What Is the File Extension of Excel? A Complete Guide to Excel File Formats

If you've ever saved a spreadsheet and noticed the letters after the dot in the filename, you've encountered a file extension — the short code that tells your operating system what kind of file it is and which program should open it. Excel uses several extensions depending on the version of the software, the type of content in the file, and how you intend to use it.

Understanding which extension is which helps you avoid compatibility issues, protect your data, and share files more effectively.

The Default Excel Extension: .xlsx

The most common Excel file extension today is .xlsx. This has been the default format for Microsoft Excel since Excel 2007, when Microsoft introduced the Office Open XML standard.

The "x" at the end stands for XML — the underlying structure of the file. An .xlsx file is actually a compressed package of XML files zipped together, which is why the file sizes tend to be smaller than older formats.

.xlsx files:

• Support up to 1,048,576 rows and 16,384 columns
• Are compatible with Excel 2007 and later
• Can be opened by Google Sheets, LibreOffice Calc, and most modern spreadsheet tools
• Do not support macros

The Older Format: .xls

Before 2007, Excel used the .xls extension — a binary file format based on the BIFF (Binary Interchange File Format) standard.

You'll still encounter .xls files when working with legacy systems, older shared files, or organizations that haven't fully updated their software stack.

.xls files:

• Support up to 65,536 rows and 256 columns
• Are compatible with Excel 97 through Excel 2003 natively
• Can be opened in modern Excel versions, but may trigger a compatibility warning
• Are generally larger than equivalent .xlsx files

If someone sends you an .xls file, modern Excel will open it in Compatibility Mode, flagging any features not supported in the older format.

Macro-Enabled Files: .xlsm

When an Excel file contains VBA macros — automated scripts that perform tasks within the spreadsheet — it must be saved as .xlsm (Excel Macro-Enabled Workbook).

This distinction exists for a security reason: Excel deliberately separates macro-capable files from standard spreadsheets so that users aren't unknowingly running code. If you try to save a macro in a .xlsx file, Excel will warn you and prompt you to switch formats.

.xlsm files are commonly used for:

• Automated data entry or reporting workflows
• Custom buttons and interactive dashboards
• Business tools built inside Excel

Binary Workbook: .xlsb

The .xlsb format is Excel's binary workbook format — a lesser-known option that stores data in binary rather than XML.

This format is worth knowing about because it can dramatically reduce file size and improve performance for very large spreadsheets with complex formulas, pivot tables, or large datasets.

.xlsb is often preferred when:

• Files contain hundreds of thousands of rows
• Opening and saving speed is a priority
• The file is used internally and doesn't need to be shared widely

The trade-off is reduced compatibility — not all non-Microsoft tools handle .xlsb reliably.

Template Formats: .xltx and .xltm

Excel also uses template-specific extensions:

Templates are useful for creating standardized formats — invoices, reports, trackers — that users fill in without overwriting the original structure.

Other Excel-Related Extensions 📁

A few additional extensions appear in specific contexts:

• .csv — Comma-Separated Values. Technically not an Excel format, but Excel opens and creates them. Plain text, no formatting, widely compatible.
• .xlam — Excel Add-In. Used to package tools and macros that extend Excel's functionality.
• .prn / .txt — Text-based exports Excel can generate for use in other systems.

What Determines Which Extension You're Working With?

Several factors influence which format applies to a given file:

Version of Excel installed — Older installations may default to .xls. Modern installations default to .xlsx.

Whether macros are present — Any file with VBA code requires .xlsm or .xlsb, not .xlsx.

File size and performance needs — Large, performance-sensitive files may warrant .xlsb despite the compatibility trade-offs.

Who you're sharing with — If collaborators use Google Sheets or LibreOffice, .xlsx is the safest choice. If they're using a legacy system, .xls may be required.

Organizational or system requirements — Some enterprise software, databases, or government portals specify which format they accept for uploads or imports.

The Format Compatibility Spectrum 🔄

At one end, you have .csv — the most universally compatible format, readable by nearly any software, but stripped of all formatting, formulas, and structure. At the other end, .xlsb offers peak Excel performance but limited cross-platform support.

In the middle sits .xlsx — the sweet spot for most modern use cases. It balances broad compatibility, reasonable file size, and full support for Excel's feature set (minus macros). Then .xlsm steps in when automation is part of the picture.

The format that makes sense depends on what the file does, where it's going, and who's opening it. A personal budget tracker saved locally operates under completely different constraints than a macro-heavy reporting tool shared across a corporate network — and the right extension reflects that difference.