Why upgrade to XLSX
XLS is the binary format of Excel 97–2003, limited to 65,536 rows and 256 columns. XLSX, Excel’s standard since 2007, handles far larger tables, is more compact and robust, and works better with Google Sheets, Numbers and current Excel without compatibility mode.
What to check
Data, formulas, formatting and multiple sheets are converted by LibreOffice. VBA macros aren’t stored in XLSX (that would be XLSM). Very complex old formulas, charts or pivot tables might need a quick check. Everything runs on your device, so business data isn’t uploaded.
How to convert
Choose or drop the XLS above; the office page opens automatically. Click “Convert” and download the XLSX. LibreOffice (about 77 MB) is downloaded once on first use.