XLS to XLSX Converter

What is XLS to XLSX Conversion?

XLS to XLSX conversion is the process of transforming a spreadsheet from the legacy binary Microsoft Excel 97-2003 format into the modern Office Open XML format used in Microsoft Excel starting with version 2007. This is an upgrade within the Microsoft Excel ecosystem: from a proprietary binary format to a modern open standardized format.

The XLS format is a binary BIFF (Binary Interchange File Format) that Microsoft used for nearly 10 years (1997-2007) as the main spreadsheet format. Inside, an XLS file is a set of binary blocks describing sheets, cells, formulas, formatting, and charts. The structure was proprietary, poorly documented, and had strict size limitations.

The XLSX format appeared with the release of Microsoft Office 2007 and quickly became the de facto standard for spreadsheets. In 2008 it was approved as the international standard ISO/IEC 29500. Structurally, XLSX is a ZIP archive containing XML files describing the table, formulas, formatting, sheets, and charts. It is an open, transparent, size-efficient format.

PEREFILE performs XLS to XLSX conversion by transferring all spreadsheet elements from the binary format to the modern XML format while preserving structure, formulas, formatting, sheets, and charts, and also expanding to the capabilities of the new format.

Why XLS is Considered Outdated

The XLS format served almost a decade as Microsoft Excel's main spreadsheet format, but with the appearance of XLSX in 2007, its shortcomings became obvious:

Strict Technical Limitations

XLS was created in an era when even a million records seemed an astronomical quantity. Format limitations:

• 65,536 rows per sheet - even typical databases of modern companies exceed this limit
• 256 columns per sheet - insufficient for wide analytical tables
• 31 characters in sheet name - does not allow descriptive names
• 56 colors in the palette - limited styling capabilities
• up to 32,000 rows in pivot tables - a problem for large reports

Binary Structure

XLS is a monolithic binary file. This creates problems:

• Corruption vulnerability - a small error in the file can make it unreadable
• Difficulty of programmatic access - specialized libraries are needed for parsing
• Large size - no built-in compression, files take up a lot of space
• Transmission problems - binary files endure email transmission and older systems worse

Security Issues

XLS has historically been a vector for malware distribution:

• Macro viruses - VBA macros in XLS were used to create viruses
• Binary parser exploits - vulnerabilities were regularly discovered in Excel when opening XLS
• System distrust - modern email systems and antivirus software treat XLS with greater suspicion

Declining Support

• Microsoft Excel 2007 and newer work with XLS in compatibility mode with limitations
• Many modern Excel functions are unavailable for XLS files
• Cloud services (OneDrive, SharePoint) are optimized for XLSX

Comparing the XLS and XLSX Formats

Understanding the differences helps to recognize the benefits of upgrading:

The key architectural difference: XLS is a monolithic binary file, XLSX is a structured archive of open XML files. This gives XLSX advantages in size, reliability, transparency, and compatibility.

When XLS to XLSX Conversion is Necessary

Removing Table Size Limitations

If you have hit the 65,536-row or 256-column limit in XLS, this is a direct reason for conversion. XLSX supports over a million rows and 16,384 columns, allowing you to work with truly large datasets.

Updating the Document Archive

Many organizations have accumulated thousands of XLS files over years of work: reports, budgets, price lists, databases. Mass conversion to XLSX provides several advantages:

• The archive size decreases (by 30-50% due to compression)
• The risk of data loss due to corruption is reduced
• Files become compatible with modern systems

Preparing for Migration to Cloud Services

If an organization is switching to Microsoft 365, OneDrive, or SharePoint, XLSX files work there natively. XLS files can be opened but with limitations: no online editing with full functionality, problems with automation.

Using Modern Excel Features

Many Excel features are available only for XLSX files:

• Dynamic arrays (FILTER, SORT, UNIQUE)
• New functions (XLOOKUP, LET, LAMBDA)
• Power Query and Power Pivot
• Modern data types
• Extended pivot tables

Compliance with Corporate Standards

Most modern corporate document workflow standards require XLSX. The use of XLS is becoming an exception requiring justification.

Security and Compatibility

XLSX is more secure and better compatible with modern antivirus, email systems, and document management systems. Translating the archive to XLSX simplifies work with these systems.

