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Base64 String Checker

Validate Base64/Base64URL quickly: detect invalid characters, padding problems, and decode to bytes/text for a sanity check. Runs fully in your browser (no network calls).

Category: Developer · URL: /tools/base64-string-checker.html
Privacy: runs locally in your browser. No uploads, no tracking scripts.

How to use

Paste your string, choose how strict you want the validation to be, then click Check.

  1. Paste Base64/Base64URL into the input.
  2. (Optional) Enable Ignore whitespace and Auto-fix padding.
  3. Click Check to see validity, normalization, and a decode preview.
  4. Use Copy buttons to copy the normalized Base64 or the report.
Keywords this page targets (natural cluster): base64 string checker, check base64 validity, validate base64 string, base64url validator, is this base64, base64 padding checker, fix base64 padding, base64 decode preview, base64 validator online, base64 character set check, base64 whitespace ignore, normalize base64, base64url to base64, invalid base64 finder, base64 decoding error, verify base64 token, jwt base64 check, base64 strict validation, base64url without padding, base64 line breaks validator
Secondary intents covered: Confirm whether a copied value is valid Base64 or Base64URL, Find the first reason a Base64 decode fails (chars, length, padding), Normalize Base64 by stripping whitespace and adding missing padding, Convert Base64URL to standard Base64 for debugging, Preview decoded bytes as UTF-8 text and hex, Copy a normalized Base64 string for use in code or tools, Quickly sanity-check JWT segments before further parsing, Detect whether the input contains non-printable binary data

FAQ

What’s the difference between Base64 and Base64URL?

Base64 uses + and /, while Base64URL uses - and _ so it’s URL-safe; padding = is often omitted in Base64URL.

Why does my Base64 string fail to decode?

Common causes are invalid characters, length not being a multiple of 4, or incorrect/missing = padding.

Is it safe to paste secrets here?

The checker runs locally in your browser and doesn’t send data over the network, but you should still avoid sharing sensitive values on untrusted devices.

Can I validate Base64 that contains line breaks?

Yes—enable Ignore whitespace to strip newlines and spaces before validation/decoding.

What does “Auto-fix padding” do?

It adds the required = characters when the unpadded length is valid (mod 4 of 2 or 3); it can’t fix an impossible mod 4 of 1.

Why does the UTF-8 preview look weird?

Not all Base64 decodes to UTF-8 text—many values are binary, so the tool also shows a hex preview of the decoded bytes.

Does “Valid” guarantee the decoded content is correct?

No—“Valid” means the string is well-formed Base64 and decodes without error; it doesn’t verify semantics like file type, JWT structure, or signatures.