What is XLS?
Complete guide to the XLS file format
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What is XLS?
XLS is the binary file format used by Microsoft Excel from version 2.0 in 1987 through Excel 2003. XLS files store spreadsheet data including cells, formulas, charts, pivot tables, macros, and formatting in a proprietary binary structure using the BIFF (Binary Interchange File Format) standard, which evolved through multiple versions (BIFF2 through BIFF8). The format was the dominant spreadsheet file type for two decades and remains in widespread use for compatibility with older systems.
In 2007 Microsoft replaced XLS as the default format with XLSX, based on the Office Open XML standard. XLSX files are smaller, more transparent, and easier to process programmatically. Despite this, XLS files are still encountered in enterprise environments, government systems, and legacy applications that have not migrated to newer formats. All current versions of Excel, LibreOffice Calc, and Google Sheets can open and edit XLS files.
How XLS Stores a Workbook
XLS wraps its data in the same OLE Compound File container used by other legacy Office formats, with the workbook held as a stream of BIFF records, each a typed, length-prefixed binary structure.[1] The BIFF8 generation, used from Excel 97 onward, raised limits and added Unicode text support; the LoC documents this BIFF8 variant specifically.[2]
History and Constraints
BIFF evolved through several versions (BIFF2 through BIFF8) as Excel matured, and the BIFF8 worksheet was capped at 65,536 rows by 256 columns, a limit lifted only with the XLSX format introduced in Excel 2007.[3] Microsoft published the MS-XLS specification to document the binary structure for interoperability.[1]
XLS vs XLSX
Compared with the binary XLS, the XML-based XLSX is a ZIP package of human-readable parts that is smaller, easier to inspect, and simpler to generate programmatically.[3] XLS also supports VBA macros, which, as with other Office binary formats, has made it a target for macro-based malware.[2]
Technical Details
XLS vs Other Spreadsheet Formats
| Feature | XLS | XLSX | CSV |
|---|---|---|---|
| Structure/type | Binary (BIFF8)[2] | Zipped XML | Delimited text |
| Formulas/formatting | Yes[1] | Yes | None |
| Multi-sheet | Yes | Yes | No |
| Spec published by | Microsoft[3] | ECMA / ISO | IETF |
| Open/proprietary | Proprietary | Open (vendor-led) | Open |
| Best for | Legacy spreadsheets | Modern spreadsheets | Tabular interchange |
XLS is the legacy binary Excel format, replaced by the XML-based XLSX while CSV stays format-free for plain data.
Pros and Cons
Advantages
Required for exchanging spreadsheets with older software and systems that do not support XLSX.
Supported by Excel, LibreOffice, Google Sheets, and virtually all spreadsheet applications.
All Excel formulas, functions, and chart types from the pre-2007 era are preserved in XLS.
Stores VBA macros for spreadsheet automation, though XLSM is now preferred for macro-enabled workbooks.
Disadvantages
The binary BIFF format is hard to parse, debug, or manipulate without specialized libraries.
XLS is limited to 65,536 rows and 256 columns, compared to 1,048,576 rows and 16,384 columns in XLSX.
XLS files containing macros are a well-known vector for malicious code distribution.
Lacks modern Excel features introduced after 2007 and is no longer the default save format.
When to Use XLS
Here are the most common situations where XLS is the right choice:
Legacy System Integration
Use XLS when exchanging spreadsheets with older enterprise software, ERP systems, or government portals that require XLS.
Backward Compatibility
Save as XLS when recipients use Excel versions older than 2007 or other software with limited XLSX support.
Data Migration
Convert historical XLS archives to XLSX to remove row and column limits and enable modern Excel features.
Opening Legacy Files
Open archived XLS files from legacy accounting or data management systems.
Convert XLS Files
Need to convert your XLS files? Use our free online converter.
Try Document Converter FreeFrequently Asked Questions about XLS
What is the difference between XLS and XLSX?
XLS is the binary BIFF format used before Excel 2007 with a 65,536 row limit. XLSX is the XML-based Office Open XML format with a 1,048,576 row limit, smaller file sizes, and better compatibility with modern tools.
Can I open XLS in Google Sheets?
Yes. Google Sheets can open XLS files directly. Upload the file to Google Drive and open it with Google Sheets. Some complex formatting or macros may not be fully supported.
How do I convert XLS to XLSX?
Open the file in Excel and use Save As to save it as XLSX. Alternatively, use our free online converter to convert XLS to XLSX.
Are XLS files with macros safe?
XLS files can contain VBA macros that execute code. Disable macros in your security settings and only enable them for XLS files from trusted sources.
Why does my XLS file have a row limit?
The XLS binary format (BIFF8) was designed with a hard limit of 65,536 rows. If you need more rows, convert to XLSX which supports over one million rows.