Percentage Calculator

How to use the percentage calculator

Pick the question you’re actually asking, then type two numbers. The segmented control switches between the three everyday percentage problems: “What is X% of Y” for a slice of a known total, “X is what % of Y” for turning a part into a percentage, and “% change from X to Y” for how much something rose or fell. The default — 15% of 200 — returns 30, the kind of mental tip-or-discount math people reach for most.

Use the first mode, “What is X% of Y,” whenever you know the rate and want the amount: 15% of a $200 bill, 8% sales tax on a price, 20% off a sticker. It answers “how much is that piece worth.” The second mode, “X is what % of Y,” runs it the other way — you have the part and the whole and want the rate. Ask “25 is what % of 200” and you get 12.5%; it’s how you turn 18 correct out of 24 into a score, or a $25 fee on a $200 order into a percentage.

The third mode, “% change from X to Y,” is the one people most often get backward. It measures growth or shrinkage between a starting value and an ending value, so order matters: going from 200 to 250 is a +25% change, but the reverse trip from 250 back to 200 is −20%, not −25%. The change is always measured against where you started, so the same gap in dollars is a different percentage depending on which number you began with.

A few mental-math tricks make most of this checkable without the tool. Ten percent is just the decimal moved one place — 10% of 80 is 8 — and 5% is half of that, so 5% of 80 is 4. Percentages are also commutative: X% of Y equals Y% of X, which is why an awkward 8% of 25 becomes an easy 25% of 8, which is plainly 2. Lean on these to sanity-check the result, especially for tips, taxes, and quick discounts where a wrong decimal place is the usual mistake.

One distinction is worth keeping straight: a percentage and a percentage point are not the same thing. If an interest rate moves from 5% to 7%, that’s a rise of 2 percentage points, but as a percent change it’s a +40% jump — because 2 is 40% of the original 5. Reach for “percentage points” when you’re comparing two rates, and “% change” when you want the relative size of the move. These are estimates for general information, so round to whatever precision the situation needs.

The formula

Each mode is a one-line formula — the only difference is which two numbers you know and which one you’re solving for:

what is X% of Y       →  (X ÷ 100) × Y
X is what % of Y       →  (X ÷ Y) × 100
% change from X to Y   →  ((Y − X) ÷ X) × 100

Worked example with the defaults — 15% of 200: (15 ÷ 100) × 200 = 0.15 × 200 = 30. Flip it with the second mode and 25 is what % of 200 comes out as (25 ÷ 200) × 100 = 12.5%. Both are everyday slices: a tip on a bill, a score out of a total.

Percentage change is where direction matters. From 200 to 250 the math is ((250 − 200) ÷ 200) × 100 = +25%. But the return trip from 250 back to 200 is ((200 − 250) ÷ 250) × 100 = −20%, not −25% — because the change is measured against the starting value, and you’re now starting from the larger 250. A rise and the fall that undoes it are almost never the same percentage.

Frequently asked questions

Related tools