Percentage Calculator covers the four percentage calculations people look up most often: finding what a percentage of a number is, finding what percentage one number is of another, calculating the percentage change between two values, and applying a markup or discount to a base value. Each lives in its own tab so you don't have to re-enter numbers when switching between calculation types.

All four use plain arithmetic — there's no rounding trickery or hidden assumptions, and every result updates live as you type, entirely in your browser.

The two tabs people mix up most often are "% of" and "Is % of," and it's worth being explicit about the difference, since they answer opposite questions with the same three numbers. "% of" starts from a percentage and a base and finds the resulting amount — 20% of 150 is 30. "Is % of" starts from an amount and a base and finds the percentage that connects them — 30 is 20% of 150. They're inverses of each other, and picking the wrong one is the single most common source of a percentage calculation that "looks wrong" even though the arithmetic is technically correct for the question actually being asked.

The markup/discount tab treats a negative percentage as a discount rather than requiring a separate mode toggle — enter -15 to see a base value reduced by 15%, or 15 to see it increased by the same amount. This mirrors how markup and discount are actually the same operation with opposite signs mathematically, which is also why they're combined into one tool here instead of two separate ones.

Because there's no submit button, you can also use this tool to sanity-check a number as you type rather than committing to a single calculation — adjust the base value and watch the result track it in real time, which is useful for quickly exploring "what if" scenarios like comparing a few different discount percentages against the same price.