Percentage Calculator

The word "percent" comes from the Latin per centum, "by the hundred." The concept appears in ancient Roman tax records — the emperor Augustus levied a 1/100 tax on goods sold at auction — and was systematized by medieval Italian merchants computing interest, profits, and tariffs. The "%" symbol itself evolved during the 17th century from the Italian "p cento" / "p 100" abbreviations, gradually compressing into the slash-with-circles form we use today.

Percentages became indispensable with the rise of standardized accounting, banking, and statistics in the 18th and 19th centuries. The U.S. tax code, introduced as a temporary Civil War measure in 1861, used percentages exclusively. By the 20th century, percentages had spread into every quantitative field — academic grading, scientific reporting, polling margins, batting averages — because they normalize values to a common 0–100 scale that humans handle relatively well.

The conventions we now take for granted — "20% off" sales, percentage-point news headlines, compound interest in basis points — are mostly innovations of the late 19th and 20th centuries. The math has always been the same: a fraction times 100. Everything else is convention and audience.

This calculator handles the four most common percentage questions in one screen: percentage of a value, what percent of one is another, percentage change between two values, and percentage error against an expected value. Type into any row and the result updates immediately. Negative answers are surfaced where they're meaningful (a percentage change can be negative; a percentage error is always positive by convention).

Everything runs entirely in your browser; no values are transmitted or stored. For special cases not covered here — markup vs. margin, weighted averages, percentile rankings — see the related tools above. For mental-math approximations, the FAQ tips work for any of these calculations within a few percent.