Base64 ⇄ Base64url Converter
Convert standard Base64 to URL-safe Base64url and back — swap +/ for -_, handle = padding, auto-detect the direction. Runs entirely in your browser, no server, no sign-up.
What this tool does
Convert a standard Base64 string into URL-safe Base64url and back, right in your browser. Nothing is uploaded — it runs locally, works offline, and needs no sign-up. This is a pure alphabet transform: it swaps the two characters that differ between the alphabets and applies a padding rule. It does not decode Base64 to plain text.
Standard Base64 uses + and /, which have special meaning in URLs, filenames,
and JWTs. Base64url replaces them with - and _ and usually drops the =
padding, so the value can be dropped straight into a query string, path segment,
cookie, or filename.
| Char 62 | Char 63 | Padding | |
|---|---|---|---|
| Standard Base64 | + | / | = (kept) |
| URL-safe Base64url | - | _ | usually dropped |
Direction
| Direction | What it does |
|---|---|
| auto (default) | If the input already contains - or _ it is treated as Base64url and converted to standard; otherwise it is treated as standard Base64 and converted to URL-safe Base64url. |
| to-url | Force standard → URL-safe (+ → -, / → _). |
| to-standard | Force URL-safe → standard (- → +, _ → /). |
Padding
| Padding | What it does |
|---|---|
| auto (default) | Pads standard output with =, leaves Base64url output unpadded — each alphabet's canonical form. |
| keep | Always pad the output to a length that is a multiple of 4. |
| strip | Remove all = padding from the output. |
Turn on Validate input is real Base64 to reject a malformed string (wrong length or stray characters) instead of silently transforming it.
Examples
| Input | Settings | Output |
|---|---|---|
c3ViamVjdHM/X2Q9MQ== | to-url · auto padding | c3ViamVjdHM_X2Q9MQ |
c3ViamVjdHM_X2Q9MQ | to-standard · auto padding | c3ViamVjdHM/X2Q9MQ== |
+/+/ | auto | -_-_ |
c3ViamVjdHM/X2Q9MQ== | to-url · keep padding | c3ViamVjdHM_X2Q9MQ== |
eyJhbGciOiJIUzI1NiJ9 | auto | eyJhbGciOiJIUzI1NiJ9 |
In the last row the JWT-style token has no +, /, -, or _, so both
alphabets agree and only the padding could differ.
Limits & edge cases
- Whitespace is ignored. Spaces, tabs, and newlines are stripped first, so line-wrapped MIME Base64 (76-column) and copy-pasted tokens convert cleanly.
- Alphabet only. Any character outside
A–Z a–z 0–9 + / - _ =is rejected with a message naming it — this is a Base64 transcoder, not a text encoder. - Not a decoder. It never turns Base64 into the underlying bytes/text; use a Base64 decoder for that. Here the output is still Base64, just in the other alphabet.
=only at the end. Padding is only valid as a trailing run; a=in the middle is treated as malformed input.
FAQ
What is the difference between Base64 and Base64url?
They encode the same bytes but use two different characters for the last two
symbols of the alphabet. Standard Base64 uses + and /; URL-safe Base64url
uses - and _ so the value is safe inside URLs, filenames, and JWTs, and it
usually omits the = padding.
Does this decode my Base64 to text?
No. It only converts between the two Base64 alphabets and adjusts padding — the output is still Base64, just URL-safe (or standard). To recover the original bytes or text you need a Base64 decoder instead.
Why did the padding disappear?
With the default auto padding, URL-safe Base64url output is left unpadded
because that is its canonical form (and how JWTs are written). Choose keep
if the system you are targeting still expects trailing = characters.
How does auto-detect decide the direction?
It looks for the URL-safe markers - and _. If either is present the input is
assumed to be Base64url and converted to standard; otherwise it is assumed to be
standard Base64 and converted to URL-safe. Set the direction explicitly if your
input is ambiguous.
Can I paste a whole JWT?
Convert one segment at a time. A JWT is dot-separated Base64url parts
(header.payload.signature); the . separator is not part of the Base64
alphabet, so paste an individual segment rather than the entire token.
Developer & Automation Access
Run it from the terminal
Same engine as this page, headless — via the gizza CLI:
gizza tool base64url-converter "c3ViamVjdHM/X2Q9MQ=="New to the CLI? Get gizza →
Open it by URL
Pre-fill and auto-run this tool with query parameters — the names match the API/CLI:
https://gizza.ai/tools/base64url-converter/?text=c3ViamVjdHM%2FX2Q9MQ%3D%3D&direction=auto&padding=auto&validate=trueMachine-readable descriptor: tool.json — title + parameters JSON Schema for agents.