1080p at 4000 kbps looks worse than 720p at 4000 kbps
Everyone streams 1080p because it is the number on the box. At a lot of common bitrates it is the wrong choice, and the maths says so in one line.
Two streams, both 4000 kbps. One is 1080p60 and one is 720p60.
The 720p one looks better. It is not close.
The number that decides it
Divide your bitrate by pixels per second. That gives bits per pixel, and bits per pixel is what quality actually tracks.
1080p60 is 1920 times 1080 times 60, which is 124.4 million pixels a second. At 4000 kbps that is 0.032 bits per pixel.
720p60 is 55.3 million pixels a second. Same 4000 kbps gives 0.072 bits per pixel. That is 2.25 times as much data describing each pixel.
More pixels, each one described worse. Or fewer pixels, each one described well. A soft 720p beats a smeared 1080p on any screen.
Where the line sits
| Bitrate | Comfortable at | Struggles at |
|---|---|---|
| 2500 kbps | 720p30 | 720p60 and anything above |
| 4000 kbps | 720p60, 900p30 | 1080p60 |
| 6000 kbps | 900p60, 1080p30 | 1080p60 with heavy motion |
| 8000 kbps and up | 1080p60 | 4K anything |
Content moves the line. A talking head at 1080p60 on 4500 kbps is fine, because almost nothing in the frame changes. A moving camera outdoors at the same settings falls apart.
Frame rate is the cheaper cut
Halving frame rate halves pixels per second and doubles bits per pixel. Halving resolution does the same thing while also making the picture smaller.
For anything where motion is not the point, 1080p30 beats 720p60. For sport, gameplay and a moving camera, take the resolution cut and keep 60.
The part that is out of your hands
Platforms re-encode. Your 1080p may be transcoded down for viewers on slower connections, and the tiers a platform offers depend on your account.
Where transcoding is not guaranteed, every viewer gets your source. Sending a resolution your bitrate cannot support then punishes the people on small screens too, because they receive the same smeared frames.
Work out your bits per pixel before you pick a resolution. It takes one division and it settles the argument.
Sources
The Rig Wire reports on streaming technology from public documentation, source repositories and vendor releases. We did not bench-test what is described here. When we have tested something, we say so.