XLS File—Open, Check Safety, and Convert to XLSX

Quick answer

XLS is a binary spreadsheet format that stores BIFF5 or BIFF8 workbook records in an OLE Compound File container; saving a newer workbook back to XLS can omit data or features.
Open XLS with a current desktop version of Microsoft Excel. Excel opens an Excel 97–2003 workbook in Compatibility Mode. On a Windows computer with desktop Excel installed, open the file from Excel and keep Protected View active when it appears. Do not enable macros or refresh connections in an untrusted file.
Treat an unexpected XLS file as high risk. XLS can contain VBA or Excel 4.0 macros and external connection definitions with server, provider, security, and command information. Keep macros disabled unless both the source and behavior are trusted, and do not refresh unknown connections. Protected View and Office File Validation reduce risk but do not prove that a workbook is safe. The OLE signature identifies a container, not XLS uniquely.

XLS File: What It Is, How to Open It, and How to Convert It

An XLS file is a Microsoft binary spreadsheet workbook. The extension covers Excel 5.0/95 BIFF5 and Excel 97–2003 BIFF8 files. BIFF5 and BIFF8 use an OLE Compound File containing storages, streams, and binary-record substreams. A BIFF8 workbook has one Workbook stream with a Globals Substream followed by at least one worksheet, chart-sheet, macro-sheet, or dialog-sheet substream. application/vnd.ms-excel is the registered generic media type for Microsoft Excel data.

Keep the original first

Current desktop Excel can convert XLS to a modern workbook format such as XLSX, XLSB, or XLSM. In the opposite direction, preserve the modern workbook and run Compatibility Checker before saving it as XLS. Saving a modern workbook as XLS may omit formatting, data, formulas, or features; unsupported formulas and limits can cause permanent data loss, permanently change results, or produce #VALUE! errors.

Open a file

Open XLS with a current desktop version of Microsoft Excel. Excel opens an Excel 97–2003 workbook in Compatibility Mode. On a Windows computer with desktop Excel installed, open the file from Excel and keep Protected View active when it appears. Do not enable macros or refresh connections in an untrusted file.

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File safety

Treat an unexpected XLS file as high risk. XLS can contain VBA or Excel 4.0 macros and external connection definitions with server, provider, security, and command information. Keep macros disabled unless both the source and behavior are trusted, and do not refresh unknown connections. Protected View and Office File Validation reduce risk but do not prove that a workbook is safe. The OLE signature identifies a container, not XLS uniquely.

Pause when something does not match

Do not open the file when its source, double extension, or actual contents do not match what you expected.

Converters

Before converting

Work from a copy, then compare image quality, metadata, and layout after conversion.

Frequently asked questions

What is an XLS file and what is it used for?

An XLS file is a Microsoft binary spreadsheet workbook used to store workbook and sheet data as BIFF records. The extension includes Excel 5.0/95 BIFF5 and Excel 97–2003 BIFF8 workbooks.

How do I open an XLS file?

Use a current desktop version of Microsoft Excel. Excel opens Excel 97–2003 XLS workbooks in Compatibility Mode; keep Protected View enabled for an untrusted file and do not enable its macros or refresh its connections.

How do I open an XLS file on Windows 11 if it opens in the wrong app?

Start the current desktop version of Excel on Windows 11 and open the XLS from inside Excel. If Protected View appears, leave it active, especially for an email attachment, download, or file from an unknown source.

Why is my XLS file reported as damaged or unable to open?

Excel may apply Office File Validation protection when an Excel 97–2003 XLS file is structurally malformed. Keep the protection active, do not enable macros to force it open, and request a fresh copy from the sender or original system.

What is the difference between XLS and XLSX?

XLS is the older binary spreadsheet family covering BIFF5 and BIFF8 workbooks in an OLE Compound File container. XLSX is a modern workbook format to which current desktop Excel can convert an XLS file.

How do I convert an XLS file to XLSX?

Open the XLS in a current desktop version of Excel and save it as XLSX. Keep the original XLS as an unchanged copy.

Why does an XLS conversion lose formatting or produce #VALUE! errors?

This risk applies when a modern workbook is saved down to XLS. Unsupported formulas, features, and older-format limits can omit formatting or data, permanently change results, or produce #VALUE! errors; run Compatibility Checker before saving as XLS.

Are macros in an XLS file safe to enable?

Not by default. XLS can contain VBA or Excel 4.0 macro content, and enabling macros can execute dangerous code. Keep them disabled unless you trust both the file's source and the macro behavior.

How can I check whether an XLS file contains macros safely?

Open the workbook in current desktop Excel with macros left disabled. If Excel presents a macro or security warning, do not enable the macros. The .xls extension and leading OLE bytes alone cannot establish whether the workbook contains macros.

Are external connections in an XLS file dangerous?

They can introduce risk in an untrusted workbook. An XLS file may store provider type, server name, security information, and commands for retrieving database or OLAP data, so do not refresh unknown connections.

Does the XLS signature prove that the file is a valid workbook?

No. D0 CF 11 E0 A1 B1 1A E1 at offset 0 identifies the generic OLE Compound File container. BIFF version and workbook records must also be examined to distinguish XLS from other compound files.

Does XLS encryption hide all workbook metadata?

No. Only certain storages and streams are encrypted, while BIFF record types and sizes remain readable without decrypting the workbook. Encryption therefore does not conceal every structural detail.