Developer
Format JSON, encode Base64 and binary, make UUIDs, check colour contrast, grab placeholder text, test regex and read JWTs.
JSON Formatter
Format, minify and validate. Data stays here.
Base64 Encode/Decode
Encode or decode Base64 text in the tab.
UUID Generator
Random UUIDs, ready to copy.
Regex Tester
Test a regex live, with groups, replace and a plain-English breakdown.
JWT Decoder
Decode a JWT, check its expiry and verify the signature privately.
Diff Checker
Compare two texts and see exactly what changed, word by word.
Cron Expression Builder
Build a cron schedule, read it in English and see the next runs.
URL Encode/Decode
Percent-encode or decode text and break a link into its parts.
Unix Timestamp Converter
Convert Unix timestamps to UK time and back, with a live clock.
Binary Encode/Decode
Turn text into binary and back again.
Colour Contrast Checker
Check text and background colours are readable.
Lorem Ipsum Generator
Placeholder copy, the length you actually need.
About these developer tools
This hub collects the small jobs that interrupt real work: an API response arrives as one unreadable line, a config value needs Base64, a test fixture wants a dozen fresh IDs, or a designer asks whether grey text on white is readable. Each tool does one of those jobs in a single page, so you can paste, get the answer and get back to what you were doing.
It suits developers and testers, but it’s just as handy for students learning how data is stored, website owners poking at a theme, marketers tagging a campaign link, and designers checking accessibility before a handover.
What’s in this hub
Data and encoding
The JSON Formatter formats, minifies and validates JSON, with a choice of indent and an option to sort keys. Base64 Encode/Decode is UTF-8 safe and has a URL-safe mode for tokens and query strings. Binary Encode/Decode shows each UTF-8 byte as eight bits, which is useful for teaching or checking how an emoji really takes up four bytes. The UUID Generator makes up to 50 version 4 IDs at a time and can check a pasted one.
Design and publishing
The Colour Contrast Checker gives the ratio between two colours and a pass or fail for WCAG 2.1 AA and AAA, and can suggest a foreground that passes. The Lorem Ipsum Generator fills a layout with words, sentences or paragraphs, optionally wrapped in <p> tags.
How the tools work
Every tool here runs entirely in your browser. JSON, encoded strings, UUIDs and colours are processed in the tab and aren’t sent to Catpool, which matters when a payload contains customer data or a key you’d rather keep local. UUIDs come from the browser’s Web Crypto random generator. If that isn’t available, the page tells you rather than quietly falling back to weaker randomness.
- No account, no install, no extension
- Nothing to set up before you start
- Works on a laptop or a phone
- Nothing you paste is sent to Catpool
When to use them
Reach for them in the middle of something else. You’re debugging a webhook and the payload is a wall of text: paste it into the JSON Formatter and the missing comma shows up straight away. A Basic Auth header looks wrong: decode it and check what was really sent. You’re seeding a test database and need unique keys: generate 50 UUIDs and copy them as a block. A client wants their brand pink for body text: the contrast checker shows whether it passes, and suggests a darker shade if not.
They’re not a replacement for proper tooling in a codebase, such as a linter or your language’s own libraries. They’re for quick checks when opening an IDE would take longer than the job. For images, colour picking from photos and text tools, head to Create. To check a suspicious or shortened link, use Link X-Ray in Security.
Common questions
Is it safe to paste real data here?
For the JSON, Base64, binary, UUID and contrast tools, yes: nothing leaves the tab. Even so, it’s good practice to remove live passwords and keys from anything you paste into a website.
Is Base64 a form of encryption?
No. It’s an encoding, and anyone can decode it in seconds. Use it for moving data around safely in text, not for hiding it.
Are version 4 UUIDs really unique?
Each one has 122 random bits, so a clash is so unlikely you can ignore it for everyday use. They aren’t secrets though, so don’t use one as a password or API key.
Which contrast level should I aim for?
AA is the usual target: 4.5:1 for normal text and 3:1 for large text. AAA (7:1) is stricter and worth aiming for on long reading pages.
How long do short links last?
That’s up to TinyURL or is.gd, not Catpool. For printed material that has to work for years, a link on your own domain is safer.