Math Calculators

The math calculators in this category handle a small set of everyday arithmetic questions that are easy to get wrong by hand: finding what percentage one number is of another, calculating a percentage increase or decrease, or working backward from a percentage change to the original value. These come up constantly outside a classroom — reading a discount, checking a raise, understanding a statistic in the news — and a mistake in the arithmetic (adding percentages instead of multiplying, for instance) is common enough that it's worth double-checking with a tool that shows its work. The same confusion shows up with percentage points versus percentages: a rate that moves from 5% to 7% is a two percentage point rise, but it's also a 40% relative increase, and treating those two descriptions as interchangeable changes the meaning of a headline number.

Percentage change is the calculator in this category people trip over most often, because increasing a number by 20% and then decreasing the result by 20% does not return the original value — the base changes between the two steps. Seeing the formula applied to your own numbers, rather than just a memorized rule, makes that kind of gap visible instead of a source of quiet errors that show up later in a spreadsheet or an invoice.

This category is intentionally narrow today and will grow as more general-purpose math calculators are added. Each one follows the same rule as the rest of the site: the formula and the substituted numbers are shown next to the result, so you can verify the answer instead of taking it on faith.