Why does Wi-Fi get slower through walls?
Walls absorb and reflect part of the radio signal, and rather than simply arriving quieter, a weakened signal makes your router switch to a slower, more repetitive way of encoding data so it can still be understood.
Simple intuition
The plain reason, in everyday words
Wi-Fi is a radio conversation between your router and your laptop, using waves rather than sound. A wall does not block those waves completely, but it soaks up part of them, the way a closed door muffles someone talking in the next room. Now think about what you do when you can barely hear someone. You do not simply carry on at normal speed and hope — you slow down, repeat yourself, and say each word more clearly. Wi-Fi does exactly the same thing automatically. When the signal gets faint, your router and laptop agree to switch to a slower, more repetitive way of sending information so that it still arrives intact. So the wall does not just make the signal quieter. It makes both devices deliberately choose to talk more slowly.
The wall blocks part of the signal, so you get proportionally less speed.
The relationship is not proportional. Throughput falls in steps as the link drops to lower modulation schemes, so a small extra loss can suddenly cost you half your speed while a larger loss elsewhere costs nothing.
5 GHz is simply better than 2.4 GHz.
5 GHz has more channels, less interference, and higher peak rates, but it is absorbed more by building materials. Through several walls, 2.4 GHz frequently wins.
A Wi-Fi extender fixes a weak signal.
A traditional repeater receives and retransmits on the same channel, roughly halving the available airtime, so it can improve coverage while reducing total throughput. A wired access point or a mesh system with a dedicated backhaul does not have this problem.
More antennas always mean more range.
Extra antennas mainly add spatial streams and diversity, which increase capacity where the signal is already decent. They do very little for a link that is failing purely on signal strength.
It changes what you actually do about bad Wi-Fi. The instinct is to buy a more powerful router, but transmit power is legally capped and rarely the constraint. Understanding that the bottleneck is signal-to-noise at the receiver points to the things that genuinely work: moving the router away from metal and out of a corner, choosing a less crowded channel, and putting a wired access point on the far side of the wall rather than a repeater on your side of it.
Who worked it out
The physics is Maxwell's, but the practical foundation is Claude Shannon's 1948 result relating channel capacity to bandwidth and signal-to-noise ratio — the reason a weaker signal has a hard mathematical ceiling on how fast it can carry data.
What problem forced it
Wi-Fi itself became possible when the US regulator opened the unlicensed ISM bands to spread-spectrum use in 1985, and 802.11 was standardised in 1997.
How it changed since
Every generation since has fought the same battle for signal-to-noise: OFDM in 802.11a/g, MIMO in 802.11n, wider channels and denser modulation in 802.11ac, and scheduled multi-user access in 802.11ax. Peak rates rose roughly a thousandfold, but the behaviour through a wall improved far less, because the underlying physics of absorption did not change.
Why 5 GHz has shorter range than 2.4 GHz
The frequency-dependence of absorption, isolated on its own.
How routers choose channels
The interference half of the problem, which the wall explanation leaves out.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.