21 free tools that run in your browser

These tools are needed at the worst possible moment: a webhook payload arrives on one unreadable line, a token has to be checked before a deploy goes out, a config file will not parse and the error message points at "line 1".

Which one to reach for is mostly about what you are trying to learn. Use the JSON and YAML formatters when you need to read a structure, and watch their error output when you need to know why a parser rejected it — the position of the first invalid character is usually the whole answer. Base64 and URL encoding look interchangeable and are not: one is a transport alphabet for binary data, the other escapes characters that would otherwise break a query string, and swapping them corrupts data silently. Reach for the regex tester before you paste a pattern into code, and the cron parser before you trust a schedule expression that runs at 3am.

Anything touching a credential — JWTs, hashes — is decoded here in your browser and never transmitted. That still is not a licence to paste live secrets into web pages: a token you have pasted anywhere is a token worth rotating.

Writing for the web means writing to limits. A meta description that runs past roughly 160 characters gets truncated in search results, a title past about 60 characters does the same, and an ad headline or a social post has a hard cap. Counting by eye does not work, and counting in a word processor gives you the wrong number because it disagrees about what a character is.

Use the word counter for those limits and for reading-time estimates. Use case conversion when text has to move between contexts that disagree about capitalisation — a database column in snake_case becoming a label in Title Case, a heading being reused as a slug. Use the diff when two versions of a document exist and nobody remembers what changed: eyeballing two paragraphs side by side reliably misses a changed number, and that is exactly the kind of edit that matters.

All three work on text you paste in, in your own browser, which makes them safe for drafts and client material that should not be sitting in someone else's logs.

The arithmetic here is not hard, which is precisely why it goes wrong. Percentage change and percentage difference are not the same calculation and answer different questions. A "20% discount then 20% off" is not 40%. Return on investment quoted without a time period cannot be compared to anything. Compound growth is the one people underestimate most, because the intuition is linear and the maths is not.

So each calculator shows the formula and the inputs it used, not just a number. That matters when you have to defend the figure to someone else, or when you come back a week later and cannot remember whether the rate you entered was annual or monthly.

Two things these tools deliberately do not do: model fees and taxes, and predict the future. A compound interest projection is arithmetic on the assumptions you typed, not a forecast — real outcomes move with fees, inflation and risk that no calculator on a website knows about. Use them to check a number, then check the number that matters with someone qualified.

Dates are where confident code breaks. A Unix timestamp in a log is seconds, the one in your JavaScript is milliseconds, and reading one as the other puts you in 1970 or the year 56000. Counting days between two dates by hand fails at month boundaries, leap years and the two days a year when daylight saving time makes a "day" 23 or 25 hours long.

Use the timestamp converter when you are reading logs, debugging an expiry claim or checking what a database actually stored — it shows the value in your local timezone and in UTC at the same time, which is usually where the confusion was. Use the date difference calculator for contract terms, notice periods, age calculations and anything where being one day out has consequences.

Both use your browser's own timezone database, so the conversion matches the machine you are sitting at rather than a server in another region.

Image tools are the ones most worth being careful about, because "upload your photo to resize it" means giving a stranger the original file and whatever metadata it carries. These two do the work with the Canvas API inside your browser: the file is read from disk by the page, processed in memory, and handed back as a download. Nothing is transmitted, which you can verify by going offline before you start.

Resize when a dimension is the problem — an image that is 4000px wide on a page that displays it at 800px wastes bandwidth and hurts Core Web Vitals for every visitor. Convert when the format is the problem: WebP is typically 25–35% smaller than a comparable JPEG, PNG is the right answer for screenshots and anything needing transparency, and JPEG remains the safe choice for photographs going somewhere old.

If both are wrong, resize first and convert second. Encoding a large image and then shrinking it throws away quality you already paid for.

Every site eventually needs a small artefact that is annoying to produce by hand. This category collects those.

The QR code generator is the current occupant. It renders the code in your browser as a static image pointing directly at whatever you entered — no shortener, no redirect through a tracking domain, no expiry. That distinction matters more than it sounds: most free QR services quietly route scans through their own server, which means the code stops working when they change their pricing, and every scan is logged by someone you did not choose. A code generated here has no owner but you.

The same applies to what you encode. Wi-Fi credentials, a private URL, contact details — none of it is transmitted, so it is safe to generate a code for something that is not meant to be public.