Upgrade legacy Excel .xls files to the modern .xlsx format. All sheets and data preserved. Runs entirely in your browser.
XLS to XLSX
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Drop an .xls file or browse
Legacy Excel 97–2003 format
XLS to XLSX upgrades a legacy Microsoft Excel binary file to the modern Office Open XML spreadsheet format. The difference between these two extensions is bigger than it looks. .xls is the Binary Interchange File Format Excel used from Excel 97 through Excel 2003 — a proprietary, closed binary structure where every cell, style, and formula is packed into custom byte layouts that only Microsoft's own code (or a reverse-engineered library) can reliably parse. .xlsx, introduced with Office 2007, replaced that with Office Open XML: the file is actually a ZIP archive containing a set of human-readable XML documents, one describing the workbook structure, one per worksheet, one for shared strings, one for styles, and so on. That architectural shift is why .xlsx files are typically 40–70% smaller than an equivalent .xls file — XML compresses extremely well inside a ZIP container, whereas the old binary format did not compress at all internally.
The row-and-column ceiling is the most consequential practical difference. The .xls binary format allocates a fixed 16-bit field for row references, which hard-caps every worksheet at 65,536 rows and 256 columns (column IY) — a limit that was already tight for serious data work by the early 2000s and is routinely hit by any modern export from a database, analytics tool, or accounting system. .xlsx, using flexible XML rather than fixed binary offsets, raises that ceiling to 1,048,576 rows and 16,384 columns (column XFD), a jump of roughly sixteen times more rows. If you have ever tried to open a large CSV export in old-format Excel and watched it silently truncate your data at row 65,536, this is exactly the limitation .xlsx removes.
Compatibility runs in the opposite direction from what people expect. Every current version of Excel, LibreOffice Calc, Google Sheets, Apple Numbers, and virtually every business intelligence and data pipeline tool reads and writes .xlsx natively as their default format. .xls support, by contrast, is increasingly an afterthought — still present for backward compatibility, but no longer where new features land, and occasionally flagged by security software because the legacy binary format has historically been a vector for macro-based malware.
Under the hood, this tool uses SheetJS, an open-source spreadsheet parsing library capable of reading the old BIFF (Binary Interchange File Format) structure that .xls files are built from. It walks the binary record structure to extract every worksheet, cell value, formula, and basic cell formatting, then re-serializes that same data as a proper Office Open XML package — writing the workbook.xml, the individual sheet XML files, the shared strings table, and the ZIP container that wraps them all into a valid .xlsx file. Because SheetJS runs as pure JavaScript, this entire read-and-rewrite process happens inside your browser tab; the spreadsheet, which might contain financial records, customer data, or internal reporting, is never transmitted to any server.
Typical situations where this matters: an accounting archive going back to the early 2000s stored entirely in .xls, a legacy export from an old point-of-sale or inventory system, a dataset that needs more than 65,536 rows and currently gets silently truncated, or simply wanting a smaller file that opens faster and plays nicely with cloud tools like Google Sheets or Microsoft 365 online, both of which handle .xlsx far more gracefully than the old binary format.
Your .xls file is read by SheetJS, which parses the legacy BIFF (Binary Interchange File Format) byte structure — walking through fixed-size binary records to extract workbook metadata, individual worksheets, cell values, formulas, and basic formatting such as fonts and number formats. Rather than treating the file as an opaque blob, SheetJS reconstructs the full logical spreadsheet model in memory: every sheet, every populated cell, every formula reference. That in-memory model is then re-serialized as Office Open XML — a set of interlinked XML documents (workbook.xml, per-sheet XML, a shared-strings table, and styling metadata) — which are packaged together into a ZIP container with a .xlsx extension, exactly matching the structure any modern spreadsheet application expects. The entire read-parse-rewrite pipeline executes as JavaScript inside your browser, so the original binary file and the resulting XML package both stay on your device throughout.
Upload your XLS file
Drop an older .xls Excel 97-2003 file onto the upload zone.
Convert
Click "Convert to XLSX". The file is converted to the modern Open XML Excel format.
Download
Save the .xlsx file and open it in Microsoft Excel 2007+, Google Sheets, or LibreOffice Calc.
Edit freely
XLSX supports all modern Excel features including larger datasets and advanced formulas.
Legacy Data Archives
Upgrade XLS spreadsheets created in the late 1990s or early 2000s to XLSX so they open cleanly in modern Excel versions.
Business Reporting
Convert old .xls financial or inventory reports to XLSX before importing them into modern BI tools or databases.
Large Datasets
XLS is limited to 65,536 rows — converting to XLSX removes that restriction and supports over a million rows.
Cloud Collaboration
Upload spreadsheets to Google Sheets or Microsoft 365 online, which require XLSX for full editing functionality.
Formula Modernization
Convert legacy XLS files to XLSX to gain access to modern Excel functions and array formulas not available in the old format.
Cross-App Sharing
Share spreadsheets with LibreOffice or Google Sheets users who handle XLSX more reliably than the binary XLS format.
Browser-Only Processing
SheetJS processes your XLS file entirely in the browser — no file is uploaded to any server.
All Data Preserved
Every sheet, cell value, formula, and basic structure is faithfully transferred to the output XLSX file.
Row Limit Removed
XLSX supports 1,048,576 rows versus the old 65,536-row XLS limit, future-proofing your spreadsheet.
Smaller File Size
XLSX uses ZIP-based compression so the output file is typically smaller than the equivalent XLS binary.
Password Protection Transparency
The tool clearly informs users that password-protected files must be unlocked first, avoiding silent failures.
Free to Start — Sign Up for More
Use free with no account for files up to 40 MB. Sign up free to raise your limit to 80 MB, or upgrade to Pro for 250 MB per file.
Also Known As
Excel to PDF
Convert XLS and XLSX spreadsheets to PDF, keeping all rows and columns.
PDF to Excel
Extract tables from PDFs and convert them to editable Excel spreadsheets.
Excel to CSV
Export an Excel spreadsheet to CSV for use in any data application.
CSV to Excel
Import a CSV file into a structured Excel XLSX spreadsheet.
Excel to JPG
Capture each sheet of an Excel file as a JPG image.
ODS to Excel
Convert LibreOffice Calc ODS files to Excel XLSX format.
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