Convert XLS to XLSX
The .xls format is Excel's binary from before 2007, and it carries two limits worth escaping: 65,536 rows per sheet and 256 columns. It is also an opaque compound file that can hold macros, which is why mail filters treat it with suspicion. Converting to .xlsx removes all three problems and keeps the data, the formulas and the formatting.
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What it does
- Row ceiling rises from 65,536 to 1,048,576
- Formulas, formatting and multiple sheets are preserved
- Macros are not carried into the plain .xlsx container
How to use XLS to XLSX
- 1
Upload the .xls
Any legacy binary Excel workbook, including ones written by very old software.
- 2
The workbook is rewritten
Sheets, formulas, number formats and cell styles are re-expressed in the newer format rather than flattened.
- 3
Check any macro-driven behavior
A plain .xlsx cannot hold macros. If the workbook depended on one, that behavior will be missing.
How it works
The binary workbook is opened and re-serialized into the newer package format. An .xlsx is a ZIP containing XML: one part per worksheet, one for shared strings, one for styles. Each sheet's cells, formulas and formats are written into that structure.
This is a re-encoding of the same workbook model rather than a rendering, so formulas remain formulas and recalculate normally afterwards.
The three reasons to leave .xls
The row ceiling. 65,536 rows and 256 columns. Software that reaches it often truncates without a warning, which is the worst possible failure mode for a dataset.
Opacity. A compound binary file cannot be inspected or repaired without Excel. An .xlsx can be unzipped and read, which matters when a file is damaged or when an archive needs to know what is in it.
Macros. The legacy format's long association with macro-borne malware is why so many mail systems quarantine it. A plain .xlsx cannot execute anything.
What to do if the macro matters
Convert to .xlsm rather than .xlsx if the workbook's behavior depends on VBA. This conversion produces the plain format, which is the right default: most old workbooks that carry a macro no longer need it, and dropping it is a feature rather than a loss.
After converting
Open the result once and recalculate. Formulas referencing external workbooks by an old path are the usual thing to fix, and they are easier to spot immediately than months later.
Two ceilings lift at once
The row limit is the one people notice: 65,536 in .xls, just over a million in
.xlsx. Columns move too, from 256 to more than sixteen thousand, which matters
for anything generated by a report writer that produces a column per day or per
product.
The less visible change is what the file is. An .xls is a proprietary binary
that essentially requires Excel or a compatible reader to interpret. An .xlsx
is a ZIP archive of XML, so it can be unpacked, read, diffed and generated by
ordinary tooling with no spreadsheet software involved at all.
That is why converting is worth doing even for small files. It moves the data out of a format only one application really understands.
Examples
An old departmental workbook
Four times smaller because .xlsx is a zipped set of XML parts rather than a binary layout. All four sheets and their formulas came across, and the sheet that was near the old row ceiling now has room.
A workbook with an attached macro
The plain .xlsx container has nowhere to store VBA. If the macro is needed, .xlsm is the format that keeps it; if it is not, this conversion is a clean way to drop it.
Frequently asked questions
Will I lose anything converting XLS to XLSX?
Data, formulas, formatting and sheet structure all transfer. What does not is macro code, because a plain .xlsx cannot contain it. A handful of very old Excel features have no modern equivalent and may be simplified. In practice a workbook that is data and formulas converts without visible change, and one built around VBA needs .xlsm instead.
Why is the XLSX so much smaller?
Because .xlsx is a ZIP archive of XML parts and .xls is an uncompressed binary layout. Spreadsheet content compresses extremely well, since it is mostly repeated structure and text, so a reduction of three or four times is typical. Nothing is discarded to achieve it; the same content is simply stored more efficiently.
Does the row limit really matter?
It does the moment a dataset grows. A legacy .xls sheet stops at 65,536 rows and 256 columns, and software that hits the ceiling usually truncates silently rather than refusing. If a workbook has ever been near that figure it is worth converting purely as insurance. The newer format allows 1,048,576 rows and 16,384 columns.