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Measure Real LAN Throughput Between Two PCs With iperf3

Copying a file at 30 MB/s says nothing about the link - the disk may be the limit. Two short commands separate slow hardware from slow storage.

by@guidox updated:10/31/26 time:25 min level:Intermediate tested on:Windows 11 24H2 works on:Windows 10, Windows 11 tools:iperf3
TL;DR

Run **iperf3 -s** on one PC and **iperf3 -c <ip> -t 30** on the other. A gigabit link lands near **940 Mbit/s**; a result around 94 Mbit/s means a 100 Mbit port, a bad cable or power saving.

Before you start

Requirements

  • Two PCs on the same switch
  • Ethernet cables
  • iperf3 for Windows
Step by step

Instructions

01

Test wired before you judge Wi-Fi

Connect both machines to the same switch by cable. Wi-Fi results move with distance, channel and neighbours, so a wired number first gives you a reference to compare against.

02

Get iperf3 onto both PCs

Download the Windows build, unpack it into one folder and run it from there. It is a single executable with no installer, which is why it works on machines where nothing may be installed.

iperf3 --version
03

Start the server and open the port

Run the server on the first PC and allow it through the firewall for the private profile. Without the rule the client connects to nothing and reports a timeout after a minute.

iperf3 -s
netsh advfirewall firewall add rule name="iperf3" dir=in action=allow protocol=TCP localport=5201 profile=private
tipNote the IPv4 address of the server with ipconfig. Hostnames work too, but the address removes DNS from the equation.
04

Run a 30-second test in both directions

Thirty seconds are enough for a stable average. Run the test once normally and once with **-R**, because some links are fast in one direction and asymmetric in the other.

iperf3 -c 192.168.1.50 -t 30
iperf3 -c 192.168.1.50 -t 30 -R
05

Read the number correctly

Gigabit Ethernet tops out near 940 Mbit/s, about 112 MB/s, because of protocol overhead. A 2.5G link reaches roughly 2.35 Gbit/s, a 10G link around 9.4 Gbit/s. Anything at 94 Mbit/s is a 100 Mbit link.

⚠ warningA single TCP stream can also be limited by the CPU. Cross-check with -P 4 before you blame the cable.
06

Repeat with parallel streams

Four parallel streams show whether one connection is the bottleneck. If four streams reach the expected total while one does not, the link is fine and the limit sits in the session, not in the wire.

iperf3 -c 192.168.1.50 -t 30 -P 4
Troubleshooting

Common issues

The client reports connection refused

The server is not running, or the firewall rule is missing. Check the server window for the login banner.

The test completes with zero throughput

The PCs sit on different subnets or a VLAN boundary. Confirm both addresses share a prefix with ipconfig.

Results swing between runs

Energy Efficient Ethernet or Wi-Fi power saving is interfering. Disable EEE in the adapter settings and repeat the wired test.

Wired is fast, Wi-Fi is not

Check the negotiated rate and channel width in the adapter status. A 20 MHz channel on 2.4 GHz caps the result near 100 Mbit/s.

FAQ

Frequently asked

Does iperf3 measure the disk?

No, it generates data in memory. That is the point: it measures the link alone, without storage in the way.

Which port does it use?

TCP 5201 by default. Change it with -p on both sides if something else occupies it.

How to undo this

Close both windows and delete the folder. Remove the firewall rule with netsh if you created it just for the test.

Changelog · 1 entry
  1. 2026-10-31
    First version
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