Editing Base64 (section)

===MIME===
{{Main|MIME}}

The [[MIME]] (Multipurpose Internet Mail Extensions) specification lists Base64 as one of two [[binary-to-text encoding]] schemes (the other being [[quoted-printable]]).<ref name="rfc 2045"/> MIME's Base64 encoding is based on that of the {{IETF RFC|1421}} version of PEM: it uses the same 64-character alphabet and encoding mechanism as PEM and uses the <code>=</code> symbol for output padding in the same way, as described at {{IETF RFC|2045}}.

MIME does not specify a fixed length for Base64-encoded lines, but it does specify a maximum line length of 76 characters. Additionally, it specifies that any character outside the standard set of 64 encoding characters (For example CRLF sequences), must be ignored by a compliant decoder, although most implementations use a CR/LF [[newline]] pair to delimit encoded lines.

Thus, the actual length of MIME-compliant Base64-encoded binary data is usually about 137% of the original data length ({{fract|4|3}}×{{fract|78|76}}), though for very short messages the overhead can be much higher due to the overhead of the headers. Very roughly, the final size of Base64-encoded binary data is equal to 1.37 times the original data size + 814 bytes (for headers). The size of the decoded data can be approximated with this formula:
 bytes = (string_length(encoded_string) − 814) / 1.37