CONVERT
XLS → XLSX
Upgrade legacy Excel 97-2003 files to modern XLSX format.
DRAG. DROP. DONE.
Upload any file and our engines will handle format detection automatically.
Max 100 MB · Free plan · No signup required
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Converting XLS to XLSX modernises a legacy Excel binary workbook into the XML-based Office Open format current Excel, Google Sheets and LibreOffice Calc handle natively. The XLSX is typically smaller, more resilient to corruption, and supports 1M+ rows where XLS was capped at 65k.
Excel Spreadsheet (Legacy)
Source formatXLS is the legacy binary format for Microsoft Excel 97-2003 spreadsheets. While superseded by XLSX, it remains common in archived data and older business systems.
Excel Spreadsheet
Target formatXLSX is the modern Microsoft Excel format based on Open XML. It is the industry standard for spreadsheets, supporting formulas, charts, pivot tables, and conditional formatting.
Why convert XLS to XLSX
XLS hit structural limits in 2007 and Microsoft moved to XLSX as default. Legacy XLS files can still open in modern Excel but the compatibility warnings, slower performance and 65k-row cap make them painful to work with. Converting unlocks modern features and larger datasets.
HOW TO CONVERT
XLS → XLSX
Upload the XLS
Drop your legacy Excel workbook.
Convert via LibreOffice
LibreOffice Calc opens the binary XLS and writes a standards-compliant XLSX.
Download the XLSX
Grab the modern workbook — formulas, formatting and multi-sheet structure all preserved.
Common Use Cases
Legacy financial archives
Decades of accounting data stored as XLS becomes XLSX for ongoing use in modern finance tools.
Google Sheets import
Google Sheets imports XLSX cleanly; XLS imports sometimes truncate or lose formatting.
Row-count expansion
Datasets above 65k rows require XLSX; XLS simply cannot hold more.
Quality & Compatibility
Most XLS files convert losslessly. Complex features (legacy VBA macros targeting Excel-specific APIs, obscure chart types) may render slightly differently. Formulas, named ranges, conditional formatting and pivot tables transfer cleanly.
Tips for Best Results
- Verify any VBA macros in the XLSX — some Excel-only APIs do not survive LibreOffice's XLSX export; re-save in Excel if needed.
- For password-protected XLS, provide the password during upload; the XLSX inherits the protection.
Frequently Asked Questions
Frequently Asked Questions
Yes. The free tier accepts files up to 100 MB without registration, email capture or watermarks. Paid plans raise the size cap, enable batch conversions and provide a REST API for automation, but nothing on the free tier is quality-limited — the output is exactly the same as on any paid plan.
Yes. Standard formulas, named ranges and references transfer cleanly. Custom add-in formulas may not survive if they target Excel-specific APIs; plain arithmetic, SUMIFS, VLOOKUP, INDEX/MATCH and modern dynamic-array formulas all work.
Uploads run over HTTPS, files are processed in isolated containers, and both the source XLS and the XLSX output are auto-deleted within two hours. No account is required, file contents are never logged, and KaijuConverter does not use uploads for AI training. The paid plan adds a signable data-processing agreement for regulated workflows.
Typically 30-50% smaller. XLSX is a ZIP-compressed XML package; XLS is a binary format with historical bloat. The compression also makes XLSX faster to open and more resilient to corruption.
Most files finish in well under a minute. Small images and documents are typically ready in a few seconds; large video or audio files scale roughly with duration. Upload speed from your network is usually the dominant factor, not server time.
Yes. Pivot tables, conditional formatting, named ranges and multi-sheet structure all survive. Charts generally transfer with the same series and formatting, occasionally with minor styling differences.
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Secure & Private Conversion
Your files are encrypted during transfer, processed in isolated containers, and automatically deleted within 2 hours. We never read, share, or store your data.