When determining video quality, consumers often focus entirely on resolution (1080p, 4K). However, the most critical factor for visual fidelity is the video bitrate. Bitrate measures the amount of data processed per second of video, usually expressed in Megabits per second (Mbps).
A 4K video heavily compressed to 5 Mbps will look vastly worse than a 1080p video encoded at 20 Mbps. The heavily compressed 4K video lacks the data required to render complex motion, resulting in blocky pixelation (macroblocking) and smeared details, particularly during fast-action scenes or in dark areas.
Calculating video file sizes
Bitrate is the sole variable determining the final file size of a video, independent of resolution or framerate. The formula is simple: File Size = (Bitrate × Duration) / 8. The division by 8 converts Megabits (Mb) to Megabytes (MB).
If you record a 10-minute (600-second) video at 50 Mbps, the calculation is (50 × 600) = 30,000 Megabits. Divide by 8, and the file size is exactly 3,750 MB, or 3.75 GB. If you change the resolution from 1080p to 4K but keep the bitrate at 50 Mbps, the file size remains exactly 3.75 GB (though the 4K version will look much more compressed).
VBR vs CBR and Streaming requirements
Video can be encoded using Constant Bitrate (CBR) or Variable Bitrate (VBR). CBR allocates the exact same amount of data to every second of video, which is reliable for live broadcasting but inefficient. VBR intelligently allocates more bitrate to complex, fast-moving scenes and less to static scenes (like an interview), optimizing file size and quality.
For streaming platforms like Twitch or YouTube, your upload bandwidth dictates your maximum bitrate. Twitch typically caps at 6 Mbps (6000 kbps), which is adequate for 1080p at 60fps, but struggles with high-motion games. YouTube recommends 35-45 Mbps for 4K uploads, which requires a substantial and stable fiber internet connection to upload efficiently.