The Wi-Fi icon shows three full bars, yet web pages still take an age to open. The combination is confusing because we are used to reading signal bars as a measure of speed.
They are not. The bars only show how clearly your device hears the router. That is one requirement, not a guarantee.
What the bars actually measure
The number behind them is called RSSI, expressed in dBm and always negative. The closer to zero, the stronger.
| RSSI | Bars | Condition |
|---|---|---|
| -30 to -50 dBm | Full | Excellent |
| -50 to -60 dBm | Full | Good |
| -60 to -70 dBm | 2–3 | Adequate |
| -70 to -80 dBm | 1–2 | Weak |
| Below -80 dBm | 1 | Barely usable |
Note that everything from -30 to -60 dBm displays as full bars, even though the spread inside that range is considerable. The on-screen icon is too coarse to distinguish them.
More importantly: RSSI does not measure interference. A device can hear the router very clearly while also hearing ten other networks just as clearly. That is where the trouble starts.
What matters more: SNR
SNR is the gap between the strength of the signal you want and the noise around it. The analogy is a conversation in a room: the person opposite may be speaking loudly, but if the room is full of other people speaking just as loudly, you still struggle to follow.
An SNR above 25 dB allows high speeds. Below 15 dB, devices start reducing speed so the data still arrives. Below 10 dB, the connection becomes unreliable even with full signal bars.
Unfortunately SNR is rarely shown in ordinary interfaces. Wi-Fi analyser apps, or the status page on a mid-range router, usually provide it.
The six most common causes
1. Fighting for channels with the neighbours
On the 2.4 GHz band there are only three non-overlapping channels. If fifteen networks around you use the same one, all of them must take turns.
This is the number one cause in flats and dense housing. The fix: move devices to 5 GHz, or lock 2.4 GHz to the emptiest of channels 1, 6, and 11.
2. Too many active devices
Wi-Fi works in turns. One device transmits, the rest wait. Twenty active devices means each one gets only a small slice of the time.
Idle devices matter little. What matters is the ones genuinely sending data: a download in progress, video streaming, or automatic photo backups.
3. One old device holding back the rest
On older Wi-Fi generations, a device capable only of low speeds occupies more airtime to send the same amount of data. Everyone else feels the effect.
If there is still a very old Wi-Fi device in the house, moving it to a separate 2.4 GHz network can let the newer devices breathe on 5 GHz.
4. Interference from non-Wi-Fi devices
A microwave oven transmits squarely in the 2.4 GHz band and is strong enough to cripple nearby connections while it runs. Older cordless phones, analogue cameras, and some Bluetooth devices contribute too.
Checking is simple: run a speed test while the microwave is on, then compare after switching it off.
5. A limit from the service, not from Wi-Fi
If your plan is 20 Mbps, no amount of Wi-Fi tuning turns it into 50 Mbps. This sounds obvious, yet it is the answer often enough.
Test over a LAN cable to be sure. If the cabled result is also around 20 Mbps, your Wi-Fi is fine.
6. The problem is at the destination site or service
One site slow while everything else runs fine means the problem is not in your house. The destination server may be busy, or the route to it congested.
How to check, in order
Five minutes of checking is usually enough to find the direction.
- Test from another device. If only one device is slow, that is where the problem is.
- Test over a LAN cable. This separates a Wi-Fi problem from a service problem.
- Test standing next to the router. If it is fast here but slow in another room, the problem is coverage.
- Test when the house is quiet. If it is fast at midnight but slow at eight in the evening, the problem is congestion, in your house or on the provider's network.
- Compare 2.4 GHz and 5 GHz. A large gap points to interference on the slower band.
The correct measurement method is explained in the guide to speed testing. Most wrong conclusions come from measuring inconsistently.
Matching the finding to the action
| Finding | Action |
|---|---|
| Fast near the router, slow elsewhere | Move the router, or add an access point |
| Slow everywhere, cable slow too | Check the plan, or contact the provider |
| Slow only at busy hours | Channel congestion, or the provider's capacity |
| 2.4 GHz slow, 5 GHz fast | Move devices to 5 GHz |
| Slows down whenever someone downloads | Enable Smart Queue Management on the router |
| Only one site is slow | Not your network's problem |
One note about the numbers
The "link speed" figure your operating system displays, 866 Mbps, say, is not your internet speed. It is the theoretical rate between the device and the router, before protocol overhead, waiting for turns, and the capacity from your provider are subtracted.
Real-world speed is usually 40–60 per cent of that figure in good conditions. So a link speed of 866 Mbps producing 400 Mbps is normal, not a sign of a problem.
Frequently asked questions
What is a good RSSI value?
Above -50 dBm is excellent, -50 to -60 dBm is good, -60 to -70 dBm is adequate, and below -70 dBm starts causing problems. The numbers are negative, so -45 is better than -75.
Why does speed drop when other people use the same Wi-Fi?
Wi-Fi shares the air by taking turns, not simultaneously. Only one device can transmit at a time on a channel. The more active devices there are, the fewer turns each one gets.
Does adding an access point always make things faster?
Only if the problem is coverage. If the problem is channel congestion, adding another transmitter on the same channel makes it worse. Establish the cause before buying more hardware.
