Percentage Calculator

1. What is X% of Y? Multiply Y by X and divide by 100. A 15% tip on a 2,400 bill is 2,400 × 15 ÷ 100 = 360. This is the calculation you use for discounts, tips, commissions, and tax amounts.

2. X is what percent of Y? Divide X by Y and multiply by 100. If you scored 45 marks out of 180, that is 45 ÷ 180 × 100 = 25%. Use this to convert any fraction into a percentage — test scores, market share, or completion rates.

3. Percentage change. Subtract the original value from the new value, divide by the original, and multiply by 100. If your rent rose from 250 to 320, the increase is (320 − 250) ÷ 250 × 100 = 28%. The same formula gives a negative result for decreases, which the calculator reports as a percentage decrease.

Confusing percentage points with percentages. If an interest rate moves from 4% to 6%, it has risen by 2 percentage points but by 50% in relative terms. News reports frequently blur this distinction. Reversing a percentage change incorrectly. A 20% increase followed by a 20% decrease does not return you to the original value: 100 → 120 → 96. To undo a 20% increase you need a 16.7% decrease. Adding percentages of different bases. A 10% discount plus an extra 10% off the reduced price is a 19% total discount, not 20%, because the second discount applies to a smaller base.

Keeping these pitfalls in mind — and using a calculator to verify — helps you avoid costly mistakes when comparing offers, negotiating salaries, or reading statistics.