Excel File Formats Explained: XLS vs XLSX vs CSV vs ODS

If you work with spreadsheets regularly, you've encountered a confusing alphabet soup of file formats: .xls, .xlsx, .csv, .ods, .xlsm, .xlsb. Each has a different purpose, compatibility level, and behavior. Choosing the wrong format causes lost data, broken formulas, and compatibility headaches.

This guide demystifies every major Excel-related file format and tells you exactly when to use each one.

XLS — The Legacy Format (Excel 97–2003)

XLS is the original Microsoft Excel format, introduced with Excel 97 and used through Excel 2003. It's a binary format — not human-readable in a text editor — based on Microsoft's proprietary BIFF (Binary Interchange File Format) specification.

Key limitations of XLS:

• Maximum of 65,536 rows and 256 columns per worksheet
• Larger file size than XLSX for equivalent data
• Less reliable formula support in modern versions
• Not supported by some modern tools and APIs

When to use XLS: Only when you need to share files with someone using Excel 2003 or earlier — an increasingly rare situation. For all other cases, use XLSX.

ConvertEase supports .xls files in all its conversion tools — convert to CSV, PDF, or JSON using the Excel to CSV, Excel to PDF, or Excel to JSON converters.

XLSX — The Modern Standard (Excel 2007+)

XLSX replaced XLS with the introduction of Microsoft Office 2007. XLSX is based on the Open XML standard — it's actually a ZIP archive containing XML files. This makes it more transparent, more reliable, and more interoperable with other software.

Key advantages of XLSX:

• Maximum of 1,048,576 rows and 16,384 columns — far more capacity than XLS
• Smaller file size than XLS for equivalent data
• Better compatibility with modern software, APIs, and cloud tools
• Supported by Google Sheets, LibreOffice, Apple Numbers, and all modern Excel versions
• Open standard — not locked to Microsoft proprietary format

When to use XLSX: For virtually all spreadsheet work in 2026. XLSX is the right choice unless you have a specific reason to use a different format.

CSV — The Universal Data Format

CSV (Comma-Separated Values) is the simplest data format imaginable: plain text, rows separated by newlines, values separated by commas. No formatting, no formulas, no charts, no multiple sheets — just pure data.

Key characteristics of CSV:

• Universal compatibility — every programming language, database, and analytics tool reads CSV
• Human-readable in any text editor
• Very small file sizes
• No formatting, formulas, or multiple sheets
• Some variants use semicolons or tabs instead of commas as delimiters

When to use CSV: For data exchange between systems, database imports, API data, and any situation where the data needs to be processed programmatically. Convert between CSV and Excel using ConvertEase's Excel to CSV and CSV to Excel tools.

XLSM — Excel with Macros

XLSM is XLSX with VBA macro support enabled. Microsoft splits macro-enabled workbooks into a separate format (.xlsm) from regular workbooks (.xlsx) for security reasons — this makes it clear when a file contains executable code.

When to use XLSM: Only when your workbook contains VBA macros that automate tasks. If no macros are needed, use regular XLSX. Note that many organizations block XLSM files in email for security reasons.

XLSB — Excel Binary Format

XLSB is a binary version of XLSX. It stores the same data but in a more compact binary format rather than XML. XLSB files are typically 30–50% smaller than XLSX and open faster for very large workbooks.

When to use XLSB: For very large workbooks (hundreds of thousands of rows, complex calculations) where file size and opening speed matter. The trade-off is slightly lower compatibility — some tools that work with XLSX may not support XLSB.

ODS — OpenDocument Spreadsheet

ODS is the spreadsheet format of the OpenDocument Format (ODF) standard, used natively by LibreOffice Calc and Apache OpenOffice Calc. It's an open, vendor-neutral format that works across multiple office applications.

When to use ODS: When working exclusively in LibreOffice or sharing files with LibreOffice users who prefer avoiding Microsoft formats. For files that need to work reliably in Microsoft Excel, XLSX is safer.

JSON — For Web and APIs

While not an "Excel format," JSON is increasingly important for getting data out of Excel into web applications and APIs. Convert XLSX data to JSON using ConvertEase's Excel to JSON converter — it transforms spreadsheet rows into JSON objects with column headers as keys.

Format Conversion Quick Reference