Find non Base64 characters and show their byte values
Base64 Tools
Base64 Error Fixer
Fix Base64 decode errors by finding non Base64 characters, repairing padding, normalizing Base64URL, and explaining symbol 10, symbol 61, or 3f.
- Runs in your browser
- No account required
- Copy-ready output
Explain illegal base64 character 3f, symbol 10, and symbol 61 cases
Normalize Base64URL dash and underscore characters
Repair missing padding when the length allows it
Notes
How this tool handles your input
This Base64 error fixer is built for copied values that fail in strict decoders. Paste the value and it checks whitespace, wrappers, URL-safe characters, equals-sign padding, illegal bytes such as 3f, and whether the decoded bytes are readable UTF-8 text.
How to use
- Paste the failing Base64 value, including the copied wrapper if you are unsure.
- Click Diagnose and Repair.
- Read the detected issues, repaired value, and decode result.
- Copy the repaired value or decoded text only after checking whether the source format allows that edit.
Good for
- Fix copied Base64 values from logs, headers, JSON fields, and terminal output
- Check non Base64 characters before changing application code
- Repair padding and Base64URL values for local debugging
- Understand Rust, Python, browser atob, and command-line decode errors
Examples
Input
Bearer SGVsbG8gMTAyNGJhc2U\n
Output
SGVsbG8gMTAyNGJhc2U= decodes to Hello 1024base after the wrapper and newline are removed.
Input
SGVsbG8?MTAyNGJhc2U=
Output
The question mark is byte 0x3F. It is not part of standard Base64.
Input
SGVsbG8tdXJsX3NhZmU
Output
Dash and underscore are detected as Base64URL characters before padding is checked.FAQ
What does illegal base64 character 3f mean?
3f is hexadecimal byte 0x3F, the question mark. It is not part of standard Base64, so the copied value needs to be checked at that position.
Does this change signed tokens?
No. The tool shows a repaired copy for inspection. Do not edit and reuse a signed token unless the issuer expects that representation.
Can it fix invalid padding?
It can add trailing equals signs when the cleaned length leaves a remainder of two or three. A remainder of one points to missing or truncated data.
Does it handle Base64URL?
Yes. It detects dash and underscore characters and shows the standard Base64 form used by strict decoders.
Is my data uploaded to a server?
No. The tool runs in your browser, so the text you paste stays on your device.