File to Base64

Encoding produces the full data URI — with a MIME prefix the browser detects from the file — and the raw Base64 below it, each with its own copy button. The data URI form belongs in HTML and CSS (image src attributes, font-face declarations, inline downloads); the raw form belongs in JSON payloads, XML, and database fields, which supply their own framing. Base64 always costs you a third in size: 4 output characters per 3 input bytes, so a 5 MB file becomes a 6,990,508-character string. That entire string is rendered into the page, so encoding very large files makes the tab sluggish — this workflow is built for icons, fonts, small documents, and API fixtures, not videos.

In decode mode, the byte reconstruction is exact — Base64 is a lossless transport encoding. What the string cannot always tell the tool is what the bytes are: if your input carries a data URI header, the MIME type is extracted from it and filled in automatically; if you paste raw Base64, you supply the MIME type and filename yourself. Getting the MIME type wrong does not corrupt anything, but the filename extension is what your OS and applications key off, so name the output to match the content — decoded PNG bytes saved as report.pdf will be a byte-perfect PNG that nothing opens. The decoded byte count appears once you click out of the input field, which is a quick integrity check: it should be very close to three-quarters of the string length.