What is Transferred During XLS to XLSX Conversion

This is a very high-quality conversion type because it is about updating the format within a single Microsoft Excel ecosystem:

All Cell Data

Numbers, text, dates, time, logical values, errors - everything is transferred one-to-one without losses.

Formulas

All formulas are transferred as-is. Excel formula syntax is identical in XLS and XLSX, so no transformations are required. All functions, references, absolute and relative addresses work the same.

Full Formatting

• Fonts, sizes, styles (bold, italic, underline)
• Text and fill colors (with extension to full RGB in XLSX)
• Cell borders, line types, border colors
• Alignment (horizontal, vertical, text rotation)
• Number formats (currency, percentages, dates, fractions)
• Conditional formatting
• Merged cells

All Sheets

All sheets from the XLS workbook are transferred to XLSX preserving order, names, and content. The structure of links between sheets through formulas is preserved.

Charts and Graphs

Charts are transferred in full. XLSX uses a modern chart engine; in most cases the visual result is identical to the original. Sometimes charts even look better due to improved rendering.

Images and Graphics

All embedded images, photos, illustrations, and objects are transferred to the new file.

Pivot Tables

Pivot tables are transferred preserving structure, fields, and connection to source data.

VBA Macros

If XLS had macros, they are transferred to XLSX. However, an important nuance: standard XLSX by default does not support macros for security reasons. For files with macros, the XLSM format (Excel Macro-Enabled Workbook) is used. During conversion, the file may be saved as XLSM if it contains macros.

Named Ranges

Names for cells and ranges defined in XLS are transferred to XLSX and continue to work in formulas.

Metadata

Author, creation date, modification date, title, keywords - all document metadata is preserved.

Technical Aspects of the Conversion

Identical Formula Syntax

The main advantage of XLS to XLSX conversion is the update within a single ecosystem. Excel formula syntax does not change, functions have the same names, references are written the same way. Therefore, conversion goes without losses and warnings.

Expanded Color Palette

XLS uses a limited palette of 56 colors. XLSX supports full RGB (16 million shades). During conversion, XLS colors are accurately mapped to RGB - the visual result is identical to the original, but now you can use any shades.

File Size Reduction

XLSX applies ZIP compression, which usually gives a 30-50% size reduction compared to XLS. For tables with many formulas and text, the savings can be significant. This simplifies file storage and transmission.

Improved Resilience

XLSX is a ZIP archive with XML files. If the file is partially damaged, you can open the archive, extract undamaged XML, and recover some data. There is no such possibility with binary XLS - a damaged file is often completely lost.

Saving Macros as XLSM

If VBA macros are detected in XLS, the file is saved as XLSM (Excel Macro-Enabled Workbook). This is a variant of XLSX with macro support. Modern Excel requires user confirmation to run macros in XLSM for security reasons.

Which XLS Files Convert Well

Standard Office Tables

Budgets, reports, price lists, registries, databases, financial models - all convert to XLSX without losses. This is a typical scenario where conversion goes perfectly.

Multi-Sheet Workbooks

Workbooks with multiple sheets and formulas linking sheets convert with quality preserving all links.

Documents with Charts

All types of charts transfer well. Often they even look better after conversion due to the improved XLSX rendering engine.

Files with Macros

VBA macros are transferred preserving functionality. The file will be saved as XLSM, and user confirmation will be required to run macros.

Files with Conditional Formatting

Conditional formatting is transferred correctly, including highlighting rules, color scales, and icon sets.

Advantages of the XLSX Format

Removing Size Limits

The main advantage of XLSX is the ability to work with large data. 1,048,576 rows and 16,384 columns are 16 and 64 times more than XLS. Modern business data can easily exceed XLS limits, and XLSX solves this problem.

File Size Reduction

ZIP compression in XLSX usually gives a 30-50% file size reduction. Large tables become significantly smaller, which simplifies their storage, transmission, and backup.

Better Security

XLSX is an open format; its XML structure is easily analyzed by antivirus software. Macros are separated into a distinct format (XLSM), which reduces the risk of accidentally running malicious code.

Compatibility with Modern Software

Microsoft 365, OneDrive, SharePoint, Power BI, Power Query - all modern Microsoft tools work with XLSX as the native format. XLS is supported with limitations.

Access to New Excel Features

Dynamic arrays, XLOOKUP, LET, LAMBDA, Power Query, Power Pivot - all these capabilities are available only for XLSX files.

