Guides/Percentage of a number

How to calculate percentage of a number

“What is 15% of 200?” is the classic percentage question. Once you know the formula, the same steps cover sales tax, tips, discounts, and exam scores.

The core formula

X% of Y = (X ÷ 100) × Y

Example: 15% of 200 = 0.15 × 200 = 30.

Reverse: “X is what percent of Y?”

Percent = (X ÷ Y) × 100. Example: 30 is what percent of 150? 30 ÷ 150 × 100 = 20%.

Worked examples

Discount: 20% off a $49.99 item → 0.20 × 49.99 = $10.00 saved; sale price = $39.99.
Tip: 18% of an $86.40 bill → 0.18 × 86.40 ≈ $15.55.
Exam: You scored 42 out of 50 → 42 ÷ 50 × 100 = 84%.
Commission: 7% of $2,500 sales → 0.07 × 2,500 = $175.
Part of a whole: 35% of 80 people = 0.35 × 80 = 28 people.

Common mistakes

Forgetting to divide by 100 (multiplying by 15 instead of 0.15).
Swapping X and Y in the reverse percent formula.
Applying a second discount to the original price instead of the already-reduced price.

When to use which tool

Percentage Calculator — X% of Y, reverse %, change, discount.
Percentage increase guide — old vs new values.
Tip Calculator — tip + split bill in one step.

Try the free calculator

Open thePercentage Calculatorand use the “X% of Y” tab. Math stays in your browser — no signup.