Does compressing a video twice ruin the quality? What to do on iPhone

Video compression is lossy: the encoder keeps what it judges you will notice and drops the rest. The second encoder starts from the already reduced picture, treats its blocky edges and soft detail as real content, and removes more on top. Fine detail that is gone stays gone, and small artefacts can grow with each pass.

How much you lose depends on the settings. Adobe’s guidance for its encoder says the same: each lossy export lowers quality, one generation is often acceptable, and repeatedly re-encoding already compressed video can degrade it beyond what is acceptable. Changing codec or settings between passes tends to add to it.

• Usually fine: one pass from the original recording to a sensible size, watched on a phone or laptop.
• Usually fine: a second pass that only changes something other than the picture quality, such as trimming, if the copy still looks clean.
• Often visible: a clip that already went through a chat app, a social app or a screen recording and is now compressed again at a low setting. Dark scenes, fast motion, text and faces show blockiness and smearing first.
• Never helpful: compressing again at a higher setting than the file already has. The picture does not get its detail back and the file can even get bigger, as in the guide on why a compressed video can be bigger than the original.

There is no fixed number of times that is safe. A clean, high-quality source survives a pass better than one that was already soft.

1. 1Think about where it came from. A video you recorded with the camera is an original. One you saved from a chat, a download or a social app was almost certainly re-encoded on the way.
2. 2Open it in Photos and tap the info button (the i in a circle) or swipe up to see its size, then compare it with how long the clip is. A few megabytes for a minute of video means it is already small.
3. 3Play it full screen, looking at dark areas, text and quick movement. If it already looks soft or blocky, a further pass will make that worse, so skip it or keep the settings the same.

• Always start from the original. If the first result is still too big, go back to the original video in your library and compress that again with a lower setting, instead of compressing the copy.
• Change everything in one pass. Resolution, frame rate and size target together in a single encode cost less than three encodes in a row.
• Trim before you compress, and keep the length to what the person needs. Fewer seconds is a smaller file with no quality cost at all.
• Pick the setting for where the video will be watched. 720p is usually enough for a chat or a slide; keep 1080p or higher for a big screen.
• Do not send the compressed copy through an app that squeezes it again if you can avoid it. Many chat apps re-encode video, so a smaller file that the app leaves alone often arrives looking better.
• Keep the original until you have watched the copy. Delete it only when you are sure.

Minivid, an iPhone app that compresses video on the phone, is built around the original: you pick a video, it shows the estimated size before anything starts, and the result is saved as a new copy. It never changes the original, so you can always go back to it and try a different setting from the same starting point. Menu wording can differ a little between iOS versions.

1. 1Open Minivid and choose the original video from your library, not an earlier compressed copy.
2. 2Look at the estimated size. Change the preset, the resolution or the frame rate until the number is right; the estimate updates as you do.
3. 3Compress, then play the result full screen and check dark scenes and movement.
4. 4If it is still too big, go back to the original and choose a smaller setting. If it looks good, save it to Photos or share it.

Nothing is uploaded; everything runs on the iPhone.

Seeing the estimated size is free. In the free version, Minivid compresses one video at a time, up to 100 MB, at up to 720p, with the Quick Share, Email and Save Space presets. Pro removes the size limit, adds 1080p and 4K output, the Social Media and Keep Quality presets, and lets you name an exact size.

If you tap Replace original after compressing, Minivid saves the copy first and only then asks iOS to delete the original, which then waits in Recently Deleted for 30 days. Check the copy before you do that, because with the original gone you cannot go back for a better pass.