Quoted-printable is a MIME content-transfer encoding that represents data as mostly readable text: printable US-ASCII characters can stay literal, while other octets are written as an equals sign followed by two hexadecimal digits. It is designed for content that is largely ASCII and needs to pass through mail systems reliably.
What quoted-printable encoding does
Quoted-printable changes how octets are represented; it is not a character set. A character set maps text to bytes, while a content-transfer encoding gives those bytes a transport-friendly form. RFC 2045 describes quoted-printable as intended for data largely consisting of octets that correspond to printable US-ASCII characters. RFC 2045, section 6.7
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In practice, safe printable characters generally remain visible as themselves. An octet that needs escaping is written as = followed by two hexadecimal digits. The equals sign itself is escaped, so =3D represents the equals sign.
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How to read a quoted-printable message
Decode an escaped octet
Read each =XX sequence as the octet whose hexadecimal value is XX. For example, =3D is hexadecimal 3D, the equals-sign byte. The encoding represents bytes, not necessarily complete characters: for text using a multibyte character encoding, decoding the octets is only one step in recovering the text.
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Distinguish a soft wrap from a real line break
Quoted-printable encoded lines are limited to 76 characters, excluding the ending CRLF (carriage return and line feed). If a line must continue, the encoder ends it with = and starts the continuation on the next line. That final equals sign is a soft line break: remove it and join the next encoded line when decoding.
A hard CRLF in text, by contrast, is a meaningful text line break. Do not remove it as though it were a soft wrap. The distinction matters because soft breaks only wrap the encoded representation, while hard breaks belong to the text itself. RFC 2045, section 6.7
Watch for spaces and tabs at line ends
Spaces and tabs cannot safely remain literal at the end of an encoded line: mail transports may remove or add trailing whitespace. Quoted-printable therefore requires them to be encoded in that position. This rule is one reason an encoded message may show escape sequences even when the underlying content looks ordinary.
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Quoted-printable is a good fit when content is mostly ASCII and keeping the encoded form readable is useful. RFC 2045 describes it as a compromise between readability and transport reliability. Base64 is better suited to arbitrary octets when human readability is not important, and the RFC says it offers higher confidence through some gateways. It is also the more sensible choice if changes to line breaks could corrupt the content. RFC 2045, section 6.7
Quick Recap
| Consideration | Quoted-printable | Base64 |
|---|---|---|
| Mostly ASCII content | Usually remains largely readable | Encoded representation is not meant to be human-readable |
| Arbitrary binary octets | Possible, but less natural; CR and LF must be escaped as =0D and =0A to protect the data from platform newline changes |
Designed to represent arbitrary octets |
| Transport that may alter line breaks | Less suitable if those changes could corrupt the content | RFC 2045 describes it as offering higher confidence through some gateways |
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