What is XLS?

How XLS Works

The legacy .xls file is built on the same Compound File Binary Format that underpins the other old Office formats. That means a single .xls file acts like a miniature file system, with named internal streams that hold the different parts of the workbook. Inside that container, the actual spreadsheet content is written using the BIFF record stream, short for Binary Interchange File Format.

In the BIFF stream, the workbook is stored as a long sequence of binary records. Each record has a type code and a length, followed by its data, and Excel reads these records in order to reconstruct the sheets, cells, and their relationships. Cell values, the formulas behind them, number and font formatting, and definitions for charts are all expressed as records within this stream rather than as readable text.

Because the layout is binary and record based, it was compact and fast to load on the hardware of its time, but it was also closely tied to Excel's own behavior. If the workbook included automation, the VBA macro project was stored as an additional binary part inside the same compound file, sitting alongside the worksheet data rather than in a separate file.

One consequence of this design worth knowing is scale. The classic .xls grid is capped at 65,536 rows and 256 columns per worksheet, a hard limit baked into the format. For the datasets of its era that was ample, but as spreadsheets grew this ceiling became a real constraint and was one of the motivations for a redesigned successor.

History and Standardization

The .xls extension belongs to Microsoft Excel, and the binary form most people mean is the Excel 97 to 2003 generation. For most of its life it was a proprietary format defined mainly by how Excel itself read and wrote workbooks, so other tools had to reverse engineer the BIFF records to interoperate reliably.

Later, Microsoft published documentation for its legacy binary Office formats, including the compound file structure and the BIFF record definitions. That written specification made it much more practical for third party software to open and generate .xls files correctly, even though the format itself remained a product of its original design.

The decisive change came with Office 2007, which introduced the XML based .xlsx as the new default spreadsheet format, along with the macro enabled .xlsm. This new family is built on Office Open XML, published as ECMA-376 and later adopted as ISO/IEC 29500. The redesign also lifted the old grid limits dramatically, giving worksheets a far larger row and column capacity than .xls ever allowed.

XLS Compared to XLSX, CSV, and ODS

Against its successor .xlsx, XLS trades capability for backward compatibility. XLSX is a ZIP of XML parts, so it is more transparent, usually smaller on disk, easier to repair, and it removes the tight row and column ceiling. The honest reason to keep .xls is legacy reach: some older systems, add ins, and rigid workflows still expect the binary format, and a few obscure features can behave slightly differently across the transition.

Compared with CSV, the difference is one of richness versus simplicity. A CSV file is plain text with no formulas, no formatting, no multiple sheets, and no charts; it stores only raw cell values separated by delimiters. That makes CSV wonderfully portable and easy for any program to read, but a .xls workbook preserves the live formulas, styling, and structure that a CSV simply discards.

Against ODS, the OpenDocument Spreadsheet used by LibreOffice and similar suites, XLS reflects a proprietary lineage rather than an open standard first design. ODS is a natural fit for open source and vendor neutral environments, while XLS still benefits from the enormous installed base of existing Excel workbooks and the expectation, in many offices, that a spreadsheet will simply open in Excel. For anything new, converting a legacy .xls to .xlsx is usually the most future proof choice.