If you work with spreadsheets, you have encountered both .xls and .xlsx file extensions. While they both open in Microsoft Excel, their internal architecture, file sizes, and feature support differ significantly. Here is everything you need to know.
What Is the XLS Format?
XLS is the legacy binary file format used by Microsoft Excel from 1997 through 2003. It stores data in a proprietary BIFF (Binary Interchange File Format) structure. While functional, XLS files are larger, more prone to corruption, and cannot leverage features introduced after Excel 2003.
What Is the XLSX Format?
XLSX was introduced with Office 2007 as part of the Office Open XML standard. Under the hood, an XLSX file is a ZIP archive containing XML files for each worksheet, style, and shared string. This makes XLSX files 50–75% smaller than equivalent XLS files and significantly more resistant to corruption.
Key Differences at a Glance
| Feature | XLS | XLSX |
|---|---|---|
| Max Rows | 65,536 | 1,048,576 |
| Max Columns | 256 (IV) | 16,384 (XFD) |
| File Size | Larger (binary) | Smaller (ZIP+XML) |
| Macro Support | Yes (same file) | No (use XLSM) |
| Corruption Risk | Higher | Lower |
| Modern Features | Limited | Full support |
When Should You Convert?
- Your spreadsheet has more than 65,536 rows of data.
- You are sharing files with colleagues using Excel 2007 or later.
- You need conditional formatting, pivot tables, or Power Query features.
- You want to reduce file size for email attachments or cloud storage.
- Your IT department requires the modern format for compliance.