What is the difference between percent change and percent difference?

Percent change has a direction and a reference point — it measures how much a number moved relative to its starting value: ((new − old) ÷ old) × 100. Percent difference has no direction: it compares two values of equal standing by dividing the gap by their average, (|X − Y| ÷ ((|X| + |Y|) ÷ 2)) × 100, so swapping X and Y gives the same answer.

How do I calculate what is X percent of Y?

Convert the percent to a decimal and multiply: (X ÷ 100) × Y. For example, 15% of 200 is (15 ÷ 100) × 200 = 0.15 × 200 = 30. Use the "What is X% of Y?" mode and enter 15 and 200.

How do I work out a reverse percentage?

Divide the final amount by (1 + change ÷ 100). A jacket costing 120 after a 20% increase started at 120 ÷ 1.20 = 100. For a discount, use a negative change: 80 after a −20% sale started at 80 ÷ 0.80 = 100. The reverse mode does this automatically.

Why does increasing by X% then decreasing by X% not return the original number?

The two percentages are taken from different bases. Increase 100 by 20% and you get 120; decrease that 120 by 20% and you remove 24, landing on 96, not 100. To undo a 20% increase you must divide by 1.20, not subtract 20%.

How do I convert a percentage to a decimal or a fraction?

Divide by 100 for the decimal — 15% = 0.15 — and place it over 100 for the fraction, 15/100, which simplifies to 3/20. The word percent literally means per hundred, so every percentage is already a fraction with a denominator of 100.

What happens if I divide by zero?

The tool guards every division. Asking what percent X is of 0, or the percent change from a starting value of 0, has no meaningful answer, so the result shows a dash and the summary explains why instead of printing an error or Infinity.

Is anything I type uploaded?

No. Every calculation runs in your browser with plain JavaScript arithmetic. No numbers are sent to a server, logged, or stored.

Percentage Calculator

A complete percentage calculator suite that answers every everyday percentage question in one place — what is X% of Y, X is what percent of Y, percent change from one number to another, percent difference between two numbers, increase or decrease a value by a percentage, and reverse a percentage to recover the original figure before a change was applied. It is built for shoppers checking a discount, students grinding through maths homework, analysts reading a month-on-month move, and anyone reconciling a price, a tax line, or a tip. Pick a mode, type two numbers, and the answer updates instantly — alongside a plain-English sentence and the full working so you can see exactly how the result was reached.

How it works

The word percent means “per hundred”, so any percentage P is simply the decimal P ÷ 100. Each mode applies one standard formula:

X% of Y computes (X ÷ 100) × Y.
X is what % of Y computes (X ÷ Y) × 100.
Percent change computes ((Y − X) ÷ X) × 100, showing + for a rise and − for a fall.
Percent difference computes (|X − Y| ÷ ((|X| + |Y|) ÷ 2)) × 100 — symmetric, with no “from/to” direction.
Increase computes Y × (1 + X ÷ 100) and decrease computes Y × (1 − X ÷ 100).
Reverse computes Y ÷ (1 + change ÷ 100), undoing a percentage move.

Results are rounded to six decimal places to avoid floating-point noise, every division by zero is guarded, and increase/decrease results are colour-coded so a rise reads green and a fall reads red.

Worked example

You bought a coat for 120 in a sale that took 20% off the marked-up price, and you want the original. Choose Reverse, enter 20 as the change and 120 as the final amount: 120 ÷ (1 + 20 ÷ 100) = 120 ÷ 1.20 = 100. Notice that simply subtracting 20% from 120 gives 96, not 100 — which is precisely why a reverse percentage needs division rather than subtraction. Switch to Percent difference with 96 and 100 and you get (|96 − 100| ÷ 98) × 100 ≈ 4.08%, the symmetric gap between the two figures.

Question	Formula	Example	Result
What is X% of Y	(X ÷ 100) × Y	15% of 200	30
X is what % of Y	(X ÷ Y) × 100	30 of 200	15%
Percent change X to Y	((Y − X) ÷ X) × 100	80 to 100	+25%
Percent difference	(	X − Y	÷ avg) × 100
Increase Y by X%	Y × (1 + X ÷ 100)	200 + 15%	230
Reverse a change	Y ÷ (1 + change ÷ 100)	120 after +20%	100

Everything runs entirely in your browser — no numbers are uploaded or stored.