A one-minute phone video can easily be a hundred megabytes or more, and almost every way of sending it has a limit far below that. Squeezing it under the limit is easy. Squeezing it under the limit so it still looks good takes knowing which of three things to give up.
File size is bitrate times length
A video's bitrate is how many bits it spends on each second. Size follows directly:
size in megabytes ≈ bitrate in megabits per second × seconds ÷ 8
Phones commonly record 1080p video at somewhere around 10 to 20 megabits per second. At 16 Mbit/s, a two-minute clip is 16 × 120 ÷ 8 = 240 MB. To fit that clip into 16 MB, its average bitrate has to come down to about a fifteenth.
That is the whole problem. Length is fixed, unless you trim, so the bitrate has to fall. Everything else is deciding where the reduced bits are spent.
Resolution, frame rate and quality per pixel
The bits each second are shared between every pixel of every frame. Three things decide how thinly they are spread:
- Resolution. 1080p has 2.25 times the pixels of 720p and about five times those of 480p. At the same bitrate, a smaller picture gets far more bits per pixel.
- Frame rate. A 60 frames per second video has twice as many frames to pay for as a 30 fps one, with little visible gain on a phone screen for most content.
- Quality per pixel. What is left over. When too few bits reach each pixel, the encoder smooths away detail and, in motion, breaks the picture into visible blocks.
The intuitive move, keeping full resolution and lowering the quality, is usually the wrong one. A 1080p video starved of bits looks blocky and smeared on any screen. The same file size at 720p or 480p looks clean, because each pixel is described properly. On a phone screen, where most of these videos will be watched, the lower resolution is often impossible to tell apart.
Common limits
- WhatsApp has long capped videos sent as ordinary media at 16 MB, which is why that figure is everywhere, and it usually compresses videos again itself as it sends them. Messaging apps change their limits and options from time to time, so check the app if a video is refused.
- Gmail allows 25 MB of attachments on a message; above that it offers a Google Drive link instead. Other email services set their own limits, often in the same range.
- Email attachments are encoded for sending in a way that makes them about a third larger in transit, so a file just under a server's limit can still be refused. Leave some headroom.
If the video is long, a sharing link is often better than any attachment: the recipient gets the original, and nothing has to be compressed at all.
What the video compressor here does
The compress video tool has two ways to work.
Target size is for a hard limit. Pick 8, 16 (WhatsApp), 25 (Gmail), 50 or 100 MB, or type any size. A megabyte here is 1,000,000 bytes, so the result fits whichever way the receiving site counts. From the length of your video the tool works out the bitrate that lands just under the target, keeping back a small margin for the file's own structure. It then:
- caps the frame rate at 30 fps, since 60 fps would halve the bits each frame gets;
- on Automatic resolution, picks the sharpest resolution that still gets enough bits per pixel to look acceptable, and never above 1080p;
- gives the sound a bitrate that shrinks with the budget, from 128 kbit/s down to 32 kbit/s;
- checks the finished file, and if it came out over the limit, tries again at a proportionally lower bitrate, up to three attempts in all;
- returns your original untouched if it was already under the limit.
For a typical vertical phone video at 1080 × 1920 and 30 fps, Automatic resolution chooses as follows:
| Length | Under 16 MB | Under 25 MB |
|---|---|---|
| 30 seconds | Full 1080 × 1920 | Full 1080 × 1920 |
| 1 minute | 720 × 1280 | Full 1080 × 1920 |
| 2 minutes | 480 × 852 | 720 × 1280 |
| 3 minutes | 480 × 852 | 480 × 852 |
| 5 minutes | 360 × 640 | 480 × 852 |
Resolutions are named by the short side, so a portrait video at "720p" stays portrait at 720 × 1280. Below a certain point nothing watchable is left: the tool needs at least about 72 kbit/s for picture and sound, and tells you if your target is smaller than that. A one-minute video cannot go below about 0.58 MB with its sound.
Quality level is for when there is no hard limit and you just want a smaller file. High quality looks the same as the original to most eyes and is usually a third to half the size of a phone video; Balanced is slightly softer in fast movement and much smaller; Smallest is visibly softer. You can set the resolution yourself in either mode: Original, 1080p, 720p or 480p. A video is never enlarged.
Either way the result is an MP4 with H.264 picture and AAC sound, the combination every phone, browser and app plays (see why a video won't play). The file's metadata, such as recording location and device details, is left out, and the file is arranged so it can start playing before it has fully downloaded. Turning off "Keep the sound" leaves the whole budget to the picture.
Everything happens on your device. The tool re-encodes every frame, so expect it to take roughly as long as the video plays on a computer, and longer on a phone. Files above 500 MB get a warning that they may be slow or run out of memory, and files above 2 GB are refused, because a browser tab cannot hold them and the result at the same time.
Getting the best result
- Trim first. Every second you cut gives the rest more bits. The trim tool can cut without re-encoding, so the trim itself loses nothing.
- Drop the sound if it adds nothing. On a tight target the sound can be a meaningful share of the budget.
- Let the resolution go down. Automatic is right for most videos. Force a higher resolution only when fine detail matters more than smooth motion, such as a screen recording of text.
- Compress once, from the original. Each compression throws detail away. Compressing a video that WhatsApp already compressed, or running the tool twice, stacks the losses. Keep the original and make each smaller copy from it.
- Watch the result before sending. Look at the fastest-moving part of the video. That is where a low bitrate shows first.
If the video still will not fit at a quality you accept, the problem is length, not settings. Split it, trim it, or send a link. And if the only aim is to make a video smaller for your own storage, pick the Quality level mode at High: you keep the look and still save a large share of the space.