Excel File Extension: XLSX, XLS, XLSM, and Every Format Explained

Excel file extensions tell you (and Excel) what kind of file you're dealing with. The modern default is .xlsx, the standard format since Excel 2007. But several other extensions exist for specific purposes: .xls for older Excel files, .xlsm for files containing macros, .xlsb for binary format with better performance for large files, .xltx for templates, .csv for comma-separated values, and others. Understanding when to use each format prevents compatibility problems, file size issues, and lost data when sharing across different Excel versions or with different software.

This guide covers every Excel file extension you'll encounter — what each format means, when to use it, what features it supports or doesn't support, and how to convert between formats. We'll also cover the security implications of different formats, the performance considerations for large files, and the compatibility issues that arise when sharing files with users on older Excel versions or different software entirely. Whether you're choosing a format for new files or troubleshooting compatibility issues, this overview provides comprehensive context.

XLSX has been the Excel default since 2007 and is the format you should use for nearly all new spreadsheets. It uses Open Office XML format, which is essentially a ZIP file containing XML files describing the workbook structure. This makes XLSX files smaller than the older XLS format and more recoverable when corrupted. The format is widely supported by other software including Google Sheets, LibreOffice, Apple Numbers, and various web applications. Choose XLSX unless you have specific reason to use something different.

XLSM is identical to XLSX except it allows VBA macros. The distinction exists for security — when you open an XLSM file, Excel warns you about macros and may require you to enable them explicitly. This protects users from macro-based malware. If your workbook contains any VBA code, you must save it as XLSM. Saving a macro-containing workbook as XLSX silently discards all the macros, which is rarely what you want. Always save VBA workbooks as XLSM.

The legacy XLS format was the standard from Excel 97 through Excel 2003. It's still readable and writable by modern Excel for backward compatibility. The format is binary (not XML), produces larger files than XLSX, has lower row and column limits (65,536 rows and 256 columns vs over 1 million rows and 16,384 columns in XLSX), and doesn't support some newer Excel features. Use XLS only when sending to users on Excel 2003 or earlier, which is increasingly rare. For most modern use, XLS is obsolete.

XLSB is the binary version of the modern XLSX format. It produces smaller files and opens/saves faster than XLSX, especially for very large workbooks with many formulas. The performance advantage is most noticeable for files with hundreds of thousands of rows or complex calculations. The drawback: other tools (Google Sheets, Python pandas, web applications) often can't read XLSB as easily as XLSX. For workbooks you'll keep within Excel and need performance, XLSB makes sense. For workbooks you'll share with diverse tools, stick with XLSX.

CSV (Comma-Separated Values) isn't an Excel format per se, but Excel reads and writes CSV regularly because it's the universal data exchange format. CSV files contain only data — no formulas, formatting, charts, or multiple sheets. Each row in the file is a row in the spreadsheet; columns are separated by commas. Excel offers several CSV variants (UTF-8 encoded, standard, Mac, MS-DOS) with subtle differences. For exchanging data with databases, web applications, and other systems, CSV is often the most reliable choice.

Template formats deserve more attention than most users give them. XLTX (and XLTM for macros) lets you save a workbook as a starting template. New files based on the template start with all the formatting, formulas, and structure already in place. For repetitive workflows — monthly reports, standard invoices, project plans — templates eliminate hours of setup work. Save your template to a personal templates folder, and Excel makes it available when you create new files. Build templates for any workbook structure you'll create multiple times.

Compatibility considerations matter when sharing files. Modern Excel versions (2007+) can read all the formats discussed here. The challenge comes when recipients use older Excel versions, Mac Excel (which may handle some features differently), Excel for the web (limited feature support), Excel on mobile (more limited), or non-Microsoft spreadsheet tools (Google Sheets, LibreOffice). For maximum compatibility, save in XLSX format with the Compatibility Mode option that warns about features that won't transfer to older Excel versions.

Mac Excel handles file formats nearly identically to Windows Excel for modern versions, but some legacy differences exist. The XLS format in older Mac Excel versions had subtle differences from Windows XLS. Modern Mac Excel uses the same XLSX format as Windows. Cross-platform users generally have no format compatibility issues with modern Excel installations. The differences that do exist usually involve specific features like font handling, certain chart types, or platform-specific add-ins that don't apply to most users.

Excel for the web has more limited format support than desktop Excel. It works best with XLSX files. Some XLSX features (advanced charts, certain pivot table features, some VBA functionality) don't fully work in browser. XLSM macros don't run in browser at all, though the file can still be viewed. For workbooks intended primarily for browser use, design within the supported feature set. For workbooks needing full Excel functionality, plan for desktop usage and use browser as supplementary access.

For users who exchange data with databases or programming languages, CSV is usually the right format. Databases import CSV directly into tables. Python pandas reads CSV with pd.read_csv(). R imports CSV with read.csv(). JavaScript can parse CSV with various libraries. The simplicity of CSV (just rows and columns of plain text) makes it the lingua franca of data exchange. Excel exports clean CSV that downstream systems can consume reliably, as long as you understand the encoding choices (UTF-8 for international characters) and delimiter conventions in your locale.

For programmatic Excel file manipulation, several libraries exist outside Excel itself. Python's openpyxl reads and writes XLSX files without needing Excel installed. xlsxwriter creates new XLSX files efficiently. xlrd reads older XLS format (now legacy). pandas integrates with several of these for data analysis workflows. R has openxlsx and readxl packages. Node.js has libraries like ExcelJS. These tools enable automated Excel file creation and modification for batch processing, web applications, and data pipelines that need Excel format output.

