How to Convert XLS to XLSX

An XLS is the old Excel workbook format, a binary file used through Excel 2003. XLSX is its modern replacement, introduced with Excel 2007 and based on the open OOXML standard. Converting XLS to XLSX is essentially a version upgrade: same spreadsheet, newer and better container.

Unlike converting to CSV, this keeps everything. Multiple sheets, formulas, formatting, and charts all come across, because XLSX is a full-featured spreadsheet format just like XLS, only newer. You upgrade the file without giving anything up.

That makes XLS to XLSX one of the friendliest conversions there is: there is no fidelity trade-off to weigh, no quality slider to tune, and for an ordinary workbook the two files are indistinguishable in daily use. What actually changes is invisible to you but important to every program that touches the file: the way the spreadsheet is stored on disk moves from a closed, fragile binary to an open, resilient, standardized package. The rest of this guide explains what that means under the hood, what concrete benefits you gain, and the handful of edge cases worth a glance before you retire the old file.

Binary XLS versus modern XLSX

The two formats store the same kind of thing but in very different ways. XLS is a closed binary blob. XLSX is really a zipped bundle of XML files following the open OOXML standard, which makes it more transparent, more compact, and easier for other software to read reliably.

Because XLSX is the current default, it also lifts old limits. The classic XLS row and column ceiling, for example, is far higher in XLSX, so large data sets that would not fit in the old format have room to breathe. Moving to XLSX is the modern, expected home for an Excel workbook.

The difference is easy to see for yourself. An XLS file opened in a text editor is a wall of unreadable binary; an XLSX renamed to end in .zip opens to reveal a tidy folder of plain XML files you could read line by line. That transparency is not just a curiosity: it is why so many tools, from server-side report generators to open-source spreadsheet libraries, can produce and consume XLSX reliably while few ever fully supported the old binary. When you upgrade a file, you are joining the format that the whole ecosystem now speaks fluently.

Inside XLS: the BIFF binary and its streams

The old XLS format is not one flat file but a tiny structured filesystem. It uses Microsoft's Compound File Binary format, an OLE structured-storage container that holds named streams the way a folder holds files. The workbook data lives in a stream encoded with BIFF (Binary Interchange File Format); the version used by Excel 97 through 2003 is BIFF8.^[1] A BIFF stream is a sequence of typed binary records, each with a numeric type code and a length, describing everything from cell values and formulas to fonts, column widths, and defined names.^[2]

This design was efficient for the machines of its era but has real drawbacks. Because everything is packed into opaque binary records, a single corrupted byte can make a whole workbook unreadable, and any program that wants to read XLS must implement the full record catalog exactly. Formulas are stored in a compact tokenized form rather than as text, and strings are pooled in a shared table to save space. It works, but it is closed, fragile, and hard for non-Microsoft tools to parse reliably, which is precisely the problem the modern format was designed to solve.

Inside XLSX: a zip of XML parts

An XLSX file looks like a single document but is actually a ZIP archive of plain-text XML parts, following the Office Open XML standard published as ECMA-376 and later ISO/IEC 29500.^[3] If you rename an XLSX to end in .zip and open it, you find a predictable folder structure: a content-types map at the root, a xl/ folder holding workbook.xml for the workbook-level settings, a xl/worksheets/ folder with one XML file per sheet, a shared-strings part that pools repeated text, and a xl/styles.xml part describing every format.^[4] Relationships between parts are recorded in .rels files so the pieces know how they connect.

This open, text-based, zipped design is why XLSX is simultaneously more transparent, more robust, and usually smaller than the binary it replaced. The XML is human-readable and standardized, so any conforming tool can read it; the ZIP compression shrinks the verbose XML dramatically; and because the workbook is split into separate parts, damage to one part need not destroy the whole file.

How the format changed, and why

The move from XLS to XLSX was not a quiet version bump; it was a deliberate shift in philosophy. The binary XLS format served Excel from the 1980s through the 2003 release, and by then its closed, record-based design had become a liability. Interoperability suffered because only Microsoft's own code fully understood every BIFF record, corruption was hard to recover from, and the format was difficult for the growing world of web services and other applications to read and write.

