XLS vs XLSX: Older Excel Files Versus Modern Ones

XLS was Excel's format through Excel 2003, and XLSX replaced it starting with Excel 2007 — the same year, and for essentially the same underlying reason, that DOC gave way to DOCX in Word. The switch from a proprietary binary structure to an open, XML-based one changed some real practical things about how spreadsheets behave, not just how the file is named.

XLS: the older binary format

XLS stores spreadsheet data in a binary format specific to older Excel versions — compact, but proprietary and not straightforwardly readable outside of software specifically built to parse that exact structure. XLS also carries some hard technical limits inherited from its era, most notably a maximum of 65,536 rows per sheet — a ceiling that was rarely a problem in the 1990s but becomes a genuine constraint for larger modern datasets.

XLSX: XML-based, with real practical improvements

XLSX, like DOCX, is built on OOXML — a documented, XML-based structure rather than a proprietary binary one. Beyond the more open format itself, XLSX raised the row limit dramatically (over a million rows per sheet), generally produces smaller files through better compression, and is more consistently supported across non-Excel spreadsheet software, since the format's documented structure is more straightforward for other developers to implement support for than the older binary layout was.

When the row limit actually matters

For a typical small spreadsheet — a budget, a simple tracker, a short data table — XLS's 65,536-row ceiling is irrelevant, since most everyday spreadsheets never come close to that many rows. For larger datasets — exported transaction logs, large survey results, bigger data exports — hitting that ceiling in an old XLS file is a real, hard limit that XLSX simply doesn't have. If you've ever had an Excel import silently truncate at exactly 65,536 rows, that row limit is almost certainly why.

Compatibility with non-Excel software

Modern Excel opens both formats without issue, but other spreadsheet software (various open-source and web-based alternatives) tends to handle XLSX's documented format more reliably than XLS's older proprietary structure, similar to the same dynamic as DOC versus DOCX in Word. If a spreadsheet needs to move reliably between Excel and non-Excel tools, XLSX is the safer bet specifically because of that broader implementation support.

Should you convert an old XLS file?

For a spreadsheet you're actively maintaining, converting to XLSX (open in modern Excel, save in the new format) is generally worthwhile, particularly if the dataset could ever grow toward the old row limit or if it needs to move between different spreadsheet programs. For an old file you're archiving and unlikely to touch again, there's no urgent need to convert — but the moment you plan to actively edit, share, or grow it, updating to XLSX removes a real, if narrow, technical ceiling.

Once a spreadsheet — in either format — needs to become a shareable, fixed-layout document rather than an editable file, Excel to PDF handles that conversion directly.