What is RFC 2047 B-encoding?

B-encoding is the Base64 variant of the RFC 2047 encoded-word syntax. The header value is converted to bytes in the named charset, Base64-encoded, and wrapped as =?charset?B?base64?=. The capital B marks Base64, distinguishing it from the Q quoted-printable variant.

When is B-encoding better than Q-encoding?

B-encoding is more compact when most characters are non-ASCII, such as Chinese, Japanese, Korean, or Arabic subjects, because Q-encoding would escape nearly every byte as =XX. For mostly-ASCII text with a few accents, Q-encoding stays more readable.

Does B-encoding change the bytes themselves?

No. B-encoding only changes the representation. The underlying UTF-8 bytes are preserved exactly; Base64 simply re-expresses them in a 64-character alphabet that is safe inside an email header.

What is the length limit on an encoded-word?

RFC 2047 caps a single encoded-word at 75 characters including delimiters. Longer subjects must be split into several encoded-words separated by whitespace, each independently Base64-decoded; this tool warns when output exceeds the limit.

Is my data sent to a server?

No. All encoding and decoding happens in JavaScript in your browser, so subjects, names, and any private text stay on your device.

B-Encoding (RFC 2047) Encoder

RFC 2047 B-encoding lets you put non-ASCII text into email headers using a Base64-wrapped encoded-word: =?UTF-8?B?...?=. It is the compact counterpart to Q-encoding and is preferred when most of the text is non-ASCII. This tool encodes and decodes B encoded-words in your browser.

How it works

The encoded-word format is =?charset?encoding?encoded-text?=. For the B encoding:

The text is converted to UTF-8 bytes with TextEncoder.
Those bytes are Base64-encoded using the standard alphabet A-Z a-z 0-9 + / with = padding.
The result is wrapped as =?UTF-8?B?...?=.

Decoding reverses the process: the tool parses the charset and the B marker, Base64-decodes the body back to bytes, and runs a strict UTF-8 decode.

Tips and examples

The subject 日本語 (Japanese for “Japanese language”) encodes to =?UTF-8?B?5pel5pys6Kqe?=. Compared with Q-encoding — which would escape almost every byte as =XX — the Base64 form is noticeably shorter for this kind of text. Keep each encoded-word under 75 characters: the tool flags longer output, which a real mail client would break into multiple whitespace-separated encoded-words, each decoded independently.