With Office 2007, Microsoft introduced the Office Open XML formats, and standardized them through Ecma International as ECMA-376 in 2006, with a later edition adopted by the International Organization for Standardization as ISO/IEC 29500.^[3] Publishing the specification openly meant that any vendor could implement full read and write support, which is why spreadsheets now move so freely between Excel, LibreOffice, Google Sheets, and countless libraries. The XLSX variant most files use is the Transitional form, which preserves compatibility with features carried over from the old binary while still being fully XML and ZIP based.^[4] The practical upshot is that XLSX is not just newer, it is a genuinely open standard, which is the deeper reason it has become the default everywhere.

How the conversion rebuilds the workbook

Because the two formats model a spreadsheet so differently, converting is a genuine translation rather than a copy. The converter parses the BIFF records out of the XLS compound file, reconstructing an in-memory model of every sheet, cell, formula, and style. It then serializes that model into the OOXML part structure: writing each worksheet as XML, pooling text into the shared-strings part, mapping the old binary style records onto XLSX's style definitions, and translating each tokenized formula into OOXML formula syntax. Finally it zips the parts together into the single .xlsx file.

For ordinary workbooks this mapping is complete and lossless in practice: cell values, formulas, number formats, fonts, borders, and charts all have clean equivalents in the newer model. The only elements that need care are legacy features with no direct modern counterpart, most notably macros, which is why they land in a separate variant discussed below.

Convert XLS to XLSX, step by step

1. Open the converter and add your XLS

   Open the FileFormer document converter and drop your XLS in. It converts on your device, so the workbook is never uploaded.

2. Choose XLSX as the output

   Select XLSX. The old binary workbook is rewritten into the modern OOXML format.

3. Let the whole workbook carry over

   Every sheet, formula, and format comes across automatically, so there is nothing to rebuild by hand.

4. Convert and download

   Export the XLSX and open it to confirm the sheets and formulas look right. In most workbooks the two files are indistinguishable in use.

What you gain by upgrading

This is one of the few conversions that is almost pure upside. The table below shows why XLSX is the better file to keep.

Old limits XLSX lifts

Beyond being open and compact, XLSX quietly removes ceilings that constrained the binary format for years. The BIFF8 workbook was capped at 65,536 rows and 256 columns per sheet, a limit that real data sets began bumping into as spreadsheets grew. XLSX raises this dramatically, to 1,048,576 rows and 16,384 columns per sheet, so data that simply would not fit in an XLS now loads without truncation.

This matters most when an old XLS was itself a workaround for those limits, split across sheets or files to stay under the row cap. Upgrading to XLSX often lets you consolidate that data back into a single, cleaner sheet.

XLSX, XLSM, XLSB, and when to pick which

The modern format is not a single file type but a small family, and choosing the right member avoids surprises. Each variant is built on the same Office Open XML foundation but differs in one meaningful way.

For an ordinary spreadsheet, plain XLSX is the right target and the one this conversion produces. The distinction matters only in two cases. If your old XLS contained macros, saving to plain XLSX strips them by design, because the standard XLSX format is macro-free for security reasons; you would keep them by saving as XLSM instead. And if a workbook is enormous, the binary XLSB variant trades the open-XML transparency for faster loading. For the overwhelming majority of files, none of this applies and XLSX is simply the correct modern home.

The rare things to double-check

For ordinary spreadsheets the conversion is seamless. The only things that occasionally need a look are very old macros and some niche legacy features, since the modern equivalent handles macros in a separate XLSM variant. If your XLS contains macros you depend on, confirm they still run after the upgrade, and re-save as XLSM so they are retained rather than silently dropped. A second thing worth a glance is any external data connection or very old add-in reference, since those can point at resources that no longer exist. And if the workbook used the full old row range as a deliberate design, take advantage of the far larger XLSX grid to simplify it.

Otherwise there is little reason not to move to XLSX. If instead you need to hand the raw numbers to a database or script rather than keep a workbook, converting XLS to CSV strips it down to plain data, but that loses formulas and formatting, so it is a different job entirely.

One habit is worth adopting once you have converted: treat the XLSX as the new master and stop saving back to the old binary. Some programs will happily let you save a file as XLS again for backward compatibility, but doing so reintroduces the very row limits, fragility, and closed structure you just escaped, and it can quietly drop features that only the modern format supports. Keep the original XLS as a safety copy if you like, but edit and share the XLSX from now on. That way every future save benefits from the open format instead of falling back to the one you upgraded away from.