uuencode is a UNIX utility from the 1980s that turns binary files into printable ASCII so they can pass through email and Usenet. The encoded data is framed by a begin line with permissions and filename, and an end line.

How does the line format work?

Each data line starts with a length character giving how many bytes that line encodes, followed by groups of four characters. Every group of four characters represents three original bytes using 6-bit values offset by 0x20.

What is the back-tick at the end?

A length of zero is written as the back-tick character (0x60) rather than a space, signalling the final data line. This is the modern convention that avoids trailing-space stripping problems.

Is uuencode the same as Base64?

Both map 3 bytes to 4 characters, but uuencode uses values offset by 0x20 across a contiguous ASCII range and adds per-line length bytes plus begin/end framing. Base64 uses a fixed 64-character alphabet and padding.

Can it decode files made by the real uuencode command?

Yes. The decoder ignores the begin and end lines, reads the leading length byte on each line, and stops at the zero-length terminator, matching standard uudecode behaviour for text payloads.

What is the UUencode Encoder/Decoder?

Encode text into the classic UNIX uuencode format with begin/end framing, or decode a uuencoded block back to plain text. Implements the real 3-byte-to-4-character algorithm with per-line length bytes. Free, browser-based, instant. It runs free in your browser on Gera Tools, with nothing uploaded.

UUencode Encoder/Decoder

Name: UUencode Encoder/Decoder
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

What uuencode does

uuencode (UNIX-to-UNIX encode) was the original way to send binary attachments through plain-text systems like email and Usenet before MIME and Base64 became universal. It converts bytes into printable ASCII and frames them with a begin header that carries file permissions and a name, plus an end trailer.

How it works

The data is processed three bytes at a time. Those 24 bits are split into four 6-bit groups, and each group is turned into a printable character by adding 0x20 (a space). A group of zero is written as the back-tick ` (0x60) to dodge trailing-whitespace issues. Lines hold up to 45 input bytes (60 output characters) and begin with a single length character that also follows the +0x20 rule:

begin 644 message.txt
M1V5R82!Ub V]L<P``
`
end

The leading character of each data line tells the decoder how many bytes that line really represents, so any padding in the last group can be discarded. A final line whose length character decodes to zero ends the data stream.

The begin line: what those numbers mean

The first line of any uuencoded block follows the pattern begin MODE FILENAME. The mode is a three-digit Unix file permission octet — the same format as chmod. 644 means the owner can read and write, while group and others can only read. This mode was meaningful when uuencode was used to reconstruct actual files on a Unix system; in modern use it is largely a formality, but some decoders enforce that the mode field is present and numeric.

Why the back-tick instead of a space

The 6-bit value zero maps to ASCII 32, which is a space character. Some email servers and Usenet transport systems stripped trailing spaces from lines, which would corrupt the encoded data in the final group of a line. The uuencode convention is to encode a zero-value group as the back-tick (0x60) instead of a space, since back-ticks survive line-ending cleanup.

uuencode vs Base64: the key differences

Both formats convert binary data to text using a 3-bytes-in, 4-characters-out scheme. The differences are:

Character set. uuencode uses a contiguous block of ASCII printable characters from space (0x20) to underscore (0x5F), shifted by adding 0x20 to each 6-bit value. Base64 uses a carefully chosen 64-character alphabet (A–Z, a–z, 0–9, +, /) that avoids special characters.
Per-line length prefix. uuencode prefixes each data line with a character encoding how many bytes that line represents. Base64 has no such structure.
Framing. uuencode wraps the data in begin/end with filename and mode. MIME Base64 uses a Content-Type header for metadata.

When you still encounter uuencode today

uuencode is effectively obsolete for new systems — MIME and Base64 have replaced it entirely in email and HTTP. However, it still appears in:

Legacy Usenet archives. Decades of binary newsgroup posts (software, images, patches) were shared via uuencode. Decoding archived posts requires uudecode.
Old source archives. Some Unix software distributions from the 1980s and 1990s shipped with embedded uuencoded binary components in shell scripts.
Embedded scripts. Occasionally a shell script will bundle a small binary blob in uuencode format to avoid external file dependencies.

Tips

This tool encodes the UTF-8 bytes of whatever text you type and produces a complete, copy-pasteable uuencode document. When decoding, it skips the begin/end framing automatically and honours the per-line length byte, so it interoperates with output from the classic uuencode command. Decoded bytes are interpreted as UTF-8 text, so non-text payloads may not render cleanly.