What's the difference between a container and a codec?

A container is the file structure holding the video and audio tracks together — MP4, MKV, AVI and MOV are containers. A codec determines how the tracks are compressed — H.264, H.265, VP9 and AV1 are codecs. An MP4 file can contain H.264 or H.265 video; that's why saying 'MP4 format' is only half a description. Most compatibility problems come from the codec, not the container.

Does converting video reduce quality?

If re-encoding is happening, yes, it drops somewhat — because decoding from a lossy codec and re-encoding lossily adds a second loss. But if only the container changes (a remux) there's no loss at all; the video and audio data are carried into the new container as they are. Going from MOV to MP4 can usually be done by remuxing, and in that case quality is preserved exactly.

How much does lowering the resolution shrink the file?

Going from 1080p to 720p roughly halves the pixel count, and at the same quality target the file drops to about half. Going to 480p gives close to a quarter. But bitrate is the real determinant: if you lower the resolution and leave the bitrate the same, the file doesn't shrink — you just get a needlessly high-quality small video.

Can I turn a horizontal video into a vertical one?

Unless the conversion tool crops, saving a horizontal video at a vertical ratio just creates black bars at the sides — the picture itself doesn't become vertical. To get genuinely vertical video you have to crop the picture, and that means permanently losing the content at the edges. If you need vertical content for social media, shooting vertically in the first place always gives a better result.

How to Convert Video Formats: MP4, MOV, AVI, MKV and WEBM

Your video won't play, an editing program won't accept it, it won't fit on your phone, or it won't upload to a website. Video conversion is the solution to those problems, but it involves a few decisions that need to be made right — and those decisions determine both quality and file size.

First, the most important distinction: container and codec

This is the most common confusion in the video world and you can't make the right decision without resolving it.

The container is the file's outer structure: the envelope holding the video track, audio tracks, subtitles and metadata together. MP4, MKV, AVI, MOV and WEBM are containers. The file extension indicates the container.

The codec determines how the tracks are compressed. H.264, H.265 (HEVC), VP9, AV1 and MPEG-4 Part 2 are video codecs. AAC, MP3, Opus and AC3 are audio codecs.

An MP4 file can contain H.264 video and AAC audio. Another MP4 can contain H.265 video and AC3 audio. Both are "MP4" but their compatibility is very different.

The practical consequence: the reason "my video won't open" is usually the codec, not the container. Turning an MKV file into MP4 doesn't solve the problem if the codec inside isn't supported.

Step 1: Be clear about why you're converting

The reason determines your method:

| Problem | Likely solution | |---|---| | The file won't play | Codec change (re-encoding) | | The editing program won't accept it | Container or codec change | | The file is too big | Lowering bitrate/resolution | | It won't upload to a website | Conversion to MP4 plus H.264 | | The phone won't play it | Conversion to H.264 | | Only the extension is wrong | Remux (container change) |

That last row matters: in some cases you can move the video into a new container without re-encoding at all. That's called a remux and it causes no quality loss.

Step 2: Remux or re-encode?

This is the conversion's most critical decision.

Remux (repackaging): the video and audio data are carried into the new container untouched. Only the envelope changes.

Quality loss: zero
Time: seconds (at file copy speed)
File size: almost the same
Condition: the target container has to support the existing codecs

Re-encoding (transcode): the video is decoded and encoded from scratch with a new codec.

Quality loss: yes (unavoidable with lossy codecs)
Time: minutes or hours
File size: varies with settings
Condition: none, always possible

When is a remux enough?

MOV → MP4: usually works as a remux. Both use the same underlying structure (ISO base media file format) and H.264/AAC are supported in both.
MKV → MP4: if the codecs inside are MP4-compatible (H.264/H.265 video, AAC/AC3 audio), a remux is possible. But if the MKV contains VP9 or certain subtitle formats, re-encoding is required.
AVI → MP4: usually requires re-encoding. AVIs mostly carry older codecs (DivX, Xvid, MPEG-4 Part 2) and those don't work well in the MP4 container.
WEBM → MP4: requires re-encoding. The VP8/VP9/AV1 video and Vorbis/Opus audio inside WEBM aren't MP4's standard codec set.

