About Percentage Calculator

Percentage-calculator.io is a free, fast percentage calculator: seven ways of asking a percentage question, answered live as you type. It has no accounts, no ads, and nothing to install. This page explains how it computes, how it rounds, and what standards the written content holds itself to.

Methodology: the exact-decimal engine

Most calculators compute in binary floating point, which cannot represent most decimal numbers exactly — the classic symptom is 3 × 0.165 = 0.49500000000000005. This site's engine instead keeps every value as an exact ratio of integers, so multiplication, addition and division are exact by construction. Nothing is rounded until the moment a result is displayed.

The rounding rule: one rounding step, at display time, half away from zero — the same convention spreadsheets use for display. In automatic precision, results that terminate are shown in full (16.5% of 3 is exactly 0.495) and repeating expansions are rounded to two decimals (a third shows as 33.33), extending to two significant digits for values under 0.01 so a real result never collapses to zero. The .0← / .00→ buttons switch to a fixed precision from 0 to 8 places; changing precision re-renders from the stored exact value, so rounding error can never accumulate.

Testing: a golden table of more than forty input/output pairs — including the float-trap cases, exact rounding ties, negative-base changes, and a 0-to-8 precision sweep — runs in continuous integration on every change. If a result would change, the build fails.

Documented conventions: percentage change divides by the absolute value of the starting number, keeping the sign meaningful for negative starts; percentage difference uses the average of the two magnitudes and declines opposite-sign pairs; division by zero and change-from-zero show a plain-language note instead of an invented number. The FAQ states each convention where users will actually meet it.

Content standards

Articles cite peer-reviewed research where they make empirical claims, linked by DOI at the claim itself — never as decoration. Every DOI is resolved and its metadata checked before first use, and an automated job re-checks all cited links continuously. Where the evidence is thin or doesn't cover a situation, the article says so. Every article shows its published date and, when it has been re-checked by a person, a last-reviewed date. No statistic appears without a source or a derivation you can follow.

Privacy, in one paragraph

Calculations run entirely in your browser; the numbers you type are never transmitted. The site stores two small preferences on your device (theme, decimal precision) and currently loads no analytics at all. If lightweight analytics are added, they will run only on this site's production domain and never see input values. The privacy policy is the binding version.

Who builds this

Percentage Calculator is built and maintained by the small team behind Stock Calculator, Screen Test Pro and AREALME — free browser tools with the same house rules: prerendered pages, no frameworks, honest content, and instruments that work the moment they load. The version history is public in the changelog.