guide covers the formula, five worked examples, and mental math shortcuts so you can do it in your head.
The Formula: X% of Y
Both versions give the same answer. Use whichever feels more natural. Two ways to think about it:
- Divide first: Turn X% into a decimal (X ÷ 100), then multiply by Y. Example: 15% → 0.15, then 0.15 × 200 = 30.
- Multiply first: Multiply X × Y, then divide by 100. Example: 15 × 200 = 3,000, then 3,000 ÷ 100 = 30.
Pro tip: The "divide first" method is faster with a calculator because you can type 0.15 × 200. The "multiply first" method is easier mentally because whole-number multiplication feels natural.
Example 1: A Simple Discount
What is 20% of $80?
In context: if a $80 item is 20% off, you save $16 and pay $64.
Example 2: Sales Tax
Your bill is $45 and sales tax is 8%. What's the tax amount?
Example 3: A Tip at a Restaurant
The bill is $68. You want to leave a 18% tip. How much is that?
Example 4: Exam Score
You scored 87% on a test worth 150 points. How many points did you get?
Example 5: A Company Budget
A company earns $2,400,000 per year. It spends 35% on salaries. How much is that?
Mental Math Shortcuts (No Calculator Needed)
Once you know the 10% rule, you can find almost any percentage in your head.
The 10% Rule
To find 10% of any number, move the decimal point one place to the left.
| Number | 10% = |
|---|---|
| 250 | 25 |
| 45 | 4.5 |
| 3.50 | 0.35 |
| 1,200 | 120 |
Building Other Percentages from 10%
| To find… | Do this | Example (on 80) |
|---|---|---|
| 1% | 10% ÷ 10 | 0.80 |
| 5% | 10% ÷ 2 | 4 |
| 15% | 10% + 5% | 12 |
| 20% | 10% × 2 | 16 |
| 25% | Divide by 4 | 20 |
| 50% | Divide by 2 | 40 |
| 75% | 50% + 25% | 60 |
Practice example: What is 35% of 80? Break it into 10% + 10% + 10% + 5% = 8 + 8 + 8 + 4 = 28. You just did it in your head.
Reverse: Finding the Original Number
Sometimes you know the result and the percentage, but not the original number. Use this formula:
Common Mistakes to Avoid
- Forgetting to divide by 100. If you multiply 15 × 200 you get 3,000 — not 30. That's why ÷ 100 matters.
- Mixing up the percentage and the number. In "15% of 200," 15 is the percentage and 200 is the number. Order matters in your head.
- Confusing "of" with "increase by." "15% of 200" = 30 (a new value). "Increase 200 by 15%" = 230 (the original plus 15%). These are different.
- Assuming all percentages of the same number add up. "50% of 80 + 50% of 80" is 80 — the total. But "50% increase then 50% decrease" is not break-even (it's 75% of the original).
Order matters for stacking percentages: A 20% discount followed by a 10% discount is NOT a 30% discount. It's 1 − (0.80 × 0.90) = 28%. The second discount applies to the already-reduced price.
Skip the Math — Use a Calculator
If you're doing this often — for shopping, budgets, or homework — our free percentage calculator handles X% of Y in one step. It also does percentage increase, decrease, and reverse calculations.
📊 Free Percentage Calculator
Calculate X% of Y, percentage increase, decrease, and more — instantly, in your browser.
Open Percentage Calculator →Frequently Asked Questions
How do I calculate X% of a number?
Multiply the number by the percentage and divide by 100. Example: 15% of 200 = (15 × 200) ÷ 100 = 30. Or convert the percentage to a decimal (0.15) and multiply: 0.15 × 200 = 30.
What is the fastest way to find 10% of a number?
Move the decimal point one place to the left. 10% of 250 is 25. 10% of 3.50 is 0.35. This is the foundation for most mental percentage math.
How do I calculate 15% of a number in my head?
Find 10% (move decimal left once), find 5% (half of 10%), then add them. Example: 15% of 80 = 10% (8) + 5% (4) = 12.
What is the formula for percentage of a number?
Result = (Percentage ÷ 100) × Number. This is the same as Percentage × Number ÷ 100. Both give the same answer.
How do I find the original number from a percentage?
Divide the known value by the percentage expressed as a decimal. Example: if 30 is 15% of a number, the number is 30 ÷ 0.15 = 200.
What's the difference between "X% of Y" and "X% off Y"?
"X% of Y" is the value of the percentage itself. "X% off Y" is the discount amount, and the new price is Y minus that. Example: 20% of $80 = $16. 20% off $80 = you save $16, pay $64.