Show the Working

HTM to HTML

These are the same file. Your page never leaves your browser — the renaming happens in it, on your own device — and there is no conversion to do, which this page does not pretend otherwise about: it hands yours back under the other name and tells you what is actually in it.

…or drop one anywhere on this page.

Why your file is called .htm

MS-DOS, and Windows up to 3.11, allowed a filename of eight characters and an extension of three. “html” is four. So every page authored on a PC in that era was saved as .htm, every web server was configured to serve it, and the habit outlasted the limit by thirty years — FrontPage defaulted to it, and plenty of software still does.

Nothing about the file is different. The same <html> tag, the same markup, the same text/html media type on the way out of a server. Apache and nginx ship with both extensions mapped to the same type as standard. A browser does not look at the extension at all — it reads what the server said, and a file opened from disk is sniffed.

So the honest answer is: rename it

What this page gives you back is your file, byte for byte, with .html on the end. Nothing is parsed, rewritten, tidied or re-encoded. If some other converter offers you a smaller HTML file than the HTM you put in, it changed your markup, and you should want to know what it changed.

You can do the same thing in your file manager, and if that is easy for you then do that instead. This exists because renaming is awkward on a phone, because Windows hides extensions by default so people end up with page.htm.html, and because the question “is it really that simple?” deserves an answer with the working shown.

And it tells you how old the page is

A .htm is usually old, and what you do next depends on how old. A document built out of <frameset> will not display in any browser released this decade, because frames were removed from the language — that is a rebuild, not a rename. <font> tags, <center> and table layout still render, but nothing will lay them out responsively. The count of each is on screen after you choose a file, so you know which problem you have before you start.

If you want it as something else

Every HTML converter here already takes a .htm — they read the markup and have never cared about the extension:

Where to go next