XLS file format
XLS is the binary Excel format used before Office 2007. Its 65,536-row ceiling is the reason most people finally leave it behind.
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What is XLS?
XLS stores a workbook in BIFF, Microsoft's Binary Interchange File Format, inside an OLE compound file. It dates to the late 1980s and served spreadsheets through the entire era when Excel became the default business tool. Every spreadsheet application still reads it, because too much of the world's data was saved in it to abandon.
Its defining limitation is size. A worksheet stops at 65,536 rows and 256 columns, and exports that exceed it are silently truncated by some systems — a genuinely dangerous failure mode, because the file opens fine and simply lacks the rest of the data. XLSX raised those limits to over a million rows.
XLS is also the format of the macro-virus era, so modern Excel opens it in Protected View and treats embedded VBA with suspicion. Legacy accounting packages, bank exports, ERP reports and government systems still emit XLS, which is why it keeps arriving in inboxes decades after its replacement shipped.
| Extension | .xls |
|---|---|
| Category | Document |
| MIME type | application/vnd.ms-excel |
| Released | 1987 |
| Developed by | Microsoft |
| Compression | None (uncompressed) |
XLS: frequently asked questions
- Why does my XLS file stop at 65,536 rows?
- That is the hard limit of the format — 65,536 rows and 256 columns per sheet. Anything beyond it cannot be stored, and some export tools truncate silently rather than warning you. Converting to XLSX raises the ceiling to 1,048,576 rows.
- Should I convert my XLS files to XLSX?
- Yes, in almost all cases. XLSX is substantially smaller, supports far more rows and columns, is readable by more software and is not treated as a security risk by default. Keep XLS only where a legacy system refuses anything else.
- Where do XLS files still come from?
- Legacy business software: accounting packages, banking exports, ERP reporting modules, laboratory instruments and government portals built in the 2000s. These systems export the format their developers targeted, and many have never been updated.
- Are macros in XLS files a security risk?
- They can be. XLS can hold VBA macros inside the same file with no visible marker, which is exactly how macro viruses spread. Modern Excel opens such files in Protected View and blocks macros from the internet unless you explicitly allow them.
- Will converting XLS to XLSX break my formulas?
- Normally not. Standard formulas, references, number formats and charts carry over cleanly. Very old functions, custom VBA and add-in-specific features are where issues appear, so check any sheet that depends on macros after converting.
- How much smaller is XLSX than XLS?
- Commonly half the size or less for the same data, because XLSX compresses its XML while XLS stores binary records uncompressed. The difference grows with the amount of formatting and the number of times the workbook has been edited.