XLS vs XLSX: What's the Difference? (With Test Results)
XLSX is Excel's modern format (Excel 2007 onwards): a ZIP archive of XML files that holds up to 1,048,576 rows. XLS is the older binary format of Excel 97 to 2003, limited to 65,536 rows and 256 columns. Use XLSX for almost everything -- saving a big sheet as XLS drops whatever doesn't fit, as our test shows.
XLS vs XLSX at a glance
The short version: XLSX is the modern Excel format and the right choice for almost everything. XLS is the older format, still worth knowing about if someone uses a very old version of Excel.
| XLS | XLSX | |
|---|---|---|
| Excel's default format | Excel 97 to Excel 2003 | Excel 2007 onwards |
| What the file is inside | One binary file | A ZIP archive of XML files (Office Open XML) |
| Rows and columns per sheet | 65,536 rows x 256 columns | 1,048,576 rows x 16,384 columns |
| Can contain macros? | Yes | No - macro workbooks use .xlsm |
| File size in our test | About 1.6 to 3.3 times larger | Smaller |
| Converts to PDF with our tool? | Yes | Yes |
Details and sources for each row are in the sections below.
Two different formats
XLS was Excel's format from Excel 97 through Excel 2003: a single binary file. XLSX replaced it as the default from Excel 2007. Like DOCX for Word, an XLSX file is really a ZIP archive of XML files, part of the Office Open XML standard.
You can see the difference in the first bytes of each file. In our test, the XLSX file started with the letters PK, the signature of a ZIP archive, and the XLS file started with D0 CF 11 E0, the marker of Microsoft's older compound binary format.
The row limit: what happens to data that doesn't fit
Microsoft's support page on using Excel with earlier versions puts it plainly: an Excel 97-2003 worksheet is only 65,536 rows tall by 256 columns wide, data beyond that "will not be saved", and formulas that refer to it return a #REF! error. Excel's Compatibility Checker warns you when you save as XLS; other programs may not.
We tested it. A sheet with 70,000 numbered rows, saved as XLS and opened again, ended at row 65,536 -- the last 4,464 rows were gone. A sheet 300 columns wide came back with 256 columns. Nothing in the reopened file shows that anything is missing, so the loss is easy to overlook if someone else saved it.
| Test sheet | Saved as XLS and reopened |
|---|---|
| 70,000 rows x 4 columns | 65,536 rows kept; 4,464 rows lost |
| 10 rows x 300 columns | 256 columns kept; 44 columns lost |
The XLS copies were saved with LibreOffice; Microsoft documents the same limits for Excel itself.
Is XLSX smaller? In our tests, yes
Microsoft says Open XML files are compressed automatically and "can be up to 75 percent smaller in some cases". Unlike our DOC vs DOCX test, where the two Word formats came out almost the same, the spreadsheet difference was large: our XLS copies were 1.6 to 3.3 times the size of the XLSX originals.
| Test workbook | XLSX | XLS |
|---|---|---|
| One sheet, 50 rows x 6 columns | 6.6 KB | 10.5 KB |
| Three sheets, 5,000 rows x 8 columns each | 632 KB | 1,702 KB |
| One sheet, 70,000 rows x 4 columns | 1,619 KB | 5,287 KB (and only 65,536 rows kept) |
Numbers, text and dates generated for the test. The XLS copies were saved with LibreOffice, not Microsoft Excel; sizes from Excel itself can differ.
When the difference actually matters
Old software. Excel 2003 and earlier can't open an .xlsx without Microsoft's compatibility pack -- the usual reason anyone still asks for .xls.
Large sheets. Anything over 65,536 rows or 256 columns has to stay XLSX.
Macros. A normal .xlsx can't contain macros; Excel uses .xlsm for macro-enabled workbooks. An .xls from an unknown sender may contain macros.
Converting to PDF. Our Excel to PDF converter accepts both .xls and .xlsx. An .xls is first converted to .xlsx on our server, then every column is fitted across the page width. Your upload and the PDF are deleted from our server as soon as your download is ready.
Which one should you use?
Use XLSX unless someone specifically needs XLS for an old program. It is Excel's default, it holds far more data, and in our tests it was the smaller file. If you must send an XLS, check first that the sheet fits within 65,536 rows and 256 columns.
If the reader only needs to see the figures, not edit them, a PDF is often better than either. Going the other way, our PDF to Excel converter always gives you an .xlsx file.
Is XLS the same as XLSX?
No. Both are Excel workbooks and both open in current versions of Excel, but they are different file formats with different limits: XLS is a single binary file with at most 65,536 rows and 256 columns per sheet, while XLSX is a ZIP archive of XML files with up to 1,048,576 rows and 16,384 columns.
How we tested
We generated test workbooks with openpyxl (numbers, text and dates), saved copies as XLS with LibreOffice 24.2, then reopened them to count rows and columns. File sizes were measured on disk. The tests ran in a test copy of this site on our own machine, using the same converter code as our Excel to PDF tool; the live server's software versions may differ slightly.