Base64 File Converter

Privacy note

Conversions happen in your browser. Nothing is uploaded by this page. (Just remember: if you paste Base64 into a third-party site, that’s a separate decision.)

A) Base64 to file (download)

B) File to Base64 (copy/paste)

Quick sanity check: does the size look reasonable?

Base64 increases size by about one third. A handy rule of thumb is:

where $n$ is the input size in bytes and $L$ is the Base64 length in characters (not counting line breaks).

Example 3: Debugging an API that returns Base64

Background: you’re not sure whether an API returns a raw Base64 payload or a full data URI. Paste it into the Base64 field. If it includes a prefix like $\text{data:...;base64,}$, the tool can use that to decide the file type.

If you only need Base64 text decoding (not files), use our Base64 String Encoder/Decoder.

Practical tips

• If your Base64 includes whitespace or line breaks, paste it anyway — Base64 often travels with formatting.
• Prefer leaving Extension blank if your Base64 is a full data URI. The MIME type is already there.
• For very large files, Base64 can be memory-heavy. If you can, use direct uploads instead of encoding.
• If a decoded file won’t open, verify the source (wrong MIME type, truncated payload, or URL-safe Base64).

Common mistakes to avoid

• Mixing Base64 and Base64URL: Base64URL replaces $+$ and $/$ with URL-safe characters.
• Truncated payloads: if you copied from logs, ensure you didn’t cut off the end (padding matters).
• Assuming Base64 is secure: it’s reversible by design.

Why padding exists

If the byte length is not a multiple of 3, Base64 adds padding characters so the output length is a multiple of 4. In many contexts, you’ll see one or two $=$ characters at the end.

Data URIs

A data URI embeds the MIME type and the Base64 payload in one string. Conceptually it looks like:$\text{data:}\langle\text{mime}\rangle\text{;base64,}\langle\text{payload}\rangle$.

Base64 vs Base64URL

Base64URL is a URL-safe variant that avoids characters that often need escaping. If you’re dealing with tokens (JWT parts, URL parameters), prefer Base64URL.

What this tool does not guarantee

• MIME inference is best-effort. Unknown signatures may not auto-detect.
• Very large inputs can hit browser memory limits.
• This is not a security product. Don’t treat Base64 as protection.