Convert XLS to XLSX Online

Why the Excel 97-2003 Limit Is Part of the Source

XLS is the binary workbook format associated with Excel 97, 2000, 2002, and 2003. Microsoft identifies its workbook format as BIFF8, a sequence of binary records carried in a Compound File Binary container. Those records can describe worksheets, cells, formulas, formats, charts, names, and other workbook features. The format's grid is a hard historical boundary: each XLS worksheet has at most 65,536 rows and 256 columns. That boundary is not merely an old screen size. Microsoft says data outside it is not saved when a workbook is written in the Excel 97-2003 format, and formulas referring to that unsaved region can return #REF!.

An XLS file often exists because an older line-of-business program exported it, a workbook was kept in compatibility mode, or an archive pre-dates Excel 2007. Converting it to XLSX creates a modern Excel workbook, but it cannot recover rows, columns, formulas, or links that were already omitted or damaged in the legacy file. The right first check is the source itself: count worksheets, check used ranges, look for hidden tabs, named ranges, external links, and formulas producing important totals. Keep the untouched XLS file during the review. That makes XLSX a new working copy rather than an irreversible replacement of the evidence of what the old system actually supplied.

BIFF8 Is Converted Into a Linked ZIP Package

XLSX uses SpreadsheetML, the spreadsheet part of Office Open XML. Microsoft describes an XLSX workbook as a ZIP package that follows Open Packaging Conventions and contains separate parts for its content and structure. At the minimum, a workbook part lists its sheets and each worksheet has its own XML part. The workbook relationship file connects sheet entries to those worksheet parts rather than relying on one binary stream of BIFF8 records. A shared-string table can store one occurrence of each unique string used across the workbook, while a formula cell can carry formula text in an <f> element. This structure is why simply changing .xls to .xlsx does not work: the bytes must be read and written into a new package with valid XML and relationships.

The modern grid is much larger. Current Excel documentation specifies 1,048,576 rows and 16,384 columns for an XLSX worksheet, compared with 65,536 by 256 for XLS. The larger destination lets later work grow beyond the legacy boundary, but it does not add missing history to an old file. It also does not turn cached formula results into verified calculations. After conversion, compare source and target sheet counts, formulas, named ranges, data validation, chart source ranges, and key totals before adding new data. Then make one harmless input change and confirm a dependent formula updates in XLSX. That test distinguishes a workbook whose formula logic survived from one that merely displays the old answer.

Macros require a separate decision. Microsoft says the macro-enabled workbook format is .xlsm, while XLSX preserves worksheet and chart data and features available in earlier Excel versions. If the XLS source includes VBA code or macro sheets, an ordinary XLSX target is not a safe way to retain those macros. Identify macros before selecting the target; choose XLSM where macro retention is needed and where the receiving environment permits it. Do not solve a macro warning by hiding it in a file with the wrong extension. A workbook can look normal until a button or event-driven calculation is needed, which is exactly when an untested conversion becomes a business error.

Five Practical Changes After Moving to XLSX

• A larger future grid: XLSX supports 1,048,576 rows and 16,384 columns per worksheet, but the converted file still contains only data that existed in the 65,536-by-256 XLS source.
• Separate package parts: sheets, strings, styles, and relationships can be stored as linked parts in an Office Open XML ZIP package rather than one BIFF8 binary record stream.
• Better feature headroom: current Excel features can be used after conversion, yet a workbook intentionally kept in XLS compatibility mode may have old design limits that need revisiting.
• Formula review remains essential: formulas can be preserved, but external links, unsupported legacy behavior, and missing source workbooks can still leave stale or broken results.
• Macros need XLSM, not XLSX: a macro-bearing legacy workbook needs a macro-enabled target and a deliberate security review; XLSX is not a macro container.

Which Excel Versions Make XLSX the Native Choice

XLSX became the standard workbook format with Excel 2007. Microsoft describes the Compatibility Checker in Excel for Microsoft 365, Excel 2024, Excel 2021, Excel 2019, and Excel 2016 as the tool for finding features or information unavailable in earlier versions. In the reverse direction, Excel opens an XLS workbook in Compatibility Mode because the old format cannot represent every modern feature. Once the workbook no longer needs to travel back to Excel 97-2003, converting it to the current format removes the need to keep designing within the legacy grid. This is a compatibility decision, not just a file-size preference. Confirm the oldest real recipient before deciding that an XLSX-only delivery is safe.

LibreOffice Calc can open and save XLSX, but its handling of any Excel-specific feature should be tested in the actual recipient application. The same warning applies to online spreadsheet services and third-party import tools: they may accept an XLSX upload while not implementing every chart, data connection, macro, or advanced rule in the workbook. Use Excel's Compatibility Checker while the source is still XLS to identify legacy limits, then use the destination program to test the features the recipient will edit. For a plain data table, compare rows, columns, and types. For a working workbook, compare formulas, validation, links, charts, protected sheets, and print settings as separate categories.

Failures a Bigger Grid Cannot Undo

A familiar failure occurs when an XLS workbook has formulas that appear correct until a linked source is moved or unavailable. The legacy workbook can retain the last calculated value, so the converted XLSX may display a plausible result without being able to refresh it. Find external links and test an input whose result depends on each important linked range. If the source workbook is missing, preserve the displayed result as a documented snapshot or repair the link in a controlled copy; do not call the result live. Another failure is a formula already using a range that had been clipped to the XLS grid. XLSX has room for later rows, but formulas cannot infer what the old file never held. Compare original source exports or databases before assuming the old workbook was complete.

A second failure is an incorrect target for VBA. Some old XLS files contain code even when no macro button is visible on the first sheet. Saving that workbook as XLSX can remove the executable macro project, leaving a quiet-looking file that later fails during an event, print step, or update. Check the source for macro content and choose XLSM only when the real recipient needs the code and accepts macro-enabled files. For formula-only workbooks, test a date, lookup, cross-sheet reference, a long text cell, and a named range after conversion. The problem is usually specific to one feature, so a focused test is faster and more useful than reopening the file and assuming the conversion succeeded because the first tab looks familiar.

Capacity and Feature Checks Before and After

Questions About Retiring an XLS Workbook

Will XLS to XLSX restore rows beyond the old limit?
No. XLSX has a much larger grid, but it cannot restore data that was never saved in the XLS source. Check old exports or the source system if the legacy workbook may have been truncated.

Why is changing the extension not enough?
XLS is a BIFF8 binary workbook and XLSX is an Office Open XML ZIP package with linked parts. A converter must build a valid new package; renaming leaves the old bytes in place.

Do formulas automatically remain correct?
They can be preserved, but values that depend on missing external workbooks, damaged legacy ranges, or unsupported behavior need testing. Change a safe input and verify each important dependent result in the XLSX file.

Can an XLSX file keep VBA macros?
No. Choose the macro-enabled XLSM format when macros must be retained. Inspect the XLS source first, because macro failures may not appear until someone uses a button or triggers an event.

What should be compared before accepting the result?
Compare sheet count, used ranges, hidden sheets, names, formulas, key totals, external links, validation, charts, and print settings. Keep the source XLS unchanged until the intended receiving application has opened and tested the XLSX copy.