About PercentSwift
A free set of percentage calculators that show their working, with guides that explain the math behind everyday percentages.
What the site does
PercentSwift answers percentage questions: a share of a number, a change between two values, a price after a discount, the tax inside a total, a tip split between friends. Every answer comes with the formula and the steps using your numbers, so you can check the result or copy the working into your own notes.
There is one main percentage calculator with 17 modes and 12 smaller pages that each handle one kind of question, such as the discount calculator and the reverse percentage calculator.
How the numbers are handled
The calculators use exact decimal arithmetic instead of the floating-point math built into browsers, which is why 0.1 + 0.2 shows as 0.3 here. Rounding happens once, at the end, using the rule you choose in Settings. The details are on How we calculate.
Everything runs in your browser. You don’t need an account, and the numbers you type are not sent to a server.
The guides
The guides section has 100 articles grouped into ten topics, from percentage basics to spreadsheet formulas. Each guide works through examples with real numbers and links to a calculator with those numbers filled in. When the site is built, a script runs every one of those links through the calculator and stops the build if a result in the text doesn’t match.
The guides explain general arithmetic. They are not financial, tax or legal advice, and tax rates in examples are there to show the method, not to tell you the rate that applies to you.
Corrections
If you find a wrong result or a confusing explanation, email hello@percentswift.com. The contact page lists what to include.
Publication and maintenance
PercentSwift was launched on , to provide fast, verifiable percentage arithmetic without sign-ups or paywalls. Core calculator pages and reference tables are maintained continuously and verified against automated decimal test suites.