Convert XLS to XLSX

From a 1997 binary format to a modern one

The two formats have almost nothing in common underneath. An .xls is a compound binary file — a small filesystem inside a single file, with records laid out for the way computers worked in 1997 — while an .xlsx is a ZIP archive of XML documents. So this is a genuine rebuild rather than a relabel, and the gains are worth having: the same workbook is typically much smaller as .xlsx, because XML compresses extremely well where a binary format stores its records literally.

The limits lift dramatically, and this is often the real reason for converting. An .xls stops at 65,536 rows and 256 columns; an .xlsx allows 1,048,576 rows and 16,384 columns. It also carries far more colours, more conditional formats and more cell styles than the older format's fixed palettes permitted — so a file that was quietly constrained by its format is no longer.

VBA macros do not survive, and that is a property of the destination. The .xlsx format cannot hold macros at all: Microsoft separated them into .xlsm precisely so that a plain workbook could be trusted not to contain executable code. An .xls with macros therefore converts as its data and formatting, with the automation left behind — for a workbook that depends on its macros, re-saving as .xlsm from Excel is the route, not a converter.

One thing to check afterwards on genuinely old files. Early Excel treated 1900 as a leap year, which it was not, and the resulting off-by-one in dates before March 1900 is still carried as a compatibility setting. Modern workbooks handle it, but a spreadsheet holding nineteenth-century dates is worth a glance. External links to other old workbooks are also worth checking, since they are rewritten and will only resolve if the referenced files are still where they were.

When you want something else

LibreOffice does the same conversion locally, with no size limit and nothing crossing the network: soffice --headless --convert-to xlsx *.xls converts a folder in one pass. For a shared drive full of legacy workbooks, that is the realistic approach — and old spreadsheets are exactly the kind of file that turns out to contain payroll.

For a workbook that relies on macros, convert in Excel itself and save as .xlsm, which keeps both the modern format and the automation. And if the file is genuinely ancient — Excel 5.0 or earlier, or a Lotus 1-2-3 file that was renamed at some point — LibreOffice is more likely to open it than current Excel, which has dropped support for several of the formats it once read.