Measuring Wi-Fi properly with iperf3

November 9, 2025 · 5 min read

After moving the access point to the hallway ceiling, the bedroom "felt faster". Feelings are not numbers, and online speed tests were useless: they topped out at my internet plan and said nothing about the radio. So I measured inside the network.

Setup

One wired machine runs the server. The laptop on Wi-Fi is the client. Both need iperf3:

# on the wired machine
sudo apt install iperf3
iperf3 -s

# on the laptop
iperf3 -c 192.168.1.10 -t 20

The server must be wired. If both ends are on Wi-Fi, you are measuring two radio links at once and the result is meaningless.

Measure both directions

By default the client uploads. Add -R to reverse it, which is usually what you care about (downloads to the laptop):

iperf3 -c 192.168.1.10 -t 20 -R

Wi-Fi is often asymmetric: the access point has better antennas than a laptop, so downloads tend to be faster.

Use several streams

A single TCP stream can be limited by latency rather than by the link. Four parallel streams give a more honest ceiling:

iperf3 -c 192.168.1.10 -t 20 -R -P 4

My numbers

The feeling was right, for once. The living room barely changed, but the far rooms more than doubled: fewer walls between the antenna and the client matter more than anything in the settings menu.

Things that skewed early results

• The laptop on battery saver throttled the Wi-Fi card. Plug it in.
• A phone streaming video on the same channel. Test when the network is quiet, or at least note it.
• 20-second runs varied by 10–15%. I ran each test three times and took the median.