This PPTX to MP4 conversion does not run here
A slide deck is sometimes easier to post as a video than as a PPTX.
This browser cannot render slides into a video.
What to do instead
Use PowerPoint's Export to Video, or record the slideshow. If people only need to read the slides, use PPT to PDF.
The page that does work is ppt-to-pdf.
When PPT to Video is the right page
PPT to Video sits on this site because PPTX and MP4 are not interchangeable, even when they look similar in a file list. A slide deck is sometimes easier to post as a video than as a PPTX. Decks are shared as PPTX when the other person will present, and as PDF when they only need to read. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4.
This browser cannot render slides into a video.
What you give up by leaving PPTX
PPTX is a modern PowerPoint file. The design lives in the file. A text export only keeps what was written on the slides. Themes, animations, masters, and embedded video are not cloned by a browser tool.
Keep the PPTX. A PDF or Word file of the words is not the deck.
What you gain, and do not gain, with MP4
The download has to be MP4, because that is the format the next step named. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4. If the other person can open only one video format, MP4 is that format.
A new encode cannot look cleaner than the MP4 you started with. A remux on a desktop is the lossless path when the streams are already compatible.
A test that matches the real file
Start from the question the other software asked. If it asked for MP4, PPT to Video is the matching page. If it asked for something else, switch tools instead of hoping a similar extension will pass. A slide deck is sometimes easier to post as a video than as a PPTX. This browser cannot render slides into a video. The limit is the part people skip, and it is the part that decides whether the download is usable. Use PowerPoint's Export to Video, or record the slideshow. If people only need to read the slides, use PPT to PDF. The file stays on your device. The first run downloads a converter, so keep this tab open. A phone runs out of memory on long files. A computer is the safer place for a big clip. Check three things in the download: it opens, the PPTX 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 PPTX as well when you are allowed to. PPT to Video is a convenience copy. It is not a new master. Video files hide the real problem inside the container. PPT to Video re-encodes on your device. It does not silently copy every audio language and every subtitle the way a desktop remux can. If you needed those extra tracks, this download is the wrong result even when the picture plays. Convert a one-minute piece before a full movie, and stay on the tab while the converter downloads the first time.
Here is the decision for PPT to Video. You start with PPTX and you stop when a real program accepts MP4. Slide text can be pulled out for a handout or a transcript. Soft subtitles such as ASS or PGS, and extra audio languages, are awkward or dropped when a tool re-encodes. 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.
Use PowerPoint's Export to Video, or record the slideshow. If people only need to read the slides, use PPT to PDF.
A related step, if you still need one, is MP4 to MOV.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.