Cloud Work

XLSX can be fully edited online in Excel Web, you can collaborate in real time, synchronize through OneDrive, and send editing links. XLS in the cloud works with limitations.

Open Standard

XLSX is the international standard ISO/IEC 29500. Its specification is documented and available. XLSX files will be readable in the future regardless of the fate of specific programs.

Improved Resilience

When an XLSX file is damaged, you can often recover some content by opening the ZIP archive and working with XML directly. With binary XLS there is no such possibility.

Limitations and Recommendations

Compatibility with Old Excel

If the file will be opened in Microsoft Excel 2003 or earlier, it will not be able to work with XLSX without installing a special compatibility pack (Microsoft Office Compatibility Pack). If the recipient uses a very old Excel, perhaps it is better to leave XLS.

File Size After Conversion

In most cases, the XLSX file will be 30-50% smaller than the source XLS. However, for files with many images, the difference may be less noticeable since images are already compressed inside.

Macros and Security

If XLS has macros, the resulting file will be XLSM. Modern Excel blocks macro execution by default - the user will have to explicitly allow them to run through a warning at the top of the window.

Complex Styling Elements

Very specific styling elements used in Excel 2003 may be displayed slightly differently in modern Excel. Standard formatting transfers without problems.

Alternatives to Online Conversion

Using Microsoft Excel

If you have Excel 2007 or newer installed, you can open XLS and save as XLSX via "File -> Save As -> Excel Workbook (.xlsx)". You can also use the "Convert" function for built-in conversion. The downside is the need to process each file manually.

Using Free Office Suites

Free open-source office suites can also open XLS and save as XLSX. Suitable if you do not have Microsoft Office.

PEREFILE Online Service

• No software installation required - conversion in the browser
• Formulas, formatting, sheets, and macros are processed correctly
• Suitable for one-time tasks and regular mass conversion
• Available from any device with internet
• Multiple files can be converted in one session

Who Benefits from XLS to XLSX Conversion

IT Specialists and System Administrators

The task of mass document archive migration during infrastructure updates. XLS to XLSX conversion ensures compatibility with modern systems and reduces the risk of data loss.

Accountants and Financial Specialists

Working with old reports, budgets, financial models. Conversion to XLSX opens access to new Excel features, simplifies exchange with modern systems, and reduces file size.

Data Analysts

Removing table size limitations is a frequent reason for conversion. XLSX allows working with millions of rows, which is critical for big data analysis.

Employees Transitioning to Microsoft 365

Corporate migration to Microsoft cloud services involves using XLSX as the main format. Converting historical XLS files is a mandatory stage of migration.

Archivists and Document Management Workers

Translating document archives to a modern format for long-term storage. XLSX as an open ISO standard is preferable for archiving.

Secretaries and Office Managers

Receiving documents from various sources. XLS to XLSX conversion ensures compatibility with recipient systems and simplifies further work.

Teachers and Methodologists

Updating educational materials and templates created in Excel 2003 and earlier. After conversion you can use modern Excel features and ensure compatibility with the modern software students use.

History of Excel Formats

The XLS Era (1997-2007)

The XLS format appeared together with Excel 97 and became the de facto standard for spreadsheets for a decade. During this time Excel 2000, Excel XP, Excel 2003 were released - all used XLS as the main format with minimal changes.

Transition to XLSX (2007-present)

In 2007, Microsoft introduced Excel 2007 with the new XLSX format based on Office Open XML. This was a cardinal step: the transition from binary to open XML format, approved as ISO/IEC 29500 in 2008. All subsequent versions (Excel 2010, 2013, 2016, 2019, 2021, Microsoft 365) use XLSX as the default format.

Current State

XLSX dominates the spreadsheet world. All modern programs support it as the native format. XLS remains as a format for compatibility with old software, but new files are practically never created in XLS.

Recommendations for Quality Conversion

Preparing the Source File

• Make sure the XLS file opens without errors
• Make a backup of the original
• If necessary, clean personal metadata through document properties

Checking the Result

After conversion, open XLSX in Excel and check:

• Correct display of data and formatting
• Operation of all formulas (no errors)
• All sheets transferred
• Visual appearance of charts
• Operation of macros, if any

After Conversion

• The converted XLSX is ready to work in all modern versions of Excel
• Can be uploaded to OneDrive and SharePoint for cloud work
• All modern Excel features are available
• The file is compatible with Power BI, Power Query, add-ins