Video
Resolution, frame rate, bitrate: what really makes a video heavy, measured
The settings of a smartphone or editing software all offer the same choices: 4K or 1080p, 30 or 60 frames per second, standard or high quality. It is tempting to assume that a 60 fps video weighs twice as much as a 30 fps one, since it contains twice as many frames. We wanted to check, by encoding the same clip at several resolutions and frame rates. Reality is very different, and it changes how to shrink a video effectively.
The three numbers in a technical sheet
Resolution is the number of pixels in each frame: 1920 × 1080 for 1080p (Full HD), 1280 × 720 for 720p, 3840 × 2160 for 4K. A 1080p frame has 2.25 times as many pixels as a 720p frame, and a 4K frame four times as many as a 1080p one.
Frame rate, in frames per second, is the smoothness of motion. Cinema traditionally uses 24 frames per second, European television 25, and smartphones 30 or 60. A high frame rate makes fast movement sharper, which matters for sport or video games, much less for an interview.
Bitrate, in megabits per second (Mbit/s), is the amount of data used for each second of video. It directly determines the size: one minute at 8 Mbit/s weighs 60 MB. Bitrate is not an independent setting: the encoder sets it according to resolution, frame rate, content and the requested quality.
Our experiment
We took the first 30 seconds of a smartphone video shot in 1080p at 60 frames per second: a desert landscape with continuous camera movement. The clip was encoded in H.264 with FileXvert's default quality setting (CRF 23), in five combinations of resolution and frame rate. Sound was removed so as to measure the picture only.
| Resolution and frame rate | Size | Compared with 1080p60 |
|---|---|---|
| 1080p, 60 fps | 63.5 MB | baseline |
| 1080p, 30 fps | 59.4 MB | − 7% |
| 720p, 60 fps | 25.3 MB | − 60% |
| 720p, 30 fps | 23.7 MB | − 63% |
| 480p, 30 fps | 11.0 MB | − 83% |
Why frame rate weighs so little
Halving the frame rate removed only 7% of the size. The explanation lies in how modern codecs work: most frames are not stored in full, but described as a difference from neighbouring frames. At 60 frames per second, two consecutive frames are only 17 milliseconds apart; they look extremely alike, and the difference to record is tiny. Dropping every other frame therefore mostly removes frames that cost almost nothing.
Resolution, by contrast, acts on every frame, including the full reference frames, which are the most expensive. Going from 1080p to 720p divides the size by two and a half, and going down to 480p divides it by almost six. It is by far the most powerful lever.
These proportions vary with content: a very busy scene, where each frame differs a lot from the previous one, will make frame rate somewhat more expensive. But the order of magnitude stays the same for most everyday videos.
What this changes in practice
To shrink a video, the first lever is therefore resolution, and it is often far less noticeable than people think. On a phone screen, 720p and 1080p videos are hard to tell apart; on a laptop, the difference remains modest. That is why FileXvert's smallest size level drops the video to 720p: that is where the gain is.
The second lever is the compression level, set in FileXvert by the balanced and smallest size levels. It acts on every frame at once, at the cost of losing fine detail.
Frame rate, finally, is not a good way to lighten a video. Going from 60 to 30 frames per second makes movement less smooth for a small gain. It is better chosen at recording time, depending on the subject.
Choosing your settings when you record
The best way to get reasonable files is often to set the device up properly from the start:
- For family memories, tutorials or meetings: 1080p at 30 frames per second is plenty.
- For sport, moving animals or planned slow motion: 60 frames per second, knowing the extra size will be moderate.
- For 4K: keep it for videos you will crop in editing or watch on a large TV; each frame holds four times as many pixels as in 1080p, and the file is much heavier, for a gain that is often invisible on a phone.
- For videos you will send: record in 1080p, then reduce to 720p when sharing, from the original you kept.
To see the technical details of an existing video, your file explorer is often enough: on Windows, right-click, Properties, Details tab; on a Mac, the Get Info window. There you will find resolution, frame rate and bitrate, enough to predict what a conversion will save before you run it.