Percentage Calculator
Percent of a number, what percent one value is of another, percent change, and adding or subtracting a percentage — worked exactly on this device.
1215% of 80 = 15 / 100 x 80 = 12Press ? for shortcuts
Do it offline:$ node -p '15 / 100 * 80'
How to use it
- Choose one of the four modes: percent of a number, what percent, percent change, or add and subtract a percentage.
- Type the two numbers the active mode asks for; the page opens on a worked fifteen-percent-of-eighty sample.
- For add and subtract, set the increase-or-decrease switch before reading the total.
- Read the answer, then open Working to see the arithmetic behind it.
- Copy the result, download a .txt, or Share to fold the inputs into the page link.
Four everyday percentage jobs share one screen: take a percent of a number, find what percent one value is of another, measure the percent change between two figures, or add and subtract a percentage for markups and discounts. Pick a mode, type the two numbers, and the answer updates as you type — worked on exact fractions, with the arithmetic shown.
How it works
Two formulas do most of the work. A percent of a number is 15 / 100 x 80 = 12; reverse it and a part over its whole, times 100, hands the percent back, 12 / 80 x 100 = 15. The other two modes reuse the same steps.
Percentage points and percent change are not the same measurement, and confusing them is the classic reporting slip. A rate that moves from 10 to 15 rises five percentage points, but the relative jump is (15 - 10) / 10 x 100 = 50 percent. This page reports the relative figure — read the label with the number.
Fields become exact fractions before any math runs, so decimals do not drift the way binary floating point can: 20 / 100 x 1.5 = 0.3, no trailing noise. The final answer is rounded half-up to six decimal places, and anything trimmed there says the exact value is longer — for instance 1 / 3 x 100 = 33.333333.
Examples
15% of a number: 8012Fifteen percent of eighty: 15 / 100 x 80 = 12 — exact, since both inputs are whole.
what percent is 3 of 1225%What share three is of twelve: a part over its whole, times 100, gives 3 / 12 x 100 = 25, wearing a percent sign because it is a rate.
percent change 200 to 25025%Growth from 200 to 250: difference over the start, times 100, is (250 - 200) / 200 x 100 = 25, tagged as an increase.
50 increased by 10 percent55A markup in one step: the base scales by one plus the percent, 50 x (1 + 10 / 100) = 55, and the amount added — a delta of 5 — sits beside the total.
Every example on this page is executed against the same engine that powers the tool, on every build.
Gotchas worth knowing
A division that never ends is rounded
One as a percent of three is 1 / 3 x 100 = 33.333333, cut to six places with a note that the exact value is longer — only the printed answer rounds.
A total of zero has no percentage
What-percent divides by the total, so a whole of zero is refused: every number would be an infinite share of nothing.
Percent change needs a non-zero start
Change is measured against where you began, so a start of zero is undefined — any rise from nothing is infinite, and the mode declines it.
A negative starting value flips the sign
Moving from -50 to -25 climbs on the number line, yet the change is (-25 - -50) / -50 x 100 = -50 percent, because dividing by a negative base inverts the sign. A note flags it.
Percentages are free to exceed 100
No ceiling at a hundred: 150 percent of 80 is a legitimate 150 / 100 x 80 = 120, larger than the base.
A discount is subtract-a-percentage
A percent-off price is the add-and-subtract mode set to decrease. Twenty percent off 50 is 50 x (1 - 20 / 100) = 40, with the money saved shown as a delta of -10.
Common errors
A number can't be expressed as a percentage of zero — enter a total that isn't zero.- What-percent divides by the total, and nothing divides by zero — give a total that isn't zero.
Percent change from zero is undefined — the starting value must not be zero.- Percent change divides by the first value, so a zero start leaves no baseline to grow against and no finite answer.
Enter a percentage as a number (for example 15 or 2.5)- A blank or non-numeric field can't enter the sum. The message names the field it means, and a comma where a decimal point belongs asks for a period instead.
Value has more than 30 significant digits — this calculator caps magnitudes there.- Fields cap at thirty significant digits to keep the exact-fraction math instant; a longer number is refused, not cut.
FAQ
What is the difference between percentage points and percent change?
Different questions. Percentage points subtract two rates: 10 to 15 percent is five points. Percent change divides that gap by the starting rate, so the same move is (15 - 10) / 10 x 100 = 50 percent relative.
How do I calculate a percent-off discount?
Pick the add-and-subtract mode, set the switch to decrease, then enter the price and the rate. Twenty percent off 50 is 50 x (1 - 20 / 100) = 40, with the saving shown as a delta of -10 beside the sale price. Increase turns the same fields into a markup.
Why does the result stop at six decimal places?
The math stays exact, but a value like one third has no finite decimal, so the display must stop. Six places, rounded half-up: 1 / 3 x 100 = 33.333333 arrives flagged as shorter than the true value.
Keyboard shortcuts
- ⌘/Ctrl + Enter
- Recompute on demand; the answer already follows every valid edit.
- ⌘/Ctrl + C
- Copy the result when nothing is selected; a focused number field keeps its native copy.
- ⌘/Ctrl + Shift + C
- Copy the result even when text on the page is selected.
- ⌘/Ctrl + S
- Save the result as a .txt in place of the browser's own save-page dialog.
- t
- Advance the color theme unless a field is focused.
- ? or ⌘/Ctrl + /
- Bring up this calculator's keyboard-help dialog.
- Esc
- Close the top dialog, leaving every field untouched.