Security implications differ significantly between formats. XLSX is generally safe because the format can't execute code. XLSM, XLSB, XLTM, and XLAM can contain VBA macros that potentially run malicious code. Email systems and security tools often quarantine these macro-enabled formats. For sharing files in enterprise environments, plain XLSX is the safest default. Use macro-enabled formats only when macros are genuinely needed for the workbook's functionality, and document what the macros do for users who will need to decide whether to enable them.

Excel's file extension display can be confusing on Windows because by default Windows hides known file extensions. You may see 'Sales' in File Explorer when the actual filename is 'Sales.xlsx'. To show extensions and avoid confusion, enable file extension display in File Explorer: View tab > File name extensions (in Windows 10/11) or Folder Options > View > uncheck Hide extensions for known file types. With extensions visible, you can always see whether a file is XLSX, XLSM, CSV, or another format at a glance.

For workbooks that need to support older Excel versions, the Compatibility Checker (Info > Check for Issues > Check Compatibility) identifies features in your workbook that won't work in older versions. Run this before sending files to recipients on older Excel. The report shows specific issues like 'this conditional formatting feature requires Excel 2010 or later'. Either remove the problem features or accept that recipients on older versions will see degraded functionality. The checker is especially useful for organizations that still have users on Excel 2007 or 2010.

The XLSX file format being a ZIP archive has interesting implications. You can rename an XLSX file to .zip and open it with any unzip tool to see the underlying XML files. This is occasionally useful for recovery — if Excel can't open a corrupted XLSX file, you can sometimes extract the XML manually and reconstruct usable content. It's also useful for understanding how Excel structures information internally. For most users this is trivia, but it's good trivia to know when you face unusual file problems.

Cloud storage and collaboration affect format choices. OneDrive and SharePoint work best with XLSX files for real-time collaboration features. Google Drive can store any format but converts XLSX to Google Sheets for editing in browser. Box, Dropbox, and other cloud services have varying levels of preview support for different Excel formats. For workbooks meant primarily for cloud collaboration, XLSX is the universal best choice. Macro-enabled formats often don't work well in cloud collaboration scenarios.

The historical evolution of Excel file formats provides useful context. Excel 2.0 through 4.0 (1987-1992) used a binary format with .xls extension. Excel 95 introduced a new XLS format. Excel 97-2003 used another evolution of XLS. Excel 2007 introduced the XML-based XLSX/XLSM formats as the new defaults, with XLS available for backward compatibility. The format change in 2007 was significant — it took several years for organizations to fully transition. Today, XLS is genuinely legacy and you'll rarely encounter it unless working with very old systems.

Looking forward, Microsoft continues evolving Excel file formats incrementally. New features may require updated minimum Excel versions to open files using those features. Cloud-native features may not save to traditional formats at all, instead existing only in the cloud Excel environment. The trend is toward more cloud integration and less reliance on local file formats. For now, XLSX remains the dominant standard and will be for the foreseeable future. The basic format knowledge in this guide stays useful even as Excel continues evolving.

For organizations managing many Excel files, establishing format standards prevents confusion. Decide what default format your organization uses (typically XLSX), when macros are appropriate (and document required security review processes), what templates exist for common workbook types, and how files should be named and organized. Documented standards help new users quickly adopt consistent practices and reduce format-related compatibility issues across the organization. The few minutes spent establishing standards pays back across thousands of files over time.

The bottom line on Excel file extensions: use XLSX as your default for nearly everything. Use XLSM when you need macros. Use CSV for data exchange. Use templates (XLTX) for recurring work patterns. Use XLSB only when performance demands it for very large files. The other formats (XLS, ODS, etc.) have specific niche uses. With this framework, you can handle any Excel file format situation confidently and choose the right format for each new file you create or workflow you build.

Beyond the standard Excel formats, several specialized formats deserve brief mention. XML Spreadsheet (.xml) is an older XML-based Excel format predating XLSX. Hardly anyone uses it today. Excel Web Page (.htm or .html) saves a workbook as HTML for web viewing. The output is rarely beautiful and most users export to PDF instead. Excel Workbook Template (.xltx) and Excel Macro-Enabled Template (.xltm) cover template scenarios. SpaceTab-delimited (.txt or .prn) saves as plain text with different delimiters. Most users only encounter these in unusual scenarios.

For developers building applications that interact with Excel files, OOXML (the official name for the modern Excel format) provides a documented standard. Microsoft published the format specification, making it possible to read and write XLSX files without using Excel itself. This enables web applications to generate Excel files for download, server processes to ingest user-uploaded spreadsheets, and integration platforms to move data between Excel and other systems. The standardization of Office Open XML was a major step forward for interoperability.

One subtle issue with file extensions: Windows associates extensions with applications. Double-clicking an XLSX file opens it in Excel by default. Double-clicking an XLS file does the same. But if you have multiple spreadsheet applications installed (Excel plus LibreOffice plus Google Drive sync), the file association can get complicated. Right-click a file and choose Open With to control which application opens it. Set defaults in Windows Settings if you need to change which application owns specific file types.

For organizations transitioning from older Excel versions, the file format migration deserves planning. Identify all the XLS files in your file shares and document repositories. Plan a conversion strategy — convert immediately, convert as files are touched, or maintain both versions during transition. Train users on the implications of saving in different formats. The transition from XLS to XLSX was a multi-year process for many organizations and similar transitions may happen in the future as Microsoft introduces newer formats. Plan ahead rather than reacting when changes happen.

One final thought on Excel file extensions: while the technical details may seem boring, getting the format right prevents many common problems. Files that won't open in older Excel versions. Macros that disappear silently. Files too large to email. Data corruption from improper conversions. Each problem has its root in mismatching the file format to the use case. Spending a few minutes understanding the formats once pays back across years of avoiding format-related problems in your Excel work going forward.