A speed test reports three figures, and they answer different questions.

• Download is how fast data reaches you. It is the one that matters for streaming, browsing and most everyday use, and it is the number providers put in their adverts.
• Upload is how fast data leaves you. It matters for video calls, backing up photos, sending large files and gaming. On part-fibre connections it is usually much lower than download. On full fibre it is often the same.
• Ping, also called latency, is the delay before data starts moving, measured in milliseconds. Lower is better. It barely affects streaming, but it is the number that decides whether a video call feels natural or whether an online game lags.

It depends on what you do online, not on the biggest number you can buy.

• For one or two people browsing, streaming and the odd video call, anything from 50 to 100 Mbps feels comfortable.
• For a busy household running several 4K streams at once, regular video calls, big uploads or working from home, 300 Mbps and up gives you headroom.
• Heavy gaming cares more about a low, steady ping than a huge download figure.

The honest version: most homes do not need the fastest package on the market. The useful question is not "what is the biggest speed" but "am I getting what I pay for, and could my address take more for the same money or less."

This is where our test does something a bare speed number cannot. It checks the result against what your address can actually take, using Ofcom Connected Nations data for your line, then sorts it into one of a few clear outcomes:

• Your speed matches your line, and there is nothing to fix.
• You are well under what you pay for, which points at WiFi, your router or a line fault.
• Your line could take more than your current package, so a faster deal is available at your address.
• The number looks fine but the connection still feels slow, which is usually WiFi rather than the line.

If you are paying for a speed you never see, that is worth chasing down. Sometimes it is a five minute fix at home. Sometimes it is a sign you are on the wrong package or the wrong provider for your address.

If the test comes back well below what your line should deliver, it is almost always one of three things.

• WiFi. Distance from the router, thick walls, interference from other devices or simply an older router struggling to keep up. A wired test rules this in or out in seconds.
• The router. Kit that is several years old often cannot deliver the speeds a modern line can, and a newer router frequently fixes it.
• A line fault. If a wired test is still far below spec, it points to a fault that needs your provider to investigate.

A few quick wins to try first: move the router into the open rather than behind the TV, keep it away from other electronics, give it a restart and test again on a device sitting right next to it.

A speed test is only as good as the conditions you run it in. For a result you can trust:

• Test wired if you can. Plug a laptop straight into the router with an ethernet cable, so you are measuring the line itself with WiFi taken out of the picture.
• Pause anything heavy. Downloads, big uploads, 4K streams and game updates running in the background will all drag the result down.
• Test more than once. Run it a few times at different points in the day, so you are looking at a pattern rather than one odd reading.

If a wired test at a quiet time still comes back well below what you pay for, that is solid evidence to take to your provider.