XLS vs XLSX: What Is the Difference and Which Should You Use?

If you have ever seen both .xls and .xlsx files and wondered what the difference is, you are not alone. Despite looking nearly identical, the two formats are structurally different — and choosing the wrong one can cause compatibility issues. Here is everything you need to know.

XLS vs XLSX: The Short Answer

• .xls — the old Excel format (Excel 97 through 2003). A proprietary binary file.
• .xlsx — the modern Excel format (Excel 2007 and later). A ZIP archive of XML files.

If you are creating new spreadsheets today, use .xlsx. It is smaller, more widely supported, and the default in all modern versions of Excel, LibreOffice, and Google Sheets.

Technical Differences Between XLS and XLSX

Feature	XLS (Excel 97-2003)	XLSX (Excel 2007+)
File structure	Binary (BIFF format)	ZIP archive of XML files
Max rows	65,536	1,048,576
Max columns	256	16,384
File size	Larger for the same data	Smaller (compressed XML)
Password security	Weak (easily cracked)	Strong (AES encryption)
Macro support	Built-in	Requires .xlsm for macros
Open standard	No (proprietary)	Yes (OOXML standard)

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When You Might Still Need XLS

The .xls format is largely obsolete, but there are still situations where you might encounter it:

• Legacy software that only exports to .xls (older ERP systems, accounting software)
• Files received from people using very old versions of Excel (2003 or earlier)
• Certain government or institutional systems that have not updated their export formats

If you receive an .xls file, modern tools (Excel, LibreOffice, Google Sheets, and browser-based viewers) all handle it fine. You can view it as-is or convert it to .xlsx.

How to Convert XLS to XLSX for Free

Converting .xls to .xlsx is simple with any modern tool:

In Microsoft Excel: Open the file, click File > Save As, choose .xlsx from the format dropdown, and save.

In LibreOffice Calc: Open the file, click File > Save As, choose "ODS - OpenDocument Spreadsheet" or "Excel 2007-365 (.xlsx)" from the format dropdown.

In Google Sheets: Import the .xls file, then File > Download > Microsoft Excel (.xlsx).

Browser-based viewers let you open either format and download as CSV, which you can then re-import into any tool and save as .xlsx.

Format Compatibility: What Opens XLS vs XLSX?

Tool	Opens XLS	Opens XLSX
Microsoft Excel (2007+)	Yes	Yes
LibreOffice Calc	Yes	Yes
Google Sheets	Yes	Yes
Apple Numbers	Yes	Yes
Browser-based viewer	Yes	Yes
Excel 2003	Yes	No (needs Compatibility Pack)

In practice, both formats open in every modern tool. The main advantage of .xlsx is smaller file size, better security, and no row/column limits.

Frequently Asked Questions

What is the difference between XLS and XLSX?

XLS is the older Excel binary format (Excel 97-2003) while XLSX is the modern XML-based format (Excel 2007+). XLSX is smaller, supports more rows and columns, has better encryption, and is the default format in all modern spreadsheet applications.

Is XLSX better than XLS?

Yes, for almost all purposes. XLSX is smaller, more secure, supports up to 1 million rows (vs 65,536 for XLS), and is based on an open standard. The only reason to use .xls today is compatibility with very old software.

Can I open an XLS file in Excel 2016 or later?

Yes. All modern versions of Excel open .xls files without issues. Excel will ask if you want to convert to .xlsx when you save — you can accept or keep the .xls format.

Can a browser-based viewer open both XLS and XLSX files?

Yes. Browser-based Excel viewers support both .xlsx and .xls (as well as .csv, .tsv, and .ods). You can view either format without installing any software.