Most online converters and calculators work the same way: you hand over your file or your figures, a server somewhere processes them, and you get a result back. That arrangement is convenient for whoever built the site, but it means a copy of your document — a payslip, a contract, a scan of something personal — sat on a machine you do not control, subject to a retention policy you did not read.

Every tool here runs entirely in your browser instead. Modern browsers can resize images, rewrite PDFs, and run any arithmetic locally, fast enough that a round trip to a server would only make things slower. Nothing you enter or open is transmitted. This is not a promise in a privacy policy: an automated test loads real images and PDFs into the tools on every build and fails the build if a single network request carries data out of the page.

The same design has a practical side effect. Because there is no server doing the work, there is no account, no queue, no upload limit, and no daily cap. The tools work offline once the page has loaded, and they behave identically whether you run one calculation or fifty.

Where to start

Finance holds the largest group: loans and mortgages, savings and retirement projections, take-home pay and VAT, currency conversion and rental property returns. Almost all of them come down to an amount, a rate, and a length of time — what changes is which one you are solving for.

Convert covers the operations that look trivial until an edge case appears: days between dates, ages across leap years, percentages and percentage points, unit conversions where the same name means different quantities, PDF merging and splitting, and QR codes.Image handles compression and format conversion, where the useful decision is usually about dimensions rather than the quality slider. Text counts characters and words — a question with several correct answers depending on how a character is defined. Health covers body mass index and daily energy estimates, with a clear account of what those figures can and cannot tell an individual.

How the numbers are checked

Each tool's calculation is written as a pure function, kept separate from its interface so that known values can be pinned down in tests. Where a result depends on figures set by law — tax brackets, contribution ceilings, thresholds — the tool page names the primary document it follows and the date those figures were last checked against it. Income tax follows the published tables of the relevant revenue authority rather than a secondary summary.

The written explanations are held to the same standard. When an article states a measured result — that merging a PDF drops its bookmarks, or that re-encoding a screenshot as a high-quality JPEG can produce a larger file than the PNG it came from — that measurement is re-run automatically against the current library and the current browser. If the underlying behaviour changes, the test fails and the article gets corrected. An article that quietly becomes untrue is the worst way for a site like this to break.

What you can expect

Every tool is free and complete: nothing is held back behind a sign-up or a paid tier, and no result is truncated to encourage an upgrade. The site is funded by advertising, placed below the tool rather than around or on top of it, and without the auto-inserted formats that appear over the controls you are trying to use.

Each tool ships with a written explanation of the method behind it, because a number without its assumptions is not much use. Pages are published in English and Japanese, and a tool appears in a language only when its interface, its explanation, and its examples are all genuinely translated — a half-translated page is worse than one that is not offered at all.

These are calculators and file utilities, not advice. They implement standard, publicly documented methods and their arithmetic is tested, but they do not know your circumstances. If a result looks wrong, please say so — corrections take priority over new work.