.xls File Extension

What kind of file is a .xls?

An .xls file is usually a legacy Microsoft Excel workbook. Excel 97 through Excel 2003 used the BIFF8 binary format by default, while older Excel releases used earlier Binary Interchange File Format versions under the same suffix. Its registered MIME type is application/vnd.ms-excel.¹²

Most Excel 5.0 and later workbooks place BIFF records inside an OLE Compound File Binary container. They often begin with these eight bytes:

D0 CF 11 E0 A1 B1 1A E1

That signature is not unique to XLS. Legacy Word and PowerPoint files and other compound documents share it. Reliable identification also checks the container for an Excel Workbook or older Book stream and validates its BIFF records.²³

How do I open a .xls on my computer?

• On Windows or macOS, open the workbook with Microsoft Excel.
• On Linux, LibreOffice Calc can import and export legacy Excel workbooks.
• In a web browser, Excel for the web can open an XLS workbook, but converts it to XLSX for editing.⁴

Keep an untouched copy before converting an important workbook. Formulas, charts, PivotTables, named ranges, macros, external links, and date handling can change when another application interprets the file.

Poly support for .xls

Poly gives compatible XLS workbooks a first-sheet thumbnail and a connected spreadsheet preview. It indexes readable cell content and rendered sheets, so you can find workbook data and ask the Poly agent about processed sheet text and images.

Poly does not run VBA or Excel 4.0 macros, refresh external data, or replace Excel's formula-auditing tools. Password-protected, damaged, or incompatible workbooks may not produce a preview or content index. Use a spreadsheet application when you need to edit cells, inspect formulas, or refresh connections.

History of .xls

The suffix spans several generations of Excel's binary workbook format. Excel 5.0 introduced the compound-file workbook structure in 1993. Excel 97 introduced BIFF8, which remained Excel's default through Excel 2003. Microsoft switched the default to the ZIP-and-XML-based XLSX format with Office 2007 while retaining XLS for compatibility.²⁵

That legacy design also carries smaller worksheet limits. Excel 97-2003 worksheets support 65,536 rows by 256 columns, compared with 1,048,576 rows by 16,384 columns in current XLSX workbooks.⁶

Working with .xls and alternatives

Treat an unexpected XLS attachment as active-capable content. A workbook can contain VBA, Excel 4.0 macro sheets, external links, embedded objects, and data connections. Open untrusted files with macros and external content disabled, and inspect hidden sheets, links, comments, and document properties before sharing sensitive data.

For routine editing, XLSX is usually the better destination. Preserve the original, save a modern copy, and review the result sheet by sheet. Conversion is not sanitization, and it cannot guarantee that every legacy feature survives.