Why convert XLS to XLSX?
- Compatibility — some newer tools and websites only accept XLSX.
- Smaller files — XLSX is compressed and usually much smaller than XLS.
- Modern limits — XLSX supports over a million rows instead of 65,536.
How the conversion works
The binary XLS workbook is read with SheetJS, including every sheet and its number formats, and written again in the Office Open XML (XLSX) format.
What is an XLS file?
XLS is the binary workbook format used by Microsoft Excel 97 to 2003. It stores sheets, values and formulas in a compound “BIFF” file rather than the XML used by XLSX. Many older reports, bank exports and archived spreadsheets still use it.
Modern Excel opens XLS files in compatibility mode, but some newer tools and websites no longer accept them, which is why converting XLS to XLSX or CSV is common.
What is an XLSX file?
XLSX is Microsoft Excel’s default workbook format since 2007, based on the Office Open XML standard. A workbook can contain several sheets, each a grid of cells holding text, numbers, dates or formulas, together with formatting, charts and comments. Inside, it is a ZIP archive of XML files.
XLSX opens in Excel, Google Sheets, LibreOffice Calc, Apple Numbers and Hancom Cell. Files ending in .xlsm are the same format with macros enabled.