Percentage Calculator

What is a percentage calculator?

A percentage is a ratio expressed as a fraction of 100: 25% means 25 out of every 100, or 0.25. Percentages show up everywhere — discounts and tax, conversion rates, growth in traffic or revenue, test scores, battery levels — but the arithmetic is easy to get wrong, especially when you have to work backwards from a final figure.

This page puts the seven calculations people need most side by side. Each card updates instantly as you type and shows the formula with your numbers plugged in, so you can check the working, reuse it in a spreadsheet, or explain it to someone else.

Inputs accept decimals, negative numbers, thousands separators (1,250.5) and a trailing % sign. Results are rounded to a sensible number of decimals for display, while the Copy button gives you a clean value without grouping separators.

How to use the Percentage Calculator

1. Find the card that matches your question — for example Percentage change from X to Y to compare last month with this month.
2. Type your numbers into the two fields. The result and the formula update as you type.
3. Read the badge on the percentage change card to see whether it is an increase or a decrease.
4. Click Copy to put the result on your clipboard, ready to paste into a report or spreadsheet.

Example: a price rise and a discount

A product goes from 80 to 100, then a 20% discount is applied to the new price. Notice that the discount does not undo the rise:

Change from 80 to 100:  (100 − 80) ÷ 80 × 100 = 25% increase
Decrease 100 by 20%:    100 × (1 − 0.20)      = 80
Reverse: 80 is 80% of  80 ÷ 0.80              = 100

Percentage change vs percentage difference

Percentage change has a direction: it measures movement from an old value to a new one, relative to the old value. Going from 50 to 75 is a 50% increase, but going from 75 back to 50 is only a 33.33% decrease, because the starting point is different.

Percentage difference is symmetric. It compares two values with no "before" or "after", dividing the absolute difference by their average: |A − B| ÷ ((|A| + |B|) ÷ 2) × 100. Use it when comparing two measurements, two products or two test results where neither is the baseline.

Common percentage mistakes

• Adding and then subtracting the same percentage does not return the original number: +10% then −10% gives 99% of the start.
• Percentage points are not percentages. A conversion rate that rises from 2% to 3% grew by 1 percentage point, but by 50%.
• Percentage change from zero is undefined, because you cannot divide by zero. Report the absolute change instead.
• To remove VAT or a markup, use the reverse percentage: a price of 120 including 20% tax is 120 ÷ 1.2 = 100, not 120 − 20% = 96.