Bandwidth Arithmetic

Compute the network bandwidth your system actually needs — most designs miss this until it's too late.

How It Works

Bandwidth is the forgotten dimension of scale estimation. The formula: peak QPS (see Back-of-Envelope Math) × average payload size × any fan-out factor. A video streaming service serving 1 million concurrent viewers at 5 Mbps per stream needs 5 Tbps of egress bandwidth — that's a major cost line item, not a footnote. For APIs: 10,000 QPS × 10 KB per response = 100 MB/sec = 800 Mbps of outbound traffic, which saturates a small cloud VM's default network allocation. Common misses: underestimating log shipping, forgetting CDN fill traffic, ignoring cross-region replication bandwidth. In interviews, state bandwidth in the same breath as QPS and storage.

Real-World Example

Netflix built its own CDN (Open Connect) specifically because its bandwidth needs outgrew what commercial CDNs could serve economically. At peak hours, Netflix accounts for a substantial fraction of global downstream internet traffic, measured in dozens of Tbps. Any design discussion for a video product that ignores bandwidth misses the primary cost driver.