XLSX to XLS moves values, not the whole workbook
An accounting tool from before 2007 still demands .xls.
The sheet is rewritten as the older Excel format in the browser. Very large sheets can hit the old row limit and drop data.
Dates and extra sheets after XLSX to XLS
Send XLSX if the other side allows it. Open the .xls and confirm the last row is still there before you delete the XLSX.
A related step, if you still need one, is XLS to XLSX.
When XLSX to XLS is the right page
XLSX to XLS sits on this site because XLSX and XLS are not interchangeable, even when they look similar in a file list. An accounting tool from before 2007 still demands .xls. Spreadsheets, charts, and CSV exports start here for most offices in 2026. You only need XLS when a named program refuses XLSX.
The sheet is rewritten as the older Excel format in the browser. Very large sheets can hit the old row limit and drop data.
What you give up by leaving XLSX
XLSX is the current Excel workbook. It can hold more than one sheet, plus formulas and formatting. A conversion aimed at CSV or JSON flattens that richness. Charts and macros are not the point.
Keep the workbook you calculated in. An export is a copy of values.
What you gain, and do not gain, with XLS
The download has to be XLS, because that is the format the next step named. You only need XLS when a named program refuses XLSX. A simple sheet of values can move into the old format.
Open the XLS and confirm the last row before you delete the XLSX.
A test that matches the real file
Start from the question the other software asked. If it asked for XLS, XLSX to XLS is the matching page. If it asked for something else, switch tools instead of hoping a similar extension will pass. An accounting tool from before 2007 still demands .xls. The sheet is rewritten as the older Excel format in the browser. Very large sheets can hit the old row limit and drop data. The limit is the part people skip, and it is the part that decides whether the download is usable. Send XLSX if the other side allows it. Open the .xls and confirm the last row is still there before you delete the XLSX. The file stays in your browser and is not uploaded. The limit is 100 MB. Keep the original until the download has been opened in the program that will use it. Check three things in the download: it opens, the XLSX material you cared about is still there, and the file size or the layout is what the form described. A new extension alone is not those three things. If a portal, a teacher, or a client rejects the file, send them the original XLSX as well when you are allowed to. XLSX to XLS is a convenience copy. It is not a new master. Spreadsheets break at quiet places: a second sheet, a date that changed, a leading zero that vanished, a comma inside a name. After XLSX to XLS, open the result in a text editor as well as Excel. Excel guesses types and hides the delimiter. The workbook you calculated in is still the master.
Here is the decision for XLSX to XLS. You start with XLSX and you stop when a real program accepts XLS. Values, dates, and several sheets can travel when the tool writes a real workbook. Very large sheets can hit the old row limit and drop data. Macros and modern charts do not survive. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
Send XLSX if the other side allows it. Open the .xls and confirm the last row is still there before you delete the XLSX.
A related step, if you still need one, is XLS to XLSX.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.