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XLS File Format – Structure, Versions and Advantages

XLS is a file extension used for spreadsheets created using Microsoft’s Excel application. XLS was the extension which was used for Microsoft Excel files created before MS Excel 2007 was released.  MS Excel 2007 introduced a new file extension known as XSLS. This new file format utilized XML file formatting.

The data storage technique in XLS is based on traditional BIFF (Binary Interchange File Format). In BIFF, data is first converted into the binary format and then it is directly stored in a file with extension .xls without any further processing. This is a very straight forward storage mechanism which does not involve any complex algorithm. This is due to its simplicity, it lacked sophisticated features and was eventually replaced by a more advanced and robust XLSX format in 2007.


In XLS file format data is organized into a series of binary streams and substreams. Each individual worksheet in the spreadsheet is composed of one binary sub-stream whereas a spreadsheet consists of the individual stream.

At the higher level, the spreadsheet contains tables where data is divided into several records, where each record contains several fields. A field is a type off attribute, for instance, Age, Gender, Salary of a person. Each field contains data type, for example, age can be an integer. The record type is specified by a 2-byte unsigned integer that is appended at the beginning of every Record and is known as RT.

Individual data is stored in Cell which is actually a container for the data. All the data is actually stored in a worksheet cell, with the exception of strings that are stored in a separate table whereas actual data cell contains pointers or references to these strings.


Although the first Microsoft version of XLS files was released in 1987 and was named version 2.0, complex and advanced features were added in version 7.0, released in 1995 with office 95. In 1990, 1992 and 1993, version 3.0, 4.0 and 5.0 were released respectively.

  • Version 7.0 (released with office 95)

This version of Excel was the most robust and faster among all the versions and internal stream rewrites were updated to 32 bits.

  • Version 8 (released with office 97)

VBA was introduced as a standard language and removed natural language labels were incorporated in this version for the first time. It also introduced a paper clip office assistant for the first time.

  • Version 9 (Released with office 2000)

There were only minor changes in Version 9 where paper clip office assistant could simultaneously hold multiple objects that were not previously possible.

  • Version 10 (Released with office XP)

This version did not contain any noticeable improvement.

  • Version 11 (Released with office 2003)

Major update in version 11, excel 2003 was the introduction of new tables.

From Office 2007 and onwards, XLS format was replaced by a new XLSX format which is considered more secure, robust and portable.

How different is XLS from XLSX?

There are a few basic differences between XLS and XLSX file formats, including in how they store information, speed, and compatibility with spreadsheets that contain Macros. XLS and XLSX differ in that the former store’s information in the binary format while the latter store’s information in text files and uses Office Open XML format.

XLS and XLSX also differ in the sense that XLS is the default format for Microsoft Excel version 2003 or older. XLSX, on the other hand, is the file format for Microsoft Excel 2007 and later versions. You can use any version of Microsoft Excel, including the latest versions, to read files in XLS format. However, only Microsoft Excel 2007 and later versions can read XLSX files. XLSX is the default file extension for Microsoft Excel.

How Do You Open an XLS File?

As noted above, you can use any version of Microsoft Excel to open XLS files. If you have no Microsoft Excel, then use any of the following programs. You should be able to open, edit, copy, save and print XLS files. Note that some are third-party apps that you need to download for your Windows, Mac, Linux, Android or iOS devices.

Programs you can use to open XLS files

  • For Windows, look at File Viewer Plus, Microsoft Excel 2019, Apache OpenOffice and Kingsoft Spreadsheets
  • For Mac: Microsoft Excel 2019, Apple Numbers
  • For Linux: Gnumeric, LibreOffice
  • XLS on the Web: Google Drive
  • iOS: Microsoft Excel, Apple Numbers, Google Drive
  • Android: Microsoft Excel, WPS Office, Google Drive

How to Convert XLS Files to XLSX

If you open the Microsoft Office suite, you will find the spreadsheet application Microsoft Excel. If you are using version 2003 and earlier, files will save with the .xls extension. To convert XLS files to XLSX (.xlsx):

  • Open the XLS file you want to convert.
  • Go to file menu labeled “Office”
  • Click and select “Save As.”
  • In the selections, pick “Excel Workbook”. This allows you to save the file in the default format.
  • Choose your storage location e.g. desktop or documents
  • Name your file and click “Save.” The new file will save as .xlsx.

Can I convert XLSX to XLS?

One easy way to convert an XLSX file to XLS is to save it as a workbook compatible with Microsoft Excel 2007 or later versions. Open the XLSX file and click on “Save As”. In the dialog display that comes up, choose Excel 97-2003. Proceed to save the file which will have the extension .xls.

Note: Choosing Excel 97-2003 Workbook would allow you to save the file as a.xls file fully compatible with the earlier versions of Microsoft Excel.


XLS was designed to be used on older computers. It lacked compatibility and portability with most of the non-Microsoft systems. It had many dependencies on external libraries and therefore it was not easy to run XLS files on systems where these libraries were not installed. XLS was rightly updated to XLSX in office 2007 and onwards.

The latest version of Excel is a subscription product called “Microsoft Excel with Office 365.” Microsoft Excel versions that followed the Excel 2007 release are Excel 2010, Excel 2013, and Excel 2016.


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