XLSX is an Excel workbook with several sheets, formulas and cell formats. HTML is the markup of the web, with native headings and tables. LibreOffice sits between the two, and this page says exactly what happens to an XLSX file on the way to becoming an HTML one.
What runs when an XLSX file becomes HTML
LibreOffice 24.2, on a machine we rent and watch. Your XLSX file is uploaded once, LibreOffice runs once, the HTML comes back, and neither file is kept. XLSX to HTML is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the XLSX into the HTML
The text, as text in a web page: no download needed to read it, and anything that can read HTML can read it.
Tables, written as HTML tables rather than as aligned spaces.
Cell formulas, carried into the HTML alongside their values.
What XLSX to HTML costs you
Page geometry. HTML has no pages, so margins, page breaks, headers and footers have nowhere to go. This pair turns a document into a web page, not into a picture of one.
Fonts, unless the reader happens to have them. The HTML names them and the browser substitutes what it has.
Where XLSX and HTML files come from
XLSX. Excel writes it, and so does nearly everything with an export to Excel button: Google Sheets, Salesforce, a bank statement download. It holds 1,048,576 rows and 16,384 columns, which is why it replaced the older binary.
If a model is going to read the HTML
Structure is what survives from the XLSX file, and structure is what a model needs. XLSX in, HTML out, with the heading tree intact instead of flattened into one long paragraph.
XLSX to HTML, measured rather than promised
Running XLSX to HTML against the real engine with a real file, LibreOffice wrote 2,269 bytes of HTML in 1.1 seconds, machine otherwise idle. That is one XLSX file on one day and not an average, which is why the number is given together with the file that produced it.
The XLSX to HTML verdict was reached by reading the internal type and finding a witness word, meaning the output was read back and recognised. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the HTML file has anything inside it.
The same XLSX file, sent somewhere else
Other outputs the probe measured out of an XLSX file, so the cost of choosing HTML can be read against something. Sizes do not compare across engines, because each family was probed with its own XLSX witness file.
XLSX to PDF: 18,965 bytes in 1.1 seconds, verified by a format signature.
Other ways into HTML, and what they measured
Among the published routes into HTML, XLSX is the ninth largest output of the 12 measured. The witness files differ, so this ranks the probe run and not your document.
PPTX to HTML: 14,373 bytes in 1.5 seconds.
XLS to HTML: 2,101 bytes in 1.0 seconds.
CSV to HTML: 3,894 bytes in 0.1 seconds.
JSON to HTML: 4,018 bytes in 0.1 seconds.
ODP to HTML: 12,295 bytes in 1.6 seconds.
What we will not pretend about XLSX to HTML
An XLSX file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An XLSX to HTML run that passes 90 seconds is killed, and the LibreOffice process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
LibreOffice is serialised on purpose, one document at a time per worker, because each soffice instance takes 150 to 300 MB on a machine with 3.8 GB. Measured ceiling: 23 XLSX conversions in flight and roughly 0.9 a second sustained. Past that the queue answers 429 rather than falling over, which was tested at 32 at once.
Two thirds of the pairs you could imagine in this family do not exist, and it is worth knowing why before you go looking for one. LibreOffice converts inside whichever application opened your file: from a spreadsheet it writes spreadsheets, flat text, HTML and PDF, and it will not write a .docx no matter how it is asked. It returns nothing at all rather than something wrong. Of 113 combinations probed on this engine, 64 came back with a real file and the rest were refused outright.
This pair is one of the 64. It was proved by sending a real XLSX file through the live route and reading the HTML that came back: not its extension, its internals, plus a marker word planted in the source and found again in the output. The traits named further up this page come from a second probe that looks for each of them one by one.
One XLSX file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
XLSX to HTML: what people ask
What actually converts my XLSX file to HTML?
LibreOffice does it, 24.2, on a machine we rent and watch. Not a browser trick and not somebody else service: the XLSX file is uploaded once, LibreOffice runs once, the HTML comes back, and neither file is kept afterwards.
How long does XLSX to HTML take?
On the file the probe used, LibreOffice took 1.1 seconds and wrote 2,269 bytes of HTML. That is one real measurement on one real XLSX file, not an average and not a promise about yours: a larger XLSX takes longer, and past 90 seconds the run is stopped.
What do I lose going from XLSX to HTML?
The one to know about first: Page geometry. HTML has no pages, so margins, page breaks, headers and footers have nowhere to go. This pair turns a document into a web page, not into a picture of one.
Are my speaker notes in the HTML?
No. What lands in the HTML is what was on the slides.
Can I open the HTML without any software?
Yes. Any browser opens it, which is the reason to pick HTML over an XLSX: no application, no plugin, no download step for whoever you send it to.
Is XLSX to HTML free?
There is a free allowance every month, and one XLSX to HTML conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse HTML.