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Base64 Format Checker

Validate whether a string is properly formatted Base64 (or Base64URL). This checks characters, padding, and length rules, and can normalize or decode a compact preview locally in your browser.

Category: Developer · URL: /tools/base64-format-checker.html
Input
Runs locally in your browser (no network calls).
Privacy: runs locally in your browser. No uploads, no tracking scripts.

How to use

Paste your Base64/Base64URL text, pick the rules you want, then validate or decode.

  1. Paste the string into the input box.
  2. Choose options (ignore whitespace, allow URL-safe characters, allow missing padding, or require strict padding).
  3. Click Validate to see format issues, or Validate + Decode Preview to also view bytes/UTF-8 preview.
  4. Use Normalize to convert Base64URL to standard Base64 and fix padding.
Keywords this page targets (natural cluster): base64 format checker, base64 validator, check base64 string, is this base64 valid, base64 padding checker, base64url validator, base64url format checker, base64 length multiple of 4, base64 invalid character check, base64 whitespace ignore, normalize base64 string, base64 decode preview, base64 padding required, base64 missing padding, base64 url safe characters, validate base64 online, base64 strict validation, base64 equals sign padding, base64 regex checker, base64 decode utf-8
Secondary intents covered: Validate a Base64/Base64URL string before decoding, Find why a Base64 string fails to decode (invalid char, bad padding, bad length), Normalize Base64URL to standard Base64 with correct padding, Strip whitespace/newlines and re-check validity, Quickly estimate decoded size (byte count) via a safe decode preview, Verify that '=' padding appears only at the end, Check if a token is Base64URL (JWT segments) vs standard Base64, Copy the validation result or normalized output

FAQ

What does this Base64 format checker validate?

It checks allowed characters, correct placement of '=' padding, and length rules (including missing padding if you allow it).

Does a valid Base64 string always have '=' at the end?

No. Padding may be 0, 1, or 2 '=' characters, and Base64URL commonly omits padding entirely.

What’s the difference between Base64 and Base64URL?

Base64URL uses '-' and '_' instead of '+' and '/', and often drops '=' padding to be URL-safe.

Why does my Base64 fail when I paste it from logs?

It may include spaces or newlines; enable “Ignore whitespace/newlines” and validate again.

Can this tool decode the text too?

Yes, “Validate + Decode Preview” shows byte length plus a short UTF-8 and hex preview (when possible).

Is decoding guaranteed to produce readable text?

No. Base64 can represent any bytes, so the decoded data might be binary, not UTF-8 text.

Is my data sent to a server?

No. The validation and decode preview run locally in your browser with no network requests.