About and how we calculate

Percentage Calculator Online is a free collection of calculators for everyday percentage questions. You can find a share of a number, compare two values, reverse a percentage change, work with VAT and discounts, or compare margin with markup. Each calculator explains its formula and shows working alongside the answer, so you can check the method rather than relying on an unexplained number.

The guides include worked examples, common mistakes and spreadsheet formulas. They explain which value is the base of a percentage and when similar-looking measures mean different things. The tools calculate from the figures you supply; they do not establish tax treatment, a school's grading policy or the financial suitability of a decision.

Interactive calculations run entirely in your browser. Typing numbers updates local calculator state and runs the maths there, without submitting a form to a server. Our calculator code does not send your inputs or results anywhere, put them in the page address, or store them on our servers. The Copy button writes the displayed result to your clipboard when you choose it.

Loading a page still involves network requests. Cloudflare hosts the site, and a Cloudflare Web Analytics beacon measures visits and page performance. As explained in our privacy policy, these analytics are cookieless and provide aggregate visit information, not individual visitor profiles. We do not attach calculator values to analytics events. Your light or dark theme preference can be saved locally in your browser.

The calculation helpers use big.js, a decimal arithmetic library. Decimal inputs are parsed from text rather than first being converted to ordinary JavaScript floating-point numbers. Addition and multiplication therefore avoid binary floating-point surprises such as 0.1 + 0.2 producing 0.30000000000000004: decimal arithmetic gives 0.3. Percentage multipliers such as 12.3456789 ÷ 100 can be retained exactly as 0.123456789.

That does not make an endlessly repeating fraction finite. Division uses a decimal precision limit: 20 decimal places in the base helpers, or 60 in the higher-precision helpers used by several comparison and reverse calculations. Display rounding is a separate step. Input fields are limited to 30 characters, and invalid numbers or undefined divisions receive errors or an unavailable note.

Plain-number amounts normally show up to six decimal places. Percentages and percentage-point changes normally show up to four. Money shows two decimal places, including trailing zeros, and uses half-up rounding. The currency selector changes formatting; it does not convert exchange rates. Very small non-zero plain amounts or percentages can instead retain four significant digits rather than disappear as zero. Money does not use that exception.

The Copy button uses the same rounded precision as the main answer, without currency symbols or thousands separators. Working can retain more digits for exact terminating multipliers. Some reference tables use two decimal places for readability. Keep full input precision when checking a rounded result, and follow any separate rounding rules required for an invoice or assignment.

Automated tests cover arithmetic, validation, long inputs, rounding and displayed working. Build-output tests inspect the generated HTML for titles, canonical links, structured data, redirects, analytics and share images. Everyday example tables call the same calculator logic used by the interactive tools. Their starting answers are generated when the site is built, not from visitors' inputs. These checks help catch regressions, but are not a promise that every possible use is error-free.