What percentage calculations does this cover?

It covers what is X% of Y, X is what percent of Y, the percentage increase or decrease between two numbers, and reverse percentage (finding the original value before a percentage change).

How do I work out a reverse percentage?

Divide the final amount by (1 + change/100). For example, £120 after a 20% increase started at £120 ÷ 1.20 = £100. The reverse mode does this for you.

How do I calculate what percent one number is of another?

Divide the part by the whole and multiply by 100. For example, 30 out of 200 is (30 ÷ 200) × 100 = 15%. Use the "X is what % of Y" mode and enter 30 as X and 200 as Y.

How do I find the percentage increase between two numbers?

Subtract the old value from the new value, divide by the old value, then multiply by 100: ((new − old) ÷ old) × 100. Going from 80 to 100 is ((100 − 80) ÷ 80) × 100 = +25%. A negative result means a decrease.

Why does adding back the same percentage not return the original number?

A percentage increase and the equal-sized decrease are taken from different bases. 100 increased by 20% is 120, but 120 decreased by 20% is 96, not 100. To undo a 20% increase you must divide by 1.20, which is what the reverse mode does.

How do I turn a percentage into a decimal or fraction?

Divide the percentage by 100 to get the decimal (15% = 0.15) and put it over 100 to get the fraction (15/100, which simplifies to 3/20). This calculator works in decimals internally and rounds the displayed result to six decimal places.

Is anything uploaded?

No — all calculations happen in your browser.

Percentage Calculator

Percentage calculator — all the modes

This calculator answers every common percentage question in one place: what is X% of Y, X is what percent of Y, the percentage increase or decrease between two numbers, and the reverse percentage to find an original value before a change was applied. It is built for shoppers checking discounts, students working through maths homework, and anyone reconciling a price, a tax line, or a change in a figure. Pick a mode, type two numbers, and the answer appears instantly.

How it works

Each mode uses a standard percentage formula:

X% of Y computes (X ÷ 100) × Y.
X is what % of Y computes (X ÷ Y) × 100.
% change from X to Y computes ((Y − X) ÷ X) × 100, showing a + for an increase and − for a decrease.
Reverse computes Y ÷ (1 + change ÷ 100), recovering the original value before a percentage change pushed it to its final amount.

Every result is rounded to six decimal places to avoid floating-point noise, and divisions by zero are guarded so the tool never shows an error.

Example

You bought a jacket for £120 in a sale that took 20% off the new price. To find the original price, use Reverse mode: 120 ÷ (1 + 20 ÷ 100) = 120 ÷ 1.20 = £100. Note that £100 minus 20% is £80, not £120 — which is exactly why a reverse percentage needs division, not subtraction.

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%
% change X → Y	((Y − X) ÷ X) × 100	80 → 100	+25%
Reverse a change	Y ÷ (1 + change ÷ 100)	120 after +20%	100

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