A file format using 64 ASCII characters to encode the six bit binary data values 0-63.

To convert data to base 64, the first byte is placed in the most significant eight bits of a 24-bit buffer, the next in the middle eight, and the third in the least significant eight bits. If there a fewer than three bytes to encode, the corresponding buffer bits will be zero. The buffer is then used, six bits at a time, most significant first, as indices into the string "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" and the indicated character output. If there were only one or two input bytes, the output is padded with two or one "=" characters respectively. This prevents extra bits being added to the reconstructed data. The process then repeats on the remaining input data.

Base 64 is used when transmitting binary data through text-only media such as electronic mail, though uuencode is more common.