How to measure
Wi-Fi coverage

It's the strength at which your device receives the signal, measured on a logarithmic scale: that's why the numbers are negative and why a difference that looks small is actually huge. Every 3 dBm drop is half the received power, so going from a good value to one a few points worse isn't a detail, it's losing a large part of the real signal, even though on paper it looks like a small change. That's why two spots that seem «almost the same» on a dBm map can behave completely differently in practice, one with plenty to spare and the other dropping out. The counter-intuitive part is the direction of the scale: the more negative, the worse, so −80 dBm sounds like a bigger number than −45 dBm and it's exactly the other way round: −45 is an excellent signal and −80 is practically nothing. Always trust the exact number, never the impression it gives when you first read it.

It depends on what you use the connection for, which is something a table of values can't tell you on its own. Browsing or sending messages needs far less than keeping a video call stable or uploading a large file, so a value that's more than enough for one person may fall short for someone who works with video all day. The practical threshold we use, −67 dBm, is designed for the most demanding typical use in a home or office: video and video calls without drop-outs, the band we call «sufficient». Below it, in the «borderline» band, browsing holds up but video calls already suffer, and further down it stops being any use at all even though the phone still shows a connection. The important thing isn't memorising a single number but relating it to what happens at each spot: the desk where you work needs more margin than the hallway you only pass through on your way to another room.

Because they don't follow any standard: each manufacturer decides how many bars to draw for each signal level, so an iPhone and an Android in the same spot can show different things without either technically lying. The practical problem is worse than it seems: almost every phone keeps showing two or three full bars with a signal that's already useless, because the bars are designed to reassure, not to diagnose. That's how someone can swear «there's loads of coverage here» while the video call keeps dropping in front of them. If you're going to decide where to put an access point or whether you need to buy something, the bars give you no data to work with, because they can't be compared between devices or between visits. The dBm figure can: it can be compared between phones, between spots in the house and between measurements taken on different days, which is exactly what you need to decide sensibly.

Any Wi-Fi analyser that shows the figure in dBm, not just quality bars, will do; you don't need to pay or create an account for this. On Android, if you don't want to install anything, the network settings menu itself usually shows the value in the details of the active connection, without any external app. The same goes for a laptop: the system's own network diagnostic tools show it, so you don't need extra software either if you have one to hand during the measuring round. On iPhone, Apple doesn't expose that figure to ordinary apps, so if there are only iPhones at home, the most practical option is to borrow an Android for a while or use a laptop to do the round. What matters when choosing a tool is that it shows the exact number in dBm, not a traffic light or a score out of ten that hides the real figure you need all over again.

No, and confusing them leads to buying things you don't need. Coverage is how much signal, in dBm, reaches a spot; speed is how many megabits actually get through that connection at that particular moment. You can have an excellent signal and still mediocre speed if the channel is congested by other networks, if lots of devices are competing at once or if the device you're measuring with is limited. Conversely, a borderline signal is sometimes enough to browse without trouble if nobody else is using the network at that instant. That's why it's worth measuring both at the spots that fail: if the signal is good and the speed isn't, buying a repeater or moving the router won't fix anything, because that's not where the problem lies. That specific case, a decent signal with poor performance, has to be diagnosed another way, by looking at how traffic is shared and at congestion, not at signal coverage.

Yes, because they tell different stories and both are useful for deciding where to put something. The 5 GHz band tells you how far high speed reaches before it collapses; the 2.4 GHz band tells you how far basic coverage reaches, even if it's slower. If your router combines both bands under the same network name, the phone chooses which to use by itself and in several spots stays stuck on the worse one without you realising, so to measure properly you should force the connection to each band separately, even if you combine them again afterwards. The usual pattern is that 5 GHz drops off a cliff as soon as it crosses the first thick wall, while 2.4 GHz is still there several rooms further on, but with less margin. That contrast between the two is exactly what tells you whether the gap can be filled by moving the router or whether you need a new point in between.

At least one per room, and always where and at the height the Wi-Fi is actually used, not in the middle of the hallway. Think about the spots that really matter: the desk where you work, the sofa where you watch series, the bed, the terrace if you work there at certain times of year, a shop's card terminal or till. A floor plan with a few well-chosen spots is worth much more than lots of random readings around the house. Note each value next to its spot on the plan, with the date and time, because if you ever repeat the measurement (after moving the router or adding an access point) you need to compare like with like, not a different snapshot of the house. If a value comes out very differently between two visits without anything having changed indoors, suspect one-off interference, such as a new neighbouring network, before taking the figure as final.

Then the problem isn't coverage, and that's already a useful result: it rules out half the list of possible causes in one go. If the dBm is good at the spot where it fails but real-world use is still poor, there are three other things to check: whether the line itself works properly by testing with a cable, whether the Wi-Fi channel is congested by networks from other flats or by lots of your own devices switched on at once, and whether some old or faulty device is hogging airtime for longer than its share. That's a different diagnosis from coverage, with its own order of checks, and it's best followed step by step rather than trying things at random, because each step rules out the next and saves you spending money on a device that wasn't going to fix anything. The dBm measurement you've just done isn't wasted: it confirms there's no need to change how the signal is distributed around the house.