CONVERT
XLS → XLSX
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Upgrade legacy Excel 97-2003 files to modern XLSX format.
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.
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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 While superseded by XLSX, it remains common in archived data and older business systems.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.
XLS vs XLSX — Strengths and limitations
What each format does best, and where it falls short.
XLS Strengths
- Universal legacy — every Excel since 1997 opens .xls natively.
- Binary format is compact and loads quickly.
- Full support for formulas, charts, pivots, and VBA macros.
- Deep integration with every accounting and ERP system of the 1990s-2000s.
Limitations
- Row/column limits are ~64× smaller than modern XLSX.
- Macro-enabled variants are a notorious malware vector.
- Binary corruption often means total data loss.
XLSX Strengths
- Much smaller than legacy .xls files thanks to ZIP + XML.
- Human-readable structure — easy to extract data programmatically.
- Supports macros (as .xlsm variant), charts, pivot tables, conditional formatting.
- Universal support: Excel, LibreOffice, Google Sheets, Numbers, pandas.
- ISO/IEC 29500 standardized.
Limitations
- Macros in .xlsm are a common malware vector — disabled by default in Office.
- 1M-row limit is a cultural problem — people put too much data in Excel.
- Subtle formula differences between Excel, LibreOffice, and Sheets.
XLS vs XLSX — Technical specifications
Side-by-side comparison of the technical details.
XLS
- MIME type
- application/vnd.ms-excel
- Max rows
- 65 536 (Excel 97-2003)
- Max columns
- 256 (A to IV)
- Container
- OLE Compound File
- Successor
- .xlsx (2007)
XLSX
- MIME type
- application/vnd.openxmlformats-officedocument.spreadsheetml.sheet
- Max rows
- 1,048,576
- Max columns
- 16,384
- Container
- ZIP (Office Open XML)
- Released in
- Microsoft Office 2007
- Variants
- .xlsx, .xlsm (macros), .xlsb (binary)
| Specification | XLS | XLSX |
|---|---|---|
| MIME type | application/vnd.ms-excel | application/vnd.openxmlformats-officedocument.spreadsheetml.sheet |
| Max rows | 65 536 (Excel 97-2003) | 1,048,576 |
| Max columns | 256 (A to IV) | 16,384 |
| Container | OLE Compound File | ZIP (Office Open XML) |
| Successor | .xlsx (2007) | — |
| Released in | — | Microsoft Office 2007 |
| Variants | — | .xlsx, .xlsm (macros), .xlsb (binary) |
XLS vs XLSX — Typical file sizes
Approximate file sizes for common scenarios.
XLS
- Simple budget (1 sheet) 30-60 KB
- Multi-sheet financial model 500 KB - 5 MB
- Data export with 65 000 rows 5-20 MB
XLSX
- Small budget spreadsheet 20–80 KB
- Financial model with charts 1–10 MB
- Large dataset (100k rows) 10–50 MB
- Enterprise model (1M+ rows) 100–500 MB
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
Yes. The free tier accepts files up to 25 MB without registration, email capture or watermarks; paid plans go up to 2 GB. 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.
Related comparisons
See these formats side by side to understand which fits your use case best.
Related Guides
XLSX Format: Inside Microsoft Excel's Open XML Spreadsheet
Complete guide to XLSX format: ZIP+XML structure, shared string table, date serial numbers, cell styles, formula caching, openpyxl programming, pandas integration, and comparison with CSV/ODS.
Read guideXLSX Format: The Complete Guide to Excel Open XML Spreadsheets
Everything about XLSX: ZIP container structure, workbook XML, cell references, formulas, pivot tables, macros vs. XLSM, and how to convert XLSX files.
Read guideHow to Convert Excel to PDF
Convert Microsoft Excel XLSX spreadsheets to PDF for sharing, printing, and reporting. Preserves formatting, charts, and tables.
Read guide
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