If you’re still saving Excel spreadsheets with a three-letter .xls extension, you’re lugging around a digital anchor from the era of dial-up. Beyond being a 1990s relic, these legacy files are slow, prone to corruption, and—most dangerously—a preferred backdoor for modern malware. It’s time to move to XLSX and never look back.
Reason 1: The binary black box vs. the modern ZIP
To understand why XLS needs to die, you have to look under the hood. In the late 1980s, Microsoft introduced the Binary Interchange File Format (BIFF)—a proprietary format that only Excel truly understands. These binary blobs are fragile: corruption in one area can render the entire workbook unreadable or extremely difficult to repair.
In 2007, Microsoft got smart and switched to Office Open XML (.xlsx), which is actually just a renamed ZIP folder. Don’t believe me? Rename an XLSX file to .zip, and double-click it. You’ll see a highly organized directory of XML text files that handle your data, your cell styles, and your workbook metadata separately.
Because the XLSX file is modular, Excel can often surgically repair a damaged workbook by replacing one broken XML part while preserving your raw data.
Beyond repairability, using XLS today limits access to modern features and can introduce performance inefficiencies in larger or more complex workbooks. XLS still exists largely for backward compatibility, not because it’s technically superior.
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Reason 2: The security minefield
If you want to make an IT manager flinch, just show them a folder full of XLS files. In the cybersecurity world, the old binary format is essentially a digital biohazard because its opaque binary structure makes it harder for security tools to inspect and isolate malicious code. This makes it a perfect “stealth” vehicle for macro-based viruses and ransomware.
Standard XLSX files do not support macros by design. If you try to save code in one, Excel will warn you that the code will be stripped away unless you switch to a macro-enabled format (XLSM). What’s more, Windows now uses Mark of the Web (MotW) to flag downloaded files with security warnings. For legacy formats like XLS, this often triggers security warnings that many users don’t fully understand.
In short, moving to XLSX removes the target from your back. In an era where corporate email filters often quarantine or block legacy attachments, using a modern format ensures your spreadsheets actually reach your recipient’s inbox.
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Reason 3: The 65,536-row speed bump
In the Excel 97-2003 era, 65,536 rows and 256 columns seemed like an infinite amount of data. However, in today’s era of big data and automated reports, it’s not only a significant limitation but also a data-loss hazard.
The real danger here is accidental truncation. If you copy 100,000 rows of data into an old XLS workbook, Excel will warn you that data beyond the limit will be lost—but that warning is easy to miss if you’re in a rush.
Modern XLSX workbooks, however, support 1,048,576 rows and 16,384 columns, and modern features, such as dynamic arrays and XLOOKUP, are built for this expansive grid. Forcing them into an old binary format is like trying to fit a V8 engine into a golf cart: eventually, something will break.
Reason 4: Storage bloat and the 75% slim-down
If your hard drive is feeling the squeeze, that might be because the old XLS format is a storage hog. Because these files store data in a dense, uncompressed binary stream, they’re notoriously heavy. Moving from XLS to XLSX is the digital equivalent of switching from an uncompressed suitcase to a vacuum-sealed bag.
Since XLSX is a compressed ZIP package, converting a legacy file can often slash its size by 30% to 75% (depending on data density and formatting) without losing a single cell of data.
As well as saving local disk space, this makes every part of your digital life faster. Smaller files mean quicker OneDrive syncing, faster AutoSave performance, and fewer “attachment too large” bounces when emailing. If you’re hoarding hundreds of old workbooks, a batch conversion could reclaim gigabytes of space in seconds.
Reason 5: The cloud-first collaboration gap
If you’re using XLS, you’re essentially locked out of the best parts of modern Office. Because XLS is a legacy format, it doesn’t support co-authoring, so if you want to work in a file at the same time as a coworker, Excel will force you to convert to XLSX or deal with the “File is in use” read-only prompt.
What’s more, whether you like it or not, the future of Excel is AI, and modern tools like Copilot and automated data insights struggle with the opaque, closed nature of XLS files. If you want to use the cutting-edge tools of the 2020s, like real-time cloud saving and AI analysis, you have to stop saving your work in the 1990s.
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Unless you’re handcuffed to a piece of 20-year-old enterprise software that refuses to export anything else, there’s no reason to keep creating new legacy files. Whether you have a single XLS file or a decade-old archive, it doesn’t take long to transport them to the modern era.
Before you start converting, check for macros. If your XLS file contains VBA code, converting it to XLSX will strip it away forever. For these files, you must save them as XLSM (Excel Macro-Enabled Workbook) to keep your automation intact.
The individual upgrade: For the occasional stray
If you happen to open an old Excel file and see “Compatibility Mode” in the title bar, go to File > Info and click “Convert.” Excel saves a new XLSX version of the file, so you can delete the original.
The batch move: For the data hoarder
If you’re staring at a folder full of hundreds of XLS files, don’t do it manually. Use Power Automate Desktop (free for Windows users) to build a simple workflow that opens, saves, and closes every file in a directory while you grab a coffee.
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Holding onto XLS files isn’t “playing it safe”—it’s a liability. Every time you save a legacy Excel file, you’re choosing a slower, heavier, and less secure way of working. By converting to modern formats, you get a workbook that is smaller, more reliable, and actually compatible with the future of work.
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