Percentages are everywhere — sale tags, tax receipts, investment returns, statistics — yet they are quietly one of the most misused tools in everyday math. The mistake is almost never arithmetic; it is conceptual. People reach for the wrong percentage calculation, and the wrong formula gives a confident, wrong answer.
There are really only three percentage calculations you need, and the Percentage Calculator handles all of them. The first is finding a percentage of a number: 20% of 150. This is the sale-tag calculation — a 20% discount on a $150 item saves $30. Multiply the rate by the whole and you have it.
The second is the reverse: what percentage is A of B? If you scored 45 out of 60 on a test, you want 45 divided by 60, times 100 — that is 75%. This is the mode people most often confuse with the first, because both involve two numbers and a percent sign. The test is which number is the whole: the whole is the denominator, the part is the numerator.
The third, and the one that causes the most real-world errors, is percentage change. If a price rises from $80 to $100, that is not a 20% increase. The increase is $20, and the base is the original $80, so the change is 25%. Percentage change always divides by the original value, never the new one. Getting this wrong inflates growth figures and understates losses.
A related trap is adding and subtracting percentages. A 10% raise followed by a 10% cut does not return you to where you started — you end up 1% lower, because the two percentages apply to different bases. The order and the base both matter.
The simplest habit is to pause before you calculate and ask which of the three cases you are in. Is this a part of a whole, a share of a total, or a change from a starting point? The right question gives the right formula, and the right formula gives an answer you can trust.