XLS and XLSX are both Excel workbook formats, but they belong to different generations of the software. XLS is the older binary workbook format; XLSX is the newer XML-based format used by modern Excel.
XLS: the legacy format
XLS was used by older versions of Microsoft Excel and remains relevant when a business system or older application specifically requires it. It can store spreadsheet data, formulas and formatting, but it uses an older binary representation.
If you are exchanging files with a legacy system, converting a modern XLSX workbook to XLS can be useful—but the target application's capabilities should be checked first.
XLSX: the modern default
XLSX became the standard workbook format for modern Excel. It uses a package of XML files and supports modern spreadsheet structures more naturally than XLS.
For most new workbooks, XLSX is the sensible default unless a downstream system specifically asks for XLS.
What can change during conversion?
A workbook conversion is not just a filename change. Software has to read the source workbook and write a new workbook using the target format's capabilities.
Check formulas, charts, conditional formatting, external links and other advanced features after converting. Features that are newer than the target format may need to be changed or removed.
When to use each
| Situation | Better choice | |---|---| | Modern Excel workflow | XLSX | | New spreadsheet you control | XLSX | | Legacy application explicitly requires it | XLS | | Data exchange with no workbook formatting | CSV |
You can use Excel XLSX to XLS when an older system requires the legacy format, or browse Spreadsheet Tools for other conversions.
Bottom line
Use XLSX by default for modern spreadsheet work. Treat XLS as a compatibility format for older software or systems that still require it.