Almost everything in this category is the same problem wearing different clothes: a character that means one thing to you and something structural to whatever is reading it. An ampersand separates query parameters, an angle bracket opens a tag, a plus sign is a space in a submitted form. Encoding is how you say "this one is data" — and the reason there is more than one scheme is that each reader draws the line in a different place. The tools here are deliberately explicit about which reader they are writing for, because encoding correctly for the wrong context is the same as not encoding at all. All of them convert as you type and none of them sends what you paste anywhere.
Tools in this category
Base64 Encode and Decode
Turn text into Base64 and back as you type, in the standard alphabet or the URL-safe one. Nothing you paste leaves the page.
URL Encoder and Decoder
Percent-encode text as you type, choosing whether you are escaping one value inside a URL or an address as a whole. Nothing is uploaded.
HTML Entity Encoder and Decoder
Escape text for HTML and read entities back, with the attribute case kept separate from the text case. Everything runs in your browser.
Suggest a tool
Tell us what is missing and where its numbers should come from.
What should it convert, calculate or generate, and where would the authoritative numbers come from? A link to the standard or publication that defines them is the most useful thing you can send.
Opens your mail app with this page already filled in.
How these tools are built
Every tool in this category runs entirely in your browser: there is no upload, no account and no request to a server once the page has loaded. The numbers behind them are transcribed from the organisation that defines them — NIST for units, the W3C for web formats, the WHO for health measures — and each page names its source next to the formula. A tool whose reference values stop reproducing fails the build and does not ship, which is the whole reason the catalogue grows slowly.