The rule: prefer a remux when possible. Quality is preserved and the operation finishes very quickly.

Step 3: If re-encoding is required, choose the settings

If you're going to re-encode, there are three main settings.

Codec choice

| Codec | Efficiency | Compatibility | Use | |---|---|---|---| | H.264 (AVC) | Good | Highest | General purpose, the safe choice | | H.265 (HEVC) | Very good (~40% less data) | Medium | 4K, saving storage | | VP9 | Very good | Good on the web | YouTube, browsers | | AV1 | Best | Limited (growing) | The future, slow encoding |

If you're not sure, choose H.264. Nearly every device, every browser and every piece of software supports it. H.265 takes about 40% less space at the same quality but some older devices and browsers can't play it.

Resolution

Don't increase the source's resolution — turning a 720p video into 1080p adds no detail, it just grows the file and enlarges blurry pixels.

Lowering it, though, is an effective way to shrink:

| Resolution | Pixel ratio | Typical use | |---|---|---| | 2160p (4K) | 4× | High-quality archive, large screens | | 1080p | 1× (reference) | The general-purpose standard | | 720p | ~0.44× | Web, mobile, sharing | | 480p | ~0.2× | Small screens, low bandwidth |

Bitrate

This is the real determinant of quality and size. Rough references (for H.264):

| Resolution | Low | Medium | High | |---|---|---|---| | 480p | 0.5 Mbps | 1 Mbps | 2 Mbps | | 720p | 1.5 Mbps | 3 Mbps | 5 Mbps | | 1080p | 3 Mbps | 6 Mbps | 10 Mbps | | 4K | 15 Mbps | 25 Mbps | 45 Mbps |

Content type matters too: a static talking-head video holds up at a low bitrate, while nature footage with fast motion and detail needs much more.

A common mistake: lowering the resolution and leaving the bitrate the same. That doesn't shrink the file — it just produces a needlessly high-quality small video. Adjust both together.

Step 4: Convert

Open the video conversion tool, upload your file, and choose the target format and settings. The processing runs in your browser; your video doesn't go to a server.

Video conversion is a far heavier operation than audio conversion. Expect it to take a while on long videos and don't close the tab.

Step 5: Check the result

After downloading, look at these:

Audio-video sync. This is the most common problem. Watch the video at a few points (start, middle, end). If there's drift it usually stems from the source file using variable frame rate (VFR); if there's a setting that forces constant frame rate (CFR), use it.

Is there an audio track? In some conversions the audio track can be dropped if the audio codec is incompatible with the target container.

Multiple audio tracks. MKV files can carry several audio tracks (different languages). When converting to MP4, usually only one is preserved or you have to choose.

Subtitles. Embedded subtitles in an MKV don't always transfer to MP4. If subtitles matter, you may need to keep them as a separate SRT file or burn them into the video.

Picture quality. Is there block degradation (macroblocking) in moving scenes? If so the bitrate is insufficient.

Duration. Is the resulting video the same length as the source?

Common mistakes

Changing the extension by hand. Renaming a .mkv file to .mp4 isn't a conversion. It sometimes works by coincidence (if the codecs inside are compatible) but usually corrupts the file.

Chained conversion. Every re-encode adds loss. Go from the original source to your target in one step.

Deleting the source. You usually notice you picked the wrong settings afterward.

Converting needlessly. If the video already works and the target device supports it, leave it alone.

Upscaling the resolution. It adds no detail, it just grows the file.

In summary

The first question in video conversion should be whether the container or the codec is changing. If only the container changes, do a remux — there's no quality loss and the operation takes seconds. If re-encoding is required, H.264 is the safest codec choice; adjust resolution and bitrate together and never upscale the source's resolution. After conversion, always check the audio-video sync, whether the audio track is present, and whether the subtitles transferred. And keep the original — you usually notice you picked the wrong settings afterward.