Video to HD, before you convert the clip again
Use this when a clip should be scaled up to 720p for a player that looks soft on a small file.
The picture is scaled to a height of 720 pixels and re-encoded. It is not a 1080p or 4K master.
What this cannot fix
Scaling does not invent detail. A low-resolution clip stays soft, and the file can get larger. A straight MP4 conversion is faster when size is already fine.
Why Video to HD is its own job
Video to HD sits on this site because Video and HD are not interchangeable, even when they look similar in a file list. Use this when a clip should be scaled up to 720p for a player that looks soft on a small file. Trim, compress, or resize a clip before you send it, or when a player rejects the container. People run an HD tool when a slot asks for a large image and they only have a small one.
The picture is scaled to a height of 720 pixels and re-encoded. It is not a 1080p or 4K master.
The Video file you already have
Video conversion runs on your device with a converter library. The first run downloads that library, so it is slower than an image tool. Long, high-resolution clips run out of memory on phones. A re-encode cannot look cleaner than the source, and extra subtitle tracks are not a feature of these tools.
Keep the original clip.
What a HD result is for
The download has to be HD, because that is the format the next step named. People run an HD tool when a slot asks for a large image and they only have a small one. The picture can be scaled so the long side meets a target such as 1920 pixels on image-to-HD tools.
Video-to-HD scales toward 720p height, not a true 1080p master. Read the tool page before you rely on the word HD.
How to tell that Video to HD worked
Video to HD is worth using only when HD is what the next step actually named. Use this when a clip should be scaled up to 720p for a player that looks soft on a small file. What you get from Video to HD: The picture is scaled to a height of 720 pixels and re-encoded. It is not a 1080p or 4K master. Before you trust it: Scaling does not invent detail. A low-resolution clip stays soft, and the file can get larger. A straight MP4 conversion is faster when size is already fine. 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. A second pass rarely helps. If the first HD file is rejected, read the error from the program that rejected it. Another conversion of a bad result usually makes the file worse, not more compatible. When you are done, you should still have the Video original and one HD download that you have opened. If those two are the same kind of failure, this was the wrong tool and the page above is the place that says so. Video files hide the real problem inside the container. Video to HD 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 Video to HD. You start with Video and you stop when a real program accepts HD. The work stays on your computer or phone. Stay on the tab until it finishes. Stretching does not recover detail. A blurry photo becomes a larger blur. 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.
Scaling does not invent detail. A low-resolution clip stays soft, and the file can get larger. A straight MP4 conversion is faster when size is already fine.
A related step, if you still need one, is Video Compressor.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.