What Is Base64?

Base64 encodes arbitrary binary data as a string of 64 printable ASCII characters (A-Z, a-z, 0-9, +, /). The = padding at the end aligns the output to a multiple of 4 characters. The name comes from the 64-character alphabet, not from the number 64 itself.

Common Use Cases

• HTTP Basic Auth headers — Credentials are sent as Authorization: Basic dXNlcjpwYXNz, which is user:pass base64-encoded. This is encoding, not encryption — never rely on it for security without HTTPS.
• Data URIs — Embedding small images in CSS: url("data:image/png;base64,iVBORw0KGgo..."). Useful for small icons that save an HTTP request.
• JSON Web Tokens — JWT header and payload segments are base64url-encoded (uses - and _ instead of + and /, no padding).
• Email attachments — MIME encodes attachments in Base64 so binary data survives email transport, which was historically 7-bit ASCII only.

Base64 vs. Base64url

Standard Base64 uses + and /, which need percent-encoding in URLs. Base64url replaces them with - and _ and omits the = padding, making it safe in URL query strings and JWT segments without further encoding. This tool uses standard Base64.

Size overhead

Base64 increases size by approximately 33%. A 300-byte image becomes a 400-character string. For anything larger than ~1 KB, loading it as a separate resource is usually more efficient than inlining as a data URI.