How Does a Speed Test Work?
An internet speed test measures four things - download, upload, ping and jitter - by sending and timing data packets to a nearby test server.
What a speed test actually does
When you tap "go" on a speed test, your browser opens a connection to the nearest test server and runs four short tests in sequence:
- Ping - sends a tiny packet and times how long the round-trip takes (lower is better).
- Jitter - sends multiple pings and measures variance.
- Download - pulls a multi-megabyte file in parallel streams and measures throughput.
- Upload - sends data the other way and measures the same.
Reading the results
- Download (Mbps): how fast you can pull data in. Should be at least 80% of your line speed.
- Upload (Mbps): how fast you can push data out. On symmetric fibre, equal to download.
- Ping (ms): how responsive your line feels. Under 20 ms is excellent.
- Jitter (ms): stability of your ping. Under 5 ms is excellent - see our jitter guide.
Why your results vary
Speed tests over Wi-Fi are limited by your router and signal strength, not your fibre line. To test your line itself, plug into the router with an Ethernet cable. Tests run on a busy home network (someone else streaming Netflix) will also under-report.
Run our in-house speed test for an instant reading.
Frequently asked questions
Last reviewed by the Fastest Fibre editorial team.
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