Percentage Calculator
ConvertEverything runs locally in your browser — nothing is uploaded
Percentage questions look like one thing but are really five: what a percentage of a number is, one number as a percent of another, the rate of change from A to B, the price after an increase or a discount, and how far apart two percentages are. Most calculators answer one of these at a time; this one answers every question a pair of numbers can support. Two traps are called out separately. The rate of change divides by the original value, so it is asymmetric: 100 to 120 is +20%, while 120 to 100 is −16.67%, not −20%. Percentage points and percentages are different things: 15% is 5 points below 20%, but it is 25% lower relative to 20 — the pair most often confused in the news. Questions in Chinese, English and Japanese are all understood, and a line that cannot be parsed raises an error naming the line instead of being skipped. Everything runs locally in your browser; nothing is uploaded.
Features
- Every common question about the same pair of numbers: share, rate of change, price after a percentage, percentage-point comparison
- The rate of change uses the original value as its denominator, and the reverse direction is given too (100→120 is +20%, 120→100 is −16.67%)
- Percentage points and percentages are written on separate rows
- Adding then subtracting the same percentage does not return to the start, which is shown with the shortfall
- Discounts understand “percent off” as well as the Chinese zhe notation
- Questions in Chinese, English and Japanese are recognised, one per line
- A line that cannot be parsed raises an error naming it, never skipped silently
- Adjustable decimal places
- Everything runs locally in your browser; nothing is uploaded
How to use
- Put one question per line, such as “15% of 120”, “18 is what percent of 120”, “from 100 to 120”
- Adjust the decimal places if you need to
- The result appears as you type, and every row can be copied on its own
FAQ
- Why is 120 to 100 not a 20% decrease?
- Because the rate of change divides by the original value. Here the change is −20 and the original is 120, giving −16.67%; going the other way, +20 over an original of 100 gives +20%. The denominators differ, so the percentages are asymmetric — that is the definition, not an error.
- What is the difference between “5 points lower” and “25% lower”?
- They are different quantities. Percentage points are the result of subtracting one percentage from another (20% − 15% = 5 points). A percentage divides that difference by the reference value (5 ÷ 20 = 25%). News reports often give only one, so both rows are shown here.
- Why is 100 plus 15% then minus 15% not 100?
- Because the second 15% is taken from 115: 115 × 0.85 = 97.75, which is 2.25 short of 100. Adding and subtracting the same percentage do not cancel, and the shortfall is called out as its own row.
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