Glossary · Performance
Throughput: definition, explanation, and why it matters
The actual sustained rate of data delivered across a network link, measured over a window of time — distinct from the link's theoretical bandwidth.
Definition
The actual sustained rate of data delivered across a network link, measured over a window of time — distinct from the link's theoretical bandwidth.
Plain-English explanation
Bandwidth is the speed limit; throughput is the speed you actually achieve. Throughput is always lower because of protocol overhead, congestion, lost packets, and CPU/disk bottlenecks on either end.
A 1 Gbps Ethernet link, after TCP overhead and a single sender, delivers around 940 Mbps of sustained file-transfer throughput in ideal conditions.
Real-world examples
- Downloading a large game over a 1 Gbps line might peak at 110 MB/s — that's the throughput. The bandwidth is 1 Gbps.
- A single TCP stream over a long-distance link often achieves a fraction of the available bandwidth; using multiple parallel streams can dramatically increase throughput.
Why it matters
Real workloads care about throughput, not advertised bandwidth. Provisioning, capacity planning, and benchmarking should all measure throughput.
Common misconceptions
- Myth: Bandwidth and throughput are the same.
Reality: Bandwidth is capacity; throughput is delivery. They diverge whenever the network has loss, latency, or contention.
Frequently asked questions
- Why is my download throughput lower than my plan?
- Single-source servers, distant origins, TCP window limits, Wi-Fi, and disk write speed all cap throughput below your line capacity.