XLSX is the modern Office Open XML format used by Excel 2007 and later. XLS is the legacy Binary Interchange File Format (BIFF8) from Excel 97-2003. Converting XLSX to XLS downgrades a modern spreadsheet to the older binary format for compatibility with legacy systems, older software installations, and automated workflows that have not been updated to handle the newer format.

Many industrial and enterprise systems were built before 2007 and have never been updated. ERP modules, legacy inventory management systems, and custom reporting tools that accept Excel input were often programmed specifically for the BIFF binary format. These systems cannot parse XLSX's XML structure. Converting to XLS is the only way to feed modern Excel data into these older pipelines.

Some organizations also maintain XLS as their standard for maximum backward compatibility. When spreadsheets must be opened on machines running Office 2003 or earlier (still common in manufacturing, healthcare, and government environments), converting to XLS ensures the file opens without any compatibility issues or format conversion prompts.

The conversion engine uses LibreOffice in headless mode to parse the XLSX Open XML package and re-serialize it as a BIFF8 binary workbook. Cell values, formulas (translated from OOXML formula syntax to BIFF), number formats, cell styles, and basic chart objects are mapped to their XLS equivalents. Data exceeding XLS limits is handled gracefully: rows beyond 65,536 and columns beyond 256 (IV) are truncated with a warning.

Rows beyond 65,536 are truncated. XLS cannot store more than 65,536 rows per sheet. Split your data across multiple sheets if necessary before converting.

Excel Table objects (ListObjects) are converted to regular cell ranges with formatting. Structured references in formulas are resolved to regular cell references.

No. Dynamic array functions (FILTER, SORT, UNIQUE, XLOOKUP) do not exist in the XLS format. These formulas will either produce errors or be replaced with their last-computed static values.

Usually yes. XLS files are 30-60% larger than their XLSX equivalents because the binary format lacks ZIP compression. A 3 MB XLSX may produce a 7 MB XLS.

If the source XLSM contains VBA macros, they can be preserved in the XLS output since both formats support VBA. However, this conversion targets XLSX (macro-free) input specifically.

Device	XLSX	XLS
Windows PC	Partial	Partial
macOS	Partial	Partial
iPhone/iPad	Partial	Partial
Android	Partial	Partial
Linux	Partial	Partial
Web Browser	No	No

Характеристика	XLSX	XLS
Повна назва	Microsoft Excel Spreadsheet	Microsoft Excel 97-2003
Розширення	.xlsx	.xls
Найкраще для	Spreadsheet	Legacy support

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