XLS vs XLSX: Differences and When to Convert Your Files

If you have worked with spreadsheets for years, you have surely come across files with the .xls extension and others with .xlsx. At first glance they look the same: they open in Excel and show cells, rows and columns. But inside they are very different formats, and choosing one or the other affects file size, how much data it can hold, security and compatibility. In this article we explain the differences and when it makes sense to convert.

The XLS format

XLS is the format Excel used from its early versions up to Excel 2003. It is a binary format: data is stored in an internal structure that only programs that know the specification can read. Its main characteristics are:

• A maximum of 65,536 rows and 256 columns per sheet.
• Heavier files, because they are not compressed.
• It can contain macros without the extension telling you.
• Compatible with very old versions of Excel and legacy systems.

The XLSX format

XLSX arrived with Excel 2007 and is based on the Office Open XML standard. An XLSX file is actually a ZIP package containing several XML files: one per sheet, another for styles, another for shared strings and so on. Its advantages are clear:

• Up to 1,048,576 rows and 16,384 columns per sheet.
• Lighter files thanks to ZIP compression; they are often 50% to 75% smaller than the same workbook saved as XLS.
• It cannot contain macros. Macro-enabled workbooks use the XLSM extension, which makes potentially dangerous files easier to identify.
• Better recoverability: if part of the file is damaged, the rest can often be rescued.
• An open format, compatible with LibreOffice, Google Sheets and many other applications.

Comparison table

Feature	XLS	XLSX
Introduced with	Excel 97-2003	Excel 2007 and later
File type	Binary	Compressed XML
Maximum rows	65,536	1,048,576
Maximum columns	256	16,384
Size	Larger	Smaller
Macros	Allowed	No (XLSM is used)
Recovery after damage	Hard	Easier

When to convert XLS to XLSX

In most cases you should work in XLSX. Convert your XLS files when:

• You need more than 65,536 rows, for example for sales records or system exports.
• You want to reduce the size to send the file by email.
• You are going to share the file with people who use Google Sheets or LibreOffice.
• You want modern features such as structured tables, XLOOKUP or dynamic arrays.
• You want better security, knowing an XLSX never runs macros.

With our XLS to XLSX converter, available on the Excel tools page, you just upload the old file and download the modern version.

When to convert XLSX to XLS

There are situations where you still need the old format:

• A legacy system (accounting software, an internal application or a government portal) that only accepts XLS files.
• A recipient using Excel 2003 or earlier without the compatibility pack.
• An old automated process that reads files with a library that does not support XLSX.

Keep in mind that when converting to XLS, any data beyond 65,536 rows or 256 columns will be lost, and some modern functions may show as errors. Check the resulting file before sending it.

What about XLSM, XLSB and ODS?

• XLSM: the same as XLSX, but with macros. Use it only if you need VBA automation.
• XLSB: a modern binary format, very fast for huge workbooks, but less compatible with other applications.
• ODS: the open format of LibreOffice and OpenOffice. It is a good alternative to XLSX if you work with free software.

Tips before converting

1. Make a copy of the original file.
2. Review formulas after converting; some functions do not exist in older versions.
3. Check conditional formatting, which may be simplified when saving as XLS.
4. Compare the original and the converted file with our Excel comparison tool if the data is critical.

Frequently asked questions

Conclusion

XLSX is the recommended format for almost everything: it is lighter, holds far more data, is safer and works with practically every current application. XLS only makes sense when an old system requires it. If you still have XLS files in your folders, converting them is a small step that will save you space, compatibility and security headaches. You will find both converters for free on our Excel tools page.