How we build and test these tools

Anyone can put a text box on a page and call it a tool. This page explains what actually sits behind the 108 tools here — the rules they are built to, how they get verified, and the things we have decided not to do even though they would bring traffic.

Rule 1 — your data stays on your device

With a small number of clearly-labelled exceptions, every tool processes your input locally in the browser. This is an architectural commitment, not a promise in a privacy policy: for most tools there is no upload endpoint in the codebase at all, so there is nothing to breach, log, or retain.

How that works in practice, by category:

• PDF tools — merging, splitting, rotating, watermarks, page numbers, form filling, and metadata editing run on pdf-lib in the browser. Page previews and text extraction use Mozilla's pdf.js, the same engine Firefox uses to display PDFs.
• Image tools — resizing, cropping, compression, filters, watermarking, and metadata stripping are Canvas operations on pixel data already in memory.
• Video editor — trimming, joining, cropping, captions, and transitions use the browser's native WebCodecs API. A trim that doesn't change the codec is a stream copy, so it is close to instant and lossless rather than a re-encode.
• Text, encoders, and formatters — plain JavaScript on the string you typed. Hashing uses the browser's built-in Web Crypto implementation.
• Device tests — keyboard, mouse, touch, gamepad, microphone, camera, and display tests read browser events and hardware APIs directly. Camera and microphone streams are rendered and discarded; they are never recorded.

You do not have to take our word for any of this. Open your browser's developer tools, switch to the Network tab, and run one of the file-based tools. You will see the page load, and then nothing further as you work.

The exceptions, stated plainly

Three kinds of tool genuinely cannot be local, and we would rather name them than bury them:

• The currency converter fetches published reference rates, because exchange rates do not exist on your machine.
• What Is My IP asks public IP services what address they see — that is the entire question being asked. The lookup goes from your browser straight to those services; our server is not in the path.
• The ping test measures round-trip time to a network endpoint, which requires network round trips by definition.

Rule 2 — logic is separated so it can be tested

For any tool that transforms data, the transformation is written as pure functions in a separate module from the interface. This is not architectural fashion; it means the logic can be executed and checked outside a browser, against real files, without clicking through a UI.

The PDF engine was exercised against real documents covering every operation it exposes — merge, split, extract, rotate, image-to-PDF, page numbering, watermarking, metadata read and write, and form filling — with the output PDFs re-opened and inspected afterwards, not just checked for “no error thrown”.

The video editor is verified the same way but harder, because a plausible video file can still be wrong. Its exports are decoded frame by frame and checked against what was asked for: that a cut lands on the right frame, that joining clips of different resolutions and frame rates produces clean letterboxing rather than stretched frames, that a 2× speed change yields exactly half the duration with correctly remapped frames, that a burned-in caption appears and disappears on the specified timestamps, and that a one-second dissolve actually blends at the midpoint. Audio output is decoded and measured to confirm tracks stay in sync after resampling.

Rule 3 — one tool, one job

Tools that share an input get combined rather than split into separate pages that make you paste the same thing repeatedly. The colour picker carries contrast checking, gradients, and palettes in one place. The time zone converter includes a world clock and a meeting planner. The image resizer applies cropping, rotation, resizing, and format conversion as a single pipeline over one loaded image.

The reverse also holds: when two jobs genuinely differ, they stay apart. Counting words and counting characters look similar but are reached for in different situations, so they remain separate tools.

Rule 4 — the interface should not trick you

Ads never sit inside the working area of a tool, and there are no fake download buttons, no countdown timers, and no “your file is ready” interstitials. Results appear where you are already looking. Settings you would resent re-entering — indent size, unit preferences, saved time zones, tracker entries — are stored in your browser's local storage and stay on your machine.

What we refuse to build

Some categories of tool reliably attract traffic and we have turned them down anyway:

• Media downloaders. Tools that rip video or audio from other people's platforms. Our page inspector is deliberately built as a developer inspector, not an extractor — it will show you the SVG code on a page so you can learn from it, but it will not hand you a one-click copy of someone else's artwork.
• Anything that needs an account to work. No tool here asks who you are.
• Upload-then-download converters. If a proposed tool can only work by shipping your file to a server, it does not get built unless the network trip is the actual point of the tool.
• Medical or financial tools that pretend to be advice. The health calculators show their formulas and carry explicit disclaimers. The period and pregnancy tools state clearly that they are not contraception and not a diagnosis.

Corrections

Guides and reference tables carry a visible last-updated date, and when we find an error we fix it rather than quietly deleting the page. If you spot something wrong — a broken edge case in a tool, an inaccurate claim in a guide, a reference row that is out of date — tell us on the contact page. Corrections from readers are the cheapest quality improvement available to a site like this, and they are genuinely welcome.