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RADIX-50, commonly called Rad-50 or RAD50, is a character encoding created by Digital Equipment Corporation for use on their DECsystem, PDP, and VAX computers. RADIX-50's 40-character repertoire (050 in octal) allows up to 3 characters to be encoded and packed into 16 bits (PDP-11, VAX) or 6 characters plus flag information into one 36-bit word (PDP-6, PDP-10/DECsystem-10, DECSYSTEM-20).

The actual encoding differed between the 36-bit and 16-bit systems.

Strings are encoded big-endian, with the first character in the most significant position. For example, using the PDP-11 encoding, the string "ABC", with character values 1, 2, and 3, would be encoded as (1*40 + 2) * 40 + 3 = 1683. When there are fewer than three characters, they are padded with trailing spaces. 16-bit encoded values range from 0 (three spaces) to 63999 ("999").

This encoding inherently "pads" strings that are not multiples of 3 characters with trailing spaces.

There were several minor variations of the encoding families. For example, the RT-11 operating system considered the character corresponding to value 011101 to be undefined, and some utility programs used that value to represent * instead.

The use of Rad-50 was the source of the filename size conventions used by the PDP-11 operating systems. Using Rad-50 encoding, six characters of filename could be stored in two sixteen-bit words while three more characters of extension (filetype) could be stored in a third sixteen-bit word. The period that separated the filename and extension was implied (not stored and always present). Rad-50 was also commonly used in the symbol tables of the various PDP-11 programming languages.

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