Broadband speed test and diagnosis

What this test measures

The test moves real data between your browser and a nearby server, and times it. That gives four figures: how fast data comes down, how fast it goes up, how long a round trip takes (ping), and how much that round-trip time varies (jitter).

Download speed gets all the attention, but it is often the least interesting of the four. Upload speed is what fails first on video calls and cloud backups. Jitter is what makes a call break up even when the speed test looks fine.

What a single test cannot tell you

It cannot tell you whose fault a slow result is. A speed test measures the whole path between your device and the test server — your Wi-Fi, your router, the line into your home, your provider’s network, and everything in between. A poor result proves something in that chain is limiting you. It does not say which link.

That is why we ask a couple of questions afterwards, and why the most useful thing you can do is run a second test over an Ethernet cable. Comparing two results tells you far more than one result ever can, because the difference between them isolates the part that changed.

Why your result is lower than the speed you pay for

Usually because it should be. UK broadband is advertised using average speeds that at least half of customers get at peak time — not a guarantee for everyone. On top of that, Wi-Fi typically costs a large share of the line’s capacity, protocol overhead takes a few percent more, and an older phone or laptop may not be able to receive the full speed however good the line is.

So we treat results within about 20% of the advertised figure as consistent with the package. Between roughly half and that point, you are below what you pay for, but the gap on its own does not say where the speed is going — so we say that, and suggest the retest that would narrow it down rather than calling it normal. Below half, the gap is too large for those explanations and is worth investigating properly.

Your privacy

The measurement runs in your browser against Cloudflare’s network, so the traffic necessarily reaches Cloudflare and it sees your IP address as the other end of that connection — as any website you visit does. We have switched off the library’s optional reporting of your results back to Cloudflare, and we do not store your results, your IP address or anything that identifies you. We cannot tell you what logs Cloudflare itself keeps; that is a question for their privacy policy, not ours.