XLS File Format (Microsoft Excel Binary Spreadsheet (Excel 97-2003))
XLS is the legacy binary spreadsheet format that served as the default for Microsoft Excel from Excel 5.0 (1993) through Excel 2003, before XLSX replaced it in Office 2007. An XLS file stores worksheets, cell values, formulas, formatting, and charts in Microsoft's proprietary Binary Interchange File Format (BIFF), wrapped inside the Compound File Binary Format (also called OLE2 structured storage) — a small file system within a single file. The most common version, BIFF8, was used by Excel 97 through 2003 and added Unicode text support. XLS worksheets are limited to 65,536 rows and 256 columns, a constraint that drove the move to the XML-based XLSX format. Despite being superseded, XLS remains widely encountered in older business archives, legacy line-of-business software, banking and government data exports, and systems that have not been updated in decades. Modern versions of Excel, Google Sheets, and LibreOffice Calc still open XLS files, and converting XLS to XLSX or CSV is a common task when migrating old spreadsheets into current tools. Because XLS is a binary format, it is harder to recover or inspect than the ZIP-based XLSX, and it can carry macros that pose security risks.
Quick Facts
- Extension: .xls
- MIME Type: application/vnd.ms-excel
- Category: document
Advantages
- Opens in virtually every spreadsheet application, including current Excel
- Single self-contained binary file with embedded charts and images
- Mature, stable format with decades of tooling support
- Backward compatibility for legacy business and government systems
- Compact for simple data compared to some text-based exports
Disadvantages
- Hard row and column limits (65,536 rows, 256 columns)
- Proprietary binary format is difficult to inspect or recover when damaged
- Larger and less efficient than XLSX for complex workbooks
- Macro-enabled XLS files can carry harmful VBA macros
- Officially superseded by XLSX since 2007; no longer the default
Common Use Cases
- Opening and migrating legacy spreadsheet archives
- Exports from older accounting, ERP, and banking software
- Data downloads from legacy government and institutional portals
- Compatibility with applications that predate Office 2007
- Reports generated by older server-side reporting tools
Technical Details
XLS uses the Binary Interchange File Format (BIFF), a stream of binary records each identified by a type code and length, stored inside Microsoft's Compound File Binary Format (CFBF / OLE2 structured storage). This container behaves like a miniature file system, holding named streams such as the Workbook stream that contains the BIFF records. BIFF8, used by Excel 97-2003, stores text as Unicode and references repeated strings through a shared string table (SST) to reduce size. Cell formulas are saved as tokenized reverse-Polish expressions (parsed expressions) alongside a cached result. Earlier BIFF5 and BIFF7 versions were used by Excel 5.0 and Excel 95. Microsoft later published the binary specification as [MS-XLS], enabling third-party libraries to read and write the format.
Frequently Asked Questions about XLS
What is the difference between XLS and XLSX?
XLS is the legacy binary format used by Excel 97-2003, limited to 65,536 rows and 256 columns. XLSX, introduced in Office 2007, is a ZIP archive of XML files that supports over a million rows and is generally smaller and easier to recover.
Can I still open XLS files today?
Yes. Modern Microsoft Excel, Google Sheets, LibreOffice Calc, and Apple Numbers all open XLS files. Current Excel may show a compatibility notice and offer to save the file as XLSX.
How do I convert XLS to XLSX?
FileChange converts XLS to XLSX directly in your browser. The worksheets, values, and formatting are read from the binary file and written into the modern XML-based XLSX format.
Is the XLS format safe to open?
Plain XLS data is safe, but XLS files can contain VBA macros that may run harmful code. Only enable macros in files from sources you trust, and keep macro execution disabled by default.
Why does my XLS file have fewer rows than my data?
XLS worksheets cap at 65,536 rows and 256 columns. If your dataset is larger, rows or columns beyond those limits are dropped. Convert to XLSX or CSV to keep all of your data.
What does the XLS file extension stand for?
XLS is short for Excel Spreadsheet, the file extension Microsoft used for its binary spreadsheet format. It refers to the BIFF binary format used through Excel 2003, not the newer XML-based XLSX.