Bandwidth Estimation Playbook
The formula
Bandwidth = QPS × bytes per payload (then mind bits vs bytes)
⚠️ The bits-vs-bytes trap
Network capacity is quoted in bits (Gbps); your payloads are in bytes. Always convert: 1 Gbps = 125 MB/s (÷8). A NIC’s speed is the hard ceiling on a single box.
Worked example — video streaming
100K concurrent HD streams at ~5 Mbps each:
- Total = 100,000 × 5 Mbps = 500 Gbps = ~62.5 GB/s egress.
- Gate: no single server does that ⇒ this is a CDN problem; serve from edge, not origin.
Don’t forget egress cost
Cloud egress (data leaving the provider) is the sneaky bill — often ~$0.09/GB. High-bandwidth designs are often cost problems before they’re capacity problems (see the Cost playbook).
Formulas are standard/public-domain engineering math. Approach and reference-table format adapted from the System Design Primer (CC BY 4.0), Jeff Dean’s latency numbers, the DesignGurus capacity-estimation guide, and Little’s Law.
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