Percentage Calculator
Three percentage problems on one page — each recalculates as you type, and the link in your address bar always reflects all your numbers.
How these calculators work
Every percentage problem is one of three shapes, so this page gives each its own mini-calculator. X% of Y multiplies: X ÷ 100 × Y — the tip, the tax, the discount amount. X is what percent of Y divides: X ÷ Y × 100 — your score, your progress toward a target. Percent change compares against the starting value: (Y − X) ÷ |X| × 100 — price moves, growth rates. The third one trips people up because the base matters: the same gap reads as a different percentage depending on which end you start from, and a change from zero has no defined percentage at all (there's no baseline to compare against — the tool says so instead of showing a misleading number).
Worked example
The defaults show all three shapes at once. 15% of 240 is 36 (0.15 × 240). 30 is 20% of 150 (30 ÷ 150 = 0.2). And going from 80 to 92 is a +15% change (12 ÷ 80) — while the return trip, 92 to 80, is −13.04% (12 ÷ 92), because each direction measures against its own starting point.
Frequently asked questions
What's the difference between a percentage and percentage points?
A percentage compares relative size; percentage points measure the raw gap between two percentages. If a rate rises from 4% to 6%, it went up 2 percentage points — but that's a 50% increase, because 6 is half again as big as 4. News reports mix these up constantly; when a change 'jumps 50%' from a small base, check whether the points moved much at all.
How do I work out a discount price in my head?
Flip it to what you keep paying. A 30% discount means you pay 70%, so take 10% of the price (shift the decimal), multiply by 7. For a $80 item: 10% is $8, times 7 is $56. For odd rates like 15%, do 10% plus half of that: $8 + $4 = $12 off, so $68.
Why is the change from 80 to 92 not the same as from 92 to 80?
Because percent change is always measured against the starting value, and the two directions start from different bases. Going 80 → 92 is +12 on a base of 80: +15%. Coming back 92 → 80 is −12 on a base of 92: −13.04%. Same $12 gap, different denominators — which is also why a 50% loss needs a 100% gain to recover.
How do I reverse a percentage increase?
Divide by the growth factor — don't subtract the same percentage. If a price rose 15% to reach $92, the original was 92 ÷ 1.15 = $80. Subtracting 15% instead gives 92 × 0.85 = $78.20, which is wrong, because the 15% you'd be removing is measured on the bigger number. Divide to undo, multiply to apply.