The one formula that explains file size
A video file's size is almost entirely its bitrate multiplied by its length:
size (megabytes) ≈ bitrate (megabits per second) × duration (seconds) ÷ 8
A 60-second clip at 8 Mbps is about 60 MB; the same clip at 2 Mbps is about 15 MB. Audio adds a little on top, 128 kbps of audio is about 1 MB per minute. Everything else in this guide is about choosing a lower bitrate without the picture falling apart. If you want to play with the numbers, the bitrate calculator works them out in both directions.
This is also why there's no such thing as making a video smaller "for free": a smaller file at the same length means fewer bits per second describing the picture.
Why phone and screen recordings shrink so much
Cameras, phones and screen recorders encode video in real time, while you're recording. They can't afford to think hard about each frame, so they use generous bitrates to be safe, often far more than the picture needs.
A compressor has time to be smarter. Re-encoding a phone video at a third of its bitrate frequently looks indistinguishable at normal viewing size. Videos downloaded from social platforms are the opposite: they've already been squeezed hard, so compressing them again costs quality rather than megabytes.
The four levers, from most to least effective
1. Resolution
Each step down in resolution cuts the number of pixels dramatically: 1080p has about 2.25× the pixels of 720p, and 4K has four times the pixels of 1080p. Fewer pixels need fewer bits to look sharp. On phones, where most video is watched, 720p or 1080p is usually all anyone can see. If a file is far too big, lowering the resolution is the most effective single change.
2. Bitrate (or quality preset)
Once resolution is set, bitrate decides how much detail survives. Too low and you'll see the classic artifacts: blocky flat areas (skies, walls), smeared fine textures, and banding in gradients. Fast motion and grain are the hardest things to compress, so a gaming clip needs more bitrate than a talking head at the same resolution.
3. Frame rate
Halving 60 fps to 30 fps removes half of the frames to encode. Talking-head videos, tutorials and slideshows look exactly the same at 30 fps; sport and gameplay lose some smoothness. It also makes compression faster.
4. Codec
The codec is the method used to describe the picture. Newer codecs are more efficient, they need fewer bits for the same quality, but take longer to encode and aren't supported everywhere:
- H.264 (AVC), plays on practically everything. The safe default.
- H.265 (HEVC), noticeably smaller files for the same quality; well supported on modern phones and TVs, less so in some browsers and older software.
- AV1, smaller still, increasingly common on the web; older devices may not play it.
- VP9, the usual choice for WebM video on websites.
If you're not sure where the file will be played, stick with H.264 in MP4.
Quality presets vs. a target size
There are two ways to tell a compressor what you want:
- A quality preset (High, Balanced, Small) lets the encoder choose a bitrate suited to the resolution and frame rate. Use this when you simply want a smaller file that still looks good.
- A target size (say, 10 MB) works backwards: the compressor divides the size by the length to get the bitrate. Use this when the number is the point, an upload or attachment limit.
Target sizes and long videos don't mix well. Ten megabytes is plenty for 30 seconds of 1080p but thin for five minutes. When the budget is tight, lower the resolution too: a clean 480p video looks better than a blocky 1080p one. Better still, trim the video to the part that matters.
A quick recipe
- Open the file in the video compressor and check the estimated size with the Balanced preset.
- If it's still too big, drop the resolution one step (4K → 1080p, or 1080p → 720p) before reaching for the Smallest file preset.
- For 50 to 60 fps footage that doesn't need smooth motion, halve the frame rate.
- Have a hard limit? Switch to Exact file size and type it in.
- Watch the result full-screen on the device it's meant for. If flat areas look blocky, go up one step.
Things that don't help
- Compressing the same file repeatedly. Each pass loses a little more. Always compress from the original.
- Raising the bitrate of an already-compressed file. Detail lost earlier can't be recovered; the file just gets bigger.
- Changing the container alone. Converting MOV to MP4 without re-encoding barely changes the size, the video converter is for compatibility, not compression.
- Zipping a video. Video is already compressed, so ZIP saves almost nothing.