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About this tool
Convert between plain text, standard Base64 (RFC 4648 §4) and URL-safe base64url (§5), with padding repair and a URL length comparison.
Base64 URL Converter moves a payload between plain UTF-8 text, standard Base64 and URL-safe base64url, the two alphabets defined in RFC 4648 §4 and §5. They differ in exactly two characters — standard uses `+` and `/` at positions 62 and 63, URL-safe uses `-` and `_` — and base64url normally omits the `=` padding, which is what makes JWT segments fail in ordinary decoders. It is for developers debugging tokens, signed URLs, webhook signatures and query-string payloads.
Open Base64 URL Converter on AltFTool — it loads instantly in your browser.
Type or paste your payload into the Input box. The line under it reports what was detected — plain text, standard Base64, URL-safe Base64 or mixed alphabets — and the limit is 2,000,000 characters.
Set 'Treat input as' and 'Convert to'; both menus offer Plain text (UTF-8), Standard Base64 and URL-safe Base64. Tick 'Keep = padding on URL-safe output' when you need the = characters kept, and press Swap to exchange the two formats and move the result back into the Input box.
There is no convert button — the Result box updates as you type, above a table giving the same payload as Standard Base64, URL-safe Base64 and Percent-encoded standard. Press Copy result or Copy URL-safe for the clipboard, or Reset, which asks 'Reset to the default example?' before replacing anything you changed.
Text, standard Base64 and base64url convert in any combination, so you never chain two tools.
Missing `=` characters are recalculated from the length, and can be stripped again on output.
A side-by-side length comparison of base64url against percent-encoded standard Base64, where +, / and = each cost 3 characters.
Two characters. Standard Base64 (RFC 4648 §4) uses `+` for value 62 and `/` for 63; base64url (§5) uses `-` and `_` instead, because `+` means a space in a query string and `/` splits a path segment. The `=` padding is also usually dropped in base64url, since `=` is reserved in URLs. Everything else — the 4:3 ratio, the first 62 characters — is identical.
Because JWT segments are base64url with the padding removed, as RFC 7515 §2 requires. A standard decoder sees `-` and `_` as illegal characters and a length that is not a multiple of 4, and refuses. Convert the alphabet and add back 1 or 2 `=` characters — this tool does both automatically.
It saves 2 characters for every `+` or `/` in the standard form, because each of those becomes a 3-character percent escape (`%2B`, `%2F`) but only a 1-character swap (`-`, `_`) in base64url. Roughly 1 in 32 Base64 characters is a `+` or `/`, so a 1,000-character token typically saves 60–70 characters from that alone. Padding is dropped entirely by default rather than swapped, so each `=` saves the full 3 characters its percent escape (`%3D`) would have cost — up to 6 more for the maximum of two padding characters.
Yes for URLs and filenames on every mainstream filesystem — that is exactly what RFC 4648 §5 calls it: "base64url", the URL and filename safe alphabet. The remaining characters are A–Z, a–z, 0–9, `-` and `_`, none of which are reserved in a path segment or illegal on Windows, macOS or Linux.
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