Base64 Encode and Decode Online: Strings, Files, and Data URIs

What Is Base64 Encoding?

Base64 encodes binary data into a string of 64 printable ASCII characters: A–Z, a–z, 0–9, +, /. The name comes from the 64-character alphabet. Every 3 bytes of input becomes 4 characters of output, increasing the size by approximately 33%.

The reason Base64 exists: many protocols (email SMTP, HTTP headers, XML, JSON) are designed for text and can't reliably transmit arbitrary binary data. Base64 converts binary into text that can safely pass through any text-based protocol.

Base64 vs URL-Safe Base64

Standard Base64 uses + and / which have special meaning in URLs. URL-safe Base64 replaces + with - and / with _. This matters when embedding Base64 in query parameters or JWT tokens (JWTs use URL-safe Base64 for their header and payload sections).

The padding character = is also sometimes omitted in URL-safe contexts. Most decoders handle both padded and unpadded input, but if you're passing Base64 in a URL parameter, always use the URL-safe variant without padding.

Common Base64 Use Cases in Web Development

Data URIs: Embed small images directly in HTML or CSS: data:image/png;base64,iVBOR... eliminates a network request. Best for icons under 2KB; larger images are more efficiently served as separate files.

Basic Authentication: HTTP Basic Auth sends credentials as Base64: Authorization: Basic base64(username:password). Note: Base64 is encoding, not encryption. The credentials are trivially recoverable. Always use HTTPS.

API payloads: Some APIs accept binary files as Base64 strings in JSON payloads — simpler than multipart/form-data for small files, though less efficient.