File extension

.br

Brotli compressed data

.br files are Brotli-compressed data: a lossless compression format developed by Google that combines LZ77, Huffman coding, and a built-in static dictionary, widely used for web content.

Known meanings

Formats and reference material for .br

Brotli compressed data

General-purpose lossless compression format standardized in RFC 7932

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Details

Format notes

Brotli

Brotli is a lossless compression format created at Google and specified in RFC 7932 (2016). It combines an LZ77-style match finder, Huffman coding, and context modeling with a static dictionary of common words and phrases from web content such as HTML, CSS, JavaScript, and several human languages. The dictionary helps compress small text resources in particular. Brotli generally compresses text better than gzip at comparable decompression speed, though its highest levels are slow to compress.

Technical Notes

A Brotli stream is a sequence of meta-blocks. Unlike many compressed formats, it has no magic number or file header, and the specification defines only the compressed stream, so the .br extension identifies the content by convention. The stream has no built-in checksum. On the web, Brotli is negotiated through the HTTP header Content-Encoding: br, which browsers advertise in Accept-Encoding.

Adoption

All major browsers support Brotli, and web servers and content delivery networks commonly offer it. Build tools often precompress static assets to .br files (for example app.js.br) at a high level for serving. The reference implementation is open source, and bindings exist for many languages. Brotli is also the compression used inside the WOFF2 font format.

Preservation And Security Notes

Brotli files are best treated as delivery copies; store originals alongside them. Because the format has no integrity check, use external hashes for important data. Decompressors for untrusted input should limit output size to avoid decompression bombs.

Further Reading

  • Brotli reference implementation: https://github.com/google/brotli
  • RFC 7932, Brotli Compressed Data Format: https://datatracker.ietf.org/doc/html/rfc7932