Percentage Calculator
What is X% of a number?
One number is what percent of another?
Increase or decrease
Find the original, before a change
Add or subtract a percentage
Percentage difference between two values
The percentage mistake that costs the most
A 50% rise followed by a 50% fall does not bring you back. Start at 100, rise to 150, then lose half of 150 and you are at 75 — a quarter down, not level. The second percentage is taken on a different base, and that is the whole trick. It matters most with investment returns, where a 50% loss needs a 100% gain to recover.
Removing a percentage that is already included
To get the price before 18% GST, divide by 1.18 — do not subtract 18%. On a bill of 1180 the right answer is 1000; subtracting 18% gives 967.60, which is out by more than 3%. The fourth box above does this correctly.
Percent versus percentage points
A rate moving from 4% to 6% has risen by two percentage points, and by 50%. Both are true and they sound very different, which is why the two get mixed up so often in reporting.
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Frequently Asked Questions
Change has a direction: it measures from the old value to the new one, so 50 to 75 is a 50% increase while 75 to 50 is a 33.3% decrease. Difference is symmetric — it compares two values against their average and gives the same answer either way round.
Because the second percentage applies to the larger number. 100 rises to 150, then 50% of 150 is 75, so you end at 75. This is the single most common percentage mistake, and it matters most with investment returns.
Use the fourth calculation, "find the original". A price of 118 that already includes 18% GST came from 100, not from 118 minus 18% — that would give 96.76, which is wrong by nearly 3%.
The plain difference between two percentages. Interest going from 4% to 6% is a rise of two percentage points, but a 50% increase. News reports mix these up constantly, and the two numbers tell very different stories.
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