biff

BIFF, short for Binary Interchange File Format, is a family of proprietary binary file formats used by Microsoft Excel and related Office applications to store spreadsheet data. The BIFF family comprises multiple versions, including BIFF2, BIFF3, BIFF4, BIFF5, and BIFF8. BIFF8 corresponds to the file format used by Excel 97 through Excel 2003; after that, Excel adopted the Office Open XML format (.xlsx) while older BIFF-based files (.xls) remained readable through compatibility modes and third-party libraries.

BIFF files are structured as a binary stream that encodes workbook data, including worksheets, cells, formatting, and metadata. The format is not a published standard; Microsoft historically provided limited documentation, and comprehensive public specifications are scarce. Because BIFF is a closed, record-based format, much of its exact structure was largely reverse-engineered by third-party developers. This work enabled libraries such as Apache POI (HSSF) and LibreOffice to read and write Excel workbooks in the BIFF era.

As a legacy format, BIFF presents interoperability challenges compared with modern XML-based formats. While new production work targets Open XML or other contemporary standards, BIFF remains relevant for maintaining and accessing legacy spreadsheets stored in .xls files.