Why is my WiFi slow in the evening but fine during the day?

This usually means channel congestion. WiFi channels are shared airspace, so when the neighbours come home and their networks get busy, everyone on the same channel takes turns transmitting and each network slows down. Moving to a less crowded channel, or to the 5 GHz band, normally fixes it.

Which WiFi channel should I use on 2.4 GHz?

Only channels 1, 6 and 11 do not overlap each other on 2.4 GHz, so pick whichever of those three has the fewest and weakest networks on it. Choosing an in-between channel such as 3 or 9 overlaps two of the clear channels and makes interference worse for everyone.

ClearWiFi

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Slow WiFi in a flat or busy street? It's probably channel congestion

The tell for this one is timing and geography. Your WiFi is fine at 10am and crawls at 8pm. It's fine at your parents' house in the countryside and hopeless in your city flat. Nothing about your equipment changed - what changed is how many other networks are competing for the same airspace.

What's actually happening

WiFi channels are like lanes on a road, and unlike a road, everyone on a lane has to take turns. Your router and every neighbouring router on the same channel politely wait for each other before transmitting. Two networks sharing a channel is fine. Fifteen networks sharing a channel means your data spends most of its time queuing for a gap.

This is why the problem tracks the neighbours' schedule rather than yours: their evening streaming fills the gaps your router was using.

The quickest fix: use 5 GHz instead

Before touching channel numbers, check which band you're on. Most homes have both:

2.4 GHz - travels furthest and through walls best, but has only three usable lanes and is shared with microwaves, cordless phones and Bluetooth. This is where congestion bites.
5 GHz - far more channels, much less crowded, considerably faster. The trade-off is shorter range and weaker wall penetration.

If you're within a room or two of the router and still on 2.4 GHz, moving to 5 GHz is usually a bigger, easier win than any channel change. If your router presents them as separate network names, join the one ending in "5G". If both bands share one name, toggling WiFi off and on near the router normally lets your phone pick the faster one. There's more detail in 2.4 GHz vs 5 GHz: which band should you use?

Choosing a clearer channel on 2.4 GHz

If you need 2.4 GHz for range, pick the emptiest lane. The critical detail most people miss:

On 2.4 GHz, only channels 1, 6 and 11 don't overlap each other. Every other channel partially covers two of them. Setting your router to channel 3 or 9 doesn't find a quiet gap - it splatters across two clear channels and makes things worse for you and everyone nearby.

So the job is simply: of channels 1, 6 and 11, which has the least traffic where you live? Scan the networks around you and count what's sitting on each - weighting by signal strength, because a strong neighbour on channel 6 interferes far more than a faint one three floors up. Then set your router to the winner.

On 5 GHz

There are many more channels, so congestion is rarer, but two things matter. Some 5 GHz channels are DFS channels, shared with weather and military radar: they're often beautifully empty, but if radar is detected your router must vacate the channel, which briefly drops every device. If you see occasional unexplained disconnects, try a non-DFS channel. Second, very wide channels (80 or 160 MHz) buy speed only when the airspace is clear - in a crowded band a narrower 40 MHz channel is frequently faster in practice because it collides with fewer neighbours.

How to change the channel

Open your router settings - your router's app if it has one, or type 192.168.1.1 (or 192.168.0.1) into a browser while on WiFi. The admin password is usually on a sticker on the router.
Find the wireless settings for the band you're changing (2.4 GHz and 5 GHz are configured separately).
Change Channel from Auto to your chosen number, and save. The WiFi will drop for a few seconds while the router restarts the radio.
Re-scan afterwards to confirm you actually landed on the channel you picked.

Why not just leave it on Auto? Auto only re-evaluates at boot on many routers, so it may have chosen months ago under completely different conditions - and it can't tell that your neighbour installed a new router last week. If Auto were reliably right, congestion wouldn't be one of the most common WiFi complaints there is.

What doesn't fix it

A faster internet plan changes nothing - the bottleneck is airtime between your device and your router, not the line into your home. An extender can make it worse: it repeats traffic on the same crowded airspace, roughly doubling the congestion it adds. And if your problem is stalls and freezes rather than general slowness, look at packet loss; if it only happens during downloads, that's bufferbloat.

The WiFi Doctor in the ClearWiFi Android app scores every channel by how crowded it actually is where you're standing - weighted by signal strength, not just network count - and tells you which channel to switch to. There's a live congestion map for 2.4, 5 and 6 GHz too. No ads, no tracking, no subscription. Get it on Google Play.