XLS vs XLSX: Key Differences, Pros & Cons Explained
XLS is the legacy binary spreadsheet format introduced with Excel 97-2003; XLSX is its XML-based successor that debuted with Excel 2007 and handles modern data more efficiently.
People still send .xls because “it always opens,” while .xlsx sometimes triggers “Compatibility Mode” warnings—so they second-guess which file to attach or save, especially when emailing older clients or banks.
Key Differences
XLS caps at 65,536 rows and 256 columns, lacks modern encryption, and inflates file size. XLSX supports 1,048,576 rows, 16,384 columns, ZIP compression, richer conditional formatting, and built-in XML recovery—while remaining readable by every Excel version from 2007 onward.
Which One Should You Choose?
Pick XLSX for new projects; its smaller files, crash recovery, and Power Query integration outweigh any edge-case legacy concern. Only resort to XLS when you must share with a pre-2007 system that can’t install the free compatibility pack.
Can I convert XLS to XLSX without losing data?
Yes—open the file in any modern Excel, click File > Save As, choose “Excel Workbook (*.xlsx),” and review the Compatibility Checker for minor feature loss like discontinued MS Query connections.
Why is my XLSX file larger than the original XLS?
Embedded images, slicers, or PivotCaches can balloon the XML structure; compress media or clear PivotCache to shrink it, or save as XLSB for binary compression without legacy limits.