Ascii85, also called Base85, is a form of binary-to-text encoding developed by Paul E. Rutter for the btoa utility. By using five ASCII characters to represent four bytes of binary data (making the encoded size 1⁄4 larger than the original, assuming eight bits per ASCII character), it is more efficient than uuencode or Base64, which use four characters to represent three bytes of data (1⁄3 increase, assuming eight bits per ASCII character).

Its main modern uses are in Adobe's PostScript and Portable Document Format file formats, as well as in the patch encoding for binary files used by Git.

The basic need for a binary-to-text encoding comes from a need to communicate arbitrary binary data over preexisting communications protocols that were designed to carry only English language human-readable text. Those communication protocols may only be 7-bit safe (and within that avoid certain ASCII control codes), and may require line breaks at certain maximum intervals, and may not maintain whitespace. Thus, only the 94 printable ASCII characters are "safe" to use to convey data.

Eighty-five is the minimum integer value of n such that n⁵ ≥ 256⁴ ${\displaystyle =}$ 2³² so any sequence of 4 bytes can be encoded as 5 symbols, as long as at least 85 distinct symbols are available. (Five radix-85 digits can represent the integers from 0 to 4,437,053,124 inclusive, which suffice to represent all 4,294,967,296 possible 4-byte sequences.)

Characters used by the encoded text are !"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstu and additionally z to mark a sequence of four zero bytes.

When encoding, each group of 4 bytes is taken as a 32-bit binary number, most significant byte first (Ascii85 uses a big-endian convention). This is converted, by repeatedly dividing by 85 and taking the remainder, into 5 radix-85 digits. Then each digit (again, most significant first) is encoded as an ASCII printable character by adding 33 to it, giving the ASCII characters 33 (!) through 117 (u).

Because all-zero data is quite common, an exception is made for the sake of data compression, and an all-zero group is encoded as a single character z instead of !!!!!.

Groups of characters that decode to a value greater than 2³² − 1 (encoded as s8W-!) will cause a decoding error, as will z characters in the middle of a group. White space between the characters is ignored and may occur anywhere to accommodate line-length limitations.