Base64 Validator
Check any string for valid Base64 with per-check diagnostics and one-click auto-fix.
Last updated: October 12, 2026
Try with a Sample:
Validation Report
Paste a string on the left to check its character set, padding, length, and decodability — with an auto-fix when possible.
Base64 Validator
Verify any string is genuine Base64 with our Base64 Validator. Instead of a bare yes or no, you get a full diagnostic report covering the four things that matter: character set (with the exact position of every illegal character), padding placement, length rules, and real decodability with the resulting byte size. Lenient options tolerate whitespace, URL-safe alphabets, and missing padding, or switch them off for strict RFC 4648 checking. When the input is broken but repairable, the tool produces a corrected string you can copy or load back into the input with one click, along with a plain-language summary of every repair. A content sniffer even hints at what the payload looks like, such as PNG or PDF. Everything runs instantly and locally in your browser.
Why This Tool Exists
What makes this useful — and why I built it this way.
Precise Diagnostics: See exactly which character and rule failed.
Three-State Verdicts: Distinguish clean input from repairable input.
Flexible Strictness: Match API, JWT, or RFC 4648 requirements.
Instant Repairs: Whitespace, alphabets, and padding fixed for you.
Local Processing: Strings never leave your browser.
When You'd Use This
Real situations where this tool saves the day.
API Debugging: Find why a token or payload field is rejected.
Data Cleaning: Repair Base64 copied from chats, emails, and PDFs.
JWT Inspection: Validate the alphabet and padding of token segments.
QA Testing: Assert encoder output meets strict Base64 rules.
Security Review: Confirm untrusted input decodes before processing.
Using Base64 Validator
It's straightforward — here's how it works.
Paste the suspected Base64 string into the input box.
Choose strict or lenient checking options.
Read the verdict and per-check diagnostics as they update live.
Copy the fixed version when the input is repairable.
Load the fix back into the input to confirm a clean valid verdict.
Questions People Ask
Honest answers about how this works.
What is the difference between fixable and invalid?
Fixable means the data is intact but the packaging is off: missing padding, stray whitespace, or a URL-safe alphabet. Those repairs are unambiguous. Invalid means characters are illegal, padding sits mid-string, or the length is impossible, so no safe repair exists.
Does my string leave the browser?
No. Every check, decode, and repair runs locally in your browser. Nothing is uploaded, logged, or stored, so tokens, keys, and private payloads never touch a server.
Why does length matter if the characters are legal?
Base64 packs 3 bytes into 4 characters, so a complete string must be a multiple of 4 (plus up to 2 padding characters). A length with remainder 1 is mathematically impossible and always means truncation or corruption.
What does 'looks like PNG' mean?
After decoding, the tool reads the first bytes, known as magic bytes, which identify common formats like PNG, JPEG, PDF, and ZIP. It is a hint about the payload content, not a guarantee, and unknown data simply shows no hint.
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