Percentage Calculator Guide: Every Formula You'll Ever Need

The four percentage formulas you'll actually use — finding a part, finding a percent, percent change, and reversing a percentage — each with worked examples.

Almost every real-world percentage question reduces to one of four problems: finding a part, finding a percent, finding a change, or finding the whole. Once you can recognize which one you're looking at, the formula is just plugging in numbers.

1. Finding a part: "What is X% of Y?"

Formula: (percent ÷ 100) × number. Example: a store takes 25% off a $80 jacket. (25 ÷ 100) × 80 = $20 off, so the sale price is $60.

2. Finding a percent: "X is what % of Y?"

Formula: (part ÷ whole) × 100. Example: you got 17 out of 20 questions right on a quiz. (17 ÷ 20) × 100 = 85%.

3. Percent change: "How much did X change to become Y?"

Formula: ((new − original) ÷ original) × 100. This is the one people get wrong most often, because it's tempting to divide by the new value instead of the original. Example: a $40 item rises to $50. ((50 − 40) ÷ 40) × 100 = 25% increase — not 20%, which is what you'd get by mistakenly dividing the $10 difference by the new price of $50 instead of the original $40.

4. Finding the whole: "X is Y% of what number?"

Formula: part ÷ (percent ÷ 100). Example: a $9 tip was 15% of a restaurant bill. 9 ÷ 0.15 = $60 total bill.

The Percentage Calculator has a dedicated tab for each of these four formulas, so you never have to remember which direction to divide.