The file I converted came out larger than the original, is that normal?

It can happen, and it usually relates to the bitrate chosen. WMV was an efficient codec in its day; if a bitrate higher than the source genuinely needs is used during re-encoding, the resulting file can come out larger. That does not make the picture better quality, because the detail in the source is already limited and excess bitrate does not bring back detail that is gone. Trying again with a more reasonable value usually gives the same visual result in a smaller file.

The process will not start, or errors out immediately, what could be causing it?

The most common cause is the file being DRM protected; in the Windows Media era that protection was very common on corporate and purchased content, and protected files cannot be converted. If your file asks for a licence when it opens, or only plays on one particular computer, this is most likely your situation. The second possibility is the file being partially corrupted; that is seen in files stored for a long time or in downloads that were left unfinished. Opening the source in a player first and verifying it from start to finish lets you tell these two cases apart.

There is audio in the result but it comes out muffled, what should I do?

WMV files carry WMA audio, and because MP4 does not accept it the audio has to be encoded to AAC. This is a second generation compression, and if the source audio's bitrate was already low the result can degrade noticeably. Trying again with a higher bitrate for the audio usually helps. But whatever the source audio's quality is, that is the ceiling, and encoding settings cannot raise that ceiling.

I have dozens of WMV files, should I convert them all at once?

It is safer to test with one first and examine the result carefully. Files from the same source have usually been saved with the same settings, so an approach that gives a good result on one will work on the others. Once you have found the right settings, moving on to batch processing is far better than converting fifty files and then realising all of them need doing again. And in the meantime, do not delete the originals.

Converting a WMV File to MP4: Step by Step

Setting expectations correctly from the start

There is a single technical fact you need to know before starting a WMV conversion, and it shapes the whole job.

The MP4 container does not carry the WMV3 and VC-1 video codecs found inside WMV files. On the audio side it does not accept WMA either. That means the 'just change the container' convenience frequently available with MKV or MOV files is not possible at all here.

Every move from WMV to MP4 is a genuine re-encode: the picture is decoded and compressed from scratch with H.264, and the audio is converted from WMA to AAC. That has two consequences — a certain amount of quality loss occurs and the operation takes time. On a long file it can run for minutes.

Starting out knowing this prevents needless disappointment when you come to judge the result.

What to do before converting

Check whether the file is DRM protected. This is the obstacle specific to WMV that most often ends the job before it starts. In the Windows Media era, corporate training videos, purchased films and subscription content were frequently distributed with licence-based protection. If a licence is requested when you open the file, or it only plays on one particular computer, it is protected and cannot be converted with any tool. Test this as your first step so you do not waste time.

Play the source from start to finish. WMV files stored for a long time may have partially degraded. Jump to the middle and to the end and confirm it plays without trouble. A problem in the source is not fixed by conversion, it is carried straight across.

Decide now whether you are keeping the originals. Because the operation is lossy, do not delete the source file. With recordings of archival value in particular this is an irreversible decision.

Look at the file's resolution. Old WMV recordings are mostly low resolution. That information gives you an idea of how much bitrate you need to allocate in the conversion — allocating a high bitrate to a low resolution source only makes the file bigger, it does not improve the picture.

Converting step by step

1. Upload the file

Open our WMV to MP4 conversion tool and drag and drop the file. Do not close the tab while it uploads.

2. Wait for the file to be read

At this stage the video and audio codecs inside are detected. If you see WMV3 or VC-1, you are in the expected situation.

3. Start the operation

Genuine compression is being performed here, so it takes noticeably longer than a container change. The progress bar may move irregularly; scenes with movement and detail are processed more slowly than static ones. Do not interrupt the operation.

4. Download it and compare with the source

Open the new file side by side with the original at the same scene.

Verifying the result

Because you have performed a lossy conversion, the checklist needs a little more care:

Is the duration the same as the original's? If there is a difference, the operation may have completed incompletely.
Is there blocking in the picture? Look at scenes with movement and detail: crowds, fast camera movement, fine patterns. Do not decide by looking at a static presentation slide.
Is the audio clear? The move from WMA to AAC is a second generation compression; listen for muffling, especially in speech-heavy recordings.
Are audio and picture in sync? Do not jump to the start, jump to the middle and the end.
Does it open on the device you actually targeted? That was the whole point of the conversion — test it on the Mac, the phone or the television.

Common problems

The operation does not start at all. The first suspect is DRM protection. The second suspect is a corrupted source file. The third is it being a very old WMV version — Windows Media's early era codecs may not be recognised by some tools.

The picture came out noticeably degraded. The bitrate may have been too low. Try again from the same source with a higher value. Do not perform a second operation on the already converted MP4 — the loss stacks up and cannot be undone.

The picture is stretched or the proportions are wrong. Old WMV files are sometimes recorded with non-square pixel ratios. If that information is not interpreted correctly the result comes out flattened or squashed. You can spot it by comparing with the original.

Audio and picture have drifted apart. Some WMV recordings, screen recordings especially, were produced at a variable frame rate. With sources of that kind, converting to a constant frame rate gives a more reliable result.

Can I just change the extension by hand? No. WMV files use a container called ASF, which is completely different from MP4. Turning .wmv into .mp4 does not make the file an MP4, it only causes it not to open.

A practical method for archive work

If you have a large number of WMVs, this order will make your job easier: first look through all of them quickly in a player and set aside the DRM protected ones, because there is nothing that can be done for those. Run a test with one of the rest, examine the result on detailed scenes and settle the settings. Only then move on to batch processing.

And again: do not delete the originals. Your WMV files are probably old recordings that cannot be produced again, and conversion is a lossy operation.

Conclusion

Moving from WMV to MP4 involves an unavoidable trade: you give up a certain amount of quality and in return you get a file that opens on every device. Because the quality of a file that will not open is in practice of no use at all, that trade makes sense in most scenarios.

The only thing you need to do is carry the process out deliberately: set the protected files aside in advance, check the source, examine the result on detailed scenes and keep the originals.