What is X% of Y?
X is what % of Y?
% change from X to Y
The three percentage formulas
Almost every everyday percentage question is one of three types, and the calculator above handles each one:
- X% of Y = Y × X ÷ 100. Use this for discounts, tips, tax, and commissions.
- X is what percent of Y = X ÷ Y × 100. Use this for scores, shares of a total, and "how much of my budget did this take?"
- Percent change from X to Y = (Y − X) ÷ X × 100. A positive answer is an increase, a negative one a decrease.
Worked examples
A sale price: a $80 jacket is 25% off. 25% of 80 = 80 × 25 ÷ 100 = $20 off, so you pay $60. To go straight to the final price, multiply by (100 − 25)% = 0.75: 80 × 0.75 = $60.
A test score: you got 27 questions right out of 30. 27 ÷ 30 × 100 = 90%.
A price rise: your rent went from $1,200 to $1,320. The change is (1320 − 1200) ÷ 1200 × 100 = 120 ÷ 1200 × 100 = 10% — a 10% increase.
Percentage increase vs. percentage points
These two are easy to confuse and the difference matters. If an interest rate moves from 4% to 6%, that is a rise of 2 percentage points, but a 50% increase in the rate itself (2 ÷ 4 = 50%). News headlines often mix these up. When you're comparing two percentages directly, you're usually talking about percentage points; when you're measuring growth relative to the starting figure, you're talking about a percentage change.
Reversing a percentage
Sometimes you know the result and want the original. If a price is $90 after a 10% discount, don't add 10% back — that overshoots. Instead divide by 0.90: 90 ÷ 0.90 = $100 was the original price. Likewise, if a bill is $110 including 10% tax, the pre-tax amount is 110 ÷ 1.10 = $100.
Frequently asked questions
How do I calculate a percentage in my head?
Find 10% first by moving the decimal one place left (10% of 60 is 6), then scale: 20% is double that, 5% is half. For a 15% tip on $60: 10% is $6, half of that is $3, so 15% ≈ $9.
Why can a percentage be more than 100%?
Because a value can be more than the whole it's compared to. If sales grew from 50 to 150, that's a 200% increase — the new figure is three times the old one.
What's the difference between "percent of" and "percent off"?
"25% of 80" is the part itself ($20). "25% off 80" is what's left after removing that part ($60). The calculator's first box gives you the "of" amount; subtract it from the original for the "off" price.
Related tools: the Tip Calculator applies a percentage to a bill automatically, and the Loan Calculator turns an interest rate into a monthly payment.