XLSX → XLS writes a BIFF8 workbook for Excel 2003-era software, with the row and column ceilings that format imposes.
From real runs
| Element | Result | What happens |
|---|---|---|
| Worksheets | Kept | Both sheets of the sample, Pipeline and Notes, were present with their names after reopening the XLS. |
| Cell values | Kept | Company names, stages and owners came back as text, and the arr column (48000, 120000, 22500, 64000) stayed numeric. |
| Dates | Kept | The close_date cells were text in the source workbook and remained text; real date serials are also written when the source has them. |
| Row and column limits | Changed | XLS holds 65,536 rows and 256 columns per sheet; anything beyond is dropped and the warning states the limit on every conversion. |
| Column widths and styles | Changed | Widths were not carried for the sample; the reopened workbook reported default column dimensions. |
| Formulas | Kept | Formulas within the Excel 2003 function set are written as formulas; newer functions such as XLOOKUP or LET are stored as values. |
| File size | Changed | The 17,759-byte XLSX became a 6,656-byte XLS; the binary format has no XML overhead for a small workbook. |
Both sheets of the sample, Pipeline and Notes, were present with their names after reopening the XLS.
Company names, stages and owners came back as text, and the arr column (48000, 120000, 22500, 64000) stayed numeric.
The close_date cells were text in the source workbook and remained text; real date serials are also written when the source has them.
XLS holds 65,536 rows and 256 columns per sheet; anything beyond is dropped and the warning states the limit on every conversion.
Widths were not carried for the sample; the reopened workbook reported default column dimensions.
Formulas within the Excel 2003 function set are written as formulas; newer functions such as XLOOKUP or LET are stored as values.
The 17,759-byte XLSX became a 6,656-byte XLS; the binary format has no XML overhead for a small workbook.
We converted our Excel XLSX sample (17 KB) with Office engine on 2026-09-11. It took 606 ms and produced a 7 KB Excel XLS file, 63% smaller than the input. The engine raised 1 warning, listed below.
Input · sample.xlsx · 17 KB
Workbook with 2 sheets (Pipeline, Notes); first sheet 5 rows, 25 cells, 0 formulas
Output · sample.xls · 7 KB
OLE2 compound document (legacy Office binary)
LibreOffice headless
soffice --headless --convert-to xls in.xlsx --outdir outSheets over 65,536 rows are truncated silently; check row counts afterwards.
The HTTP API and the MCP server run the exact engine this page uses (Office engine); the response carries the same warnings the web converter shows. Check support first with GET /api/v1/pairs?from=xlsx&to=xls.
curl -X POST https://filetypeconverters.com/api/v1/convert \
-H "Authorization: Bearer $FTC_API_KEY" \
-F "file=@input.xlsx" -F "from=xlsx" -F "to=xls" \
--output output.xlsRows past 65,536 and columns past 256 on any sheet, functions introduced after Excel 2003, and some conditional formats. Values, sheet names and classic formulas survive.
Yes; the file is written in the Excel 97-2003 binary format, the same BIFF8 container those versions save themselves.
XLSX is a ZIP of XML parts with fixed overhead, while BIFF8 stores small workbooks as compact binary records: 6,656 bytes against 17,759 for the sample.