You converted your .xls file to .xlsx. The file opens, the data looks right — but formulas are returning errors, calculating wrong values, or behaving differently than before. This guide covers the most common causes and the fix for each.
If your workbook references cells in other files using paths like ='C:\Reports\budget.xls'!Sheet1!A1, and the source file was also renamed or converted, those links are now broken.
Fix: Go to Data > Edit Links. Any links showing "Error: Source not found" need to be updated. Click "Change Source" and point to the new file location or format (.xlsx instead of .xls).
Named ranges survive the conversion, but if your workbook had sheet names with special characters or if sheets were reordered during the Save As process, named ranges may point to unexpected locations.
Fix: Go to Formulas > Name Manager. Check each named range's reference — particularly any that show a #REF! in the "Refers To" column. Update the reference to the correct cell or range.
Excel 2003 and earlier included functions like OFFSET, INDIRECT, and certain text functions with edge-case behaviors that were quietly changed in later Excel versions. The .xls to .xlsx conversion doesn't fix formula logic — it just changes the container.
Fix: Compare key formula outputs between the original .xls (opened in compatibility mode) and the new .xlsx. For any that differ, check Microsoft's documentation for that function — look for notes about behavior changes between Excel 2003 and later versions.
Legacy array formulas entered with Ctrl+Shift+Enter are preserved in .xlsx format, but they display the curly braces {} in the formula bar. These are valid. However, if any array formula was accidentally converted to a dynamic array formula (new Excel 365 feature) during the process, the output may differ.
Fix: Check formulas wrapped in {}. If the output is wrong, re-enter with Ctrl+Shift+Enter to ensure it's treated as a legacy array formula.
Excel has two date systems: the 1900 date system (default on Windows) and the 1904 date system (legacy Mac default). .xls files created on older Mac Excel may use the 1904 system. When converted on Windows, dates can shift by exactly 1462 days (4 years and 1 day).
Fix: If dates look wrong by exactly 4 years, go to File > Options > Advanced, scroll to "When calculating this workbook," and toggle "Use 1904 date system" to match the original file's setting.
Some older .xls files relied on iterative calculation to resolve circular references silently. The default setting in newer Excel versions is to show a warning rather than calculate silently.
Fix: If you need iterative calculation: File > Options > Formulas > Enable iterative calculation. Set maximum iterations and maximum change to match what the original workbook expected.
Very rarely, floating-point precision in calculations can differ slightly between the old binary .xls engine and the new Open XML engine. This typically only shows up in scientific or engineering workbooks doing many sequential floating-point operations.
Fix: Use ROUND() to explicitly control precision for critical calculations. Or compare outputs to the original and determine if the difference is within acceptable tolerance.
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