Convert XLS to XLSX — Free Online Converter
Convert Microsoft Excel Spreadsheet (.xls) to Excel Open XML Spreadsheet (.xlsx) online for free. Fast, secure document conversion with no watermarks or registration.
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How to Convert
Upload your .xls file by dragging it into the upload area or clicking to browse.
Choose your output settings. The default settings work great for most files.
Click Convert and download your .xlsx file when it's ready.
About XLS to XLSX Conversion
XLS is the Binary Interchange File Format (BIFF8) used by Microsoft Excel from 1997 to 2003. XLSX is the Office Open XML format introduced with Excel 2007, storing data as ZIP-compressed XML files conforming to ISO/IEC 29500. Converting XLS to XLSX is the most common spreadsheet format migration — it upgrades legacy files to the current standard used by Excel, Google Sheets, Numbers, and every modern spreadsheet application.
This conversion expands the technical limits of the workbook significantly. XLS supports a maximum of 65,536 rows and 256 columns per sheet, while XLSX supports over one million rows and 16,384 columns. For organizations with growing datasets that have outgrown the XLS row limit, this upgrade is not optional — it is necessary to continue working with the data.
Why Convert XLS to XLSX?
Microsoft stopped developing the XLS format in 2007. Every version of Excel since then has used XLSX as the default save format. Opening an XLS file in modern Excel triggers a compatibility mode warning that limits features and reminds users the format is outdated. Converting to XLSX removes this limitation, enables modern features like dynamic arrays and Power Query, and ensures full interoperability with cloud services like OneDrive and SharePoint.
XLSX files are also substantially smaller than XLS equivalents because of ZIP compression. A 10 MB XLS file often compresses to 3-4 MB as XLSX. For organizations maintaining thousands of legacy spreadsheets, batch conversion to XLSX can recover significant storage space while simultaneously improving compatibility and data integrity.
Common Use Cases
- Upgrade legacy Excel 97-2003 files to work seamlessly in Microsoft 365
- Remove compatibility mode warnings when opening old spreadsheets in modern Excel
- Reduce storage footprint of XLS archives through XLSX's built-in ZIP compression
- Enable editing in Google Sheets, which handles XLSX better than XLS
- Migrate spreadsheet data past the 65,536-row limit of the XLS format
How It Works
The conversion engine uses LibreOffice in headless mode to parse the XLS BIFF8 binary records — cell values, shared strings, formatting records, formulas, and named ranges — and re-serialize the workbook as an Office Open XML SpreadsheetML package. The output XLSX conforms to the Transitional variant of ISO/IEC 29500, which provides the broadest compatibility with existing software.
Quality & Performance
Cell values, formulas, number formats, cell styles (fonts, borders, fills), column widths, row heights, and sheet names convert with high fidelity. Charts, images, and basic conditional formatting rules are preserved. VBA macros are stripped since XLSX is a macro-free format — use XLSM if macros are essential. Very old BIFF5 (Excel 5.0/95) files may require a two-step conversion through a newer XLS version first.
Device Compatibility
| Device | XLS | XLSX |
|---|---|---|
| Windows PC | Partial | Partial |
| macOS | Partial | Partial |
| iPhone/iPad | Partial | Partial |
| Android | Partial | Partial |
| Linux | Partial | Partial |
| Web Browser | No |
Tips for Best Results
- 1Convert entire XLS archives in batch to save storage space and standardize on one format
- 2Check for VBA macros before converting — choose XLSM as the target if macros are critical
- 3Verify that large datasets exceeding 65,536 rows in the XLS are intact after conversion to XLSX
- 4Recalculate all formulas after conversion (Ctrl+Alt+F9 in Excel) to confirm accuracy
- 5Remove personal information and metadata from the XLS before converting if sharing externally
Related Conversions
XLS to XLSX is the essential format upgrade for anyone maintaining legacy Excel spreadsheets. It reduces file sizes, removes compatibility barriers, and unlocks modern spreadsheet capabilities.