Two meanings to keep apart
- Rate: how fast data can flow at one moment, in Mbps or Gbps. This is what limits the quality a viewer can receive and what the service must supply at peak
- Volume: how much data was delivered over a period, in GB or TB. This is what a CDN bill is based on, and is more precisely called data transfer or egress
Viewer bandwidth
A viewer's connection has to exceed the bitrate of the video with room to spare. Around 5 Mbps is comfortable for 1080p and about 25 Mbps for 4K. When the connection falls short, adaptive streaming drops to a lower quality instead of stalling.
Service bandwidth at peak
The rate a service must deliver at its busiest moment is concurrent viewers multiplied by average bitrate. 2,000 concurrent viewers at 4 Mbps is 8 Gbps. A CDN supplies this capacity, which is why peak concurrency is the figure to check against a plan's limits.
Monthly volume and cost
Volume is viewing hours multiplied by average bitrate. Multiply hours by the bitrate in Mbps and by 0.45 to get gigabytes: 50,000 hours at 3 Mbps is about 67,500 GB, or 67.5 TB.
Average bitrate is usually well below your top rendition, because many viewers are on phones and slower connections. Use a measured average if you have one.
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