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To test bandwidth with iperf3, install it on two machines, run iperf3 -s on one (it listens on TCP port 5201) and iperf3 -c 203.0.113.10 -P 4 on the other. Add -R to test the opposite direction. iperf3 measures throughput between those two hosts only, so pick a second host close to where your users are.

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Before you start

  • Throughput (bandwidth) is how much data per second gets through, shown in Mbits/sec or Gbits/sec. iperf3 measures it between a server (which listens) and a client (which connects).
  • You need a second machine: another server, or your own computer. A test to a public iperf3 server only shows the path to that server.
  • Open TCP port 5201 on the machine that runs iperf3 -s, and close it again when you finish. See UFW, firewalld or Windows Firewall.
  • Commands use sudo for a non-root user. As root, leave it out.

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How to install iperf3

If the installer asks Start Iperf3 as a daemon automatically?, choose No unless you need a permanent test server.

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How to run an iperf3 test

1

Open port 5201 on the server

2

Start the iperf3 server

On the server:
It prints Server listening on 5201 (test #1) and waits. Add -1 to exit after one test.
3

Run the client

On the other machine, replace the IP with your server’s:
-P 4 runs four parallel streams and -t 30 runs for 30 seconds (default 10).
4

Test the other direction

-R makes the server send and the client receive. Use --bidir to test both directions at once.
5

Stop and close the port

Press Ctrl+C on the server, then remove the firewall rule (sudo ufw delete allow 5201/tcp, or Remove-NetFirewallRule -DisplayName "iperf3 test" on Windows).

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How to read the result

The result is limited by the slowest part of the path: either machine’s network port, the route between them, or CPU on either side.

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Useful iperf3 options

A UDP test also reports jitter and lost datagrams, which helps for voice and game traffic:
Don’t leave iperf3 -s running on a public IP. Anyone who finds the port can use your bandwidth. Stop it and close port 5201 when you’re done, and keep heavy tests short: the traffic counts towards your server’s usage (see traffic and bandwidth).

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Frequently asked questions

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Why is my iperf3 result lower than my port speed?

Distance, the slower of the two connections, CPU limits and TCP tuning all reduce throughput. Try more streams with -P 8, and on long paths see BBR.

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Why does iperf3 say “unable to connect to server”?

If the message ends in “Connection refused”, nothing is listening: check iperf3 -s is running. A timeout means a firewall drops the connection: check port 5201 is open on the server’s firewall.

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Can iperf3 talk to iperf2?

No. iperf3 isn’t backwards compatible with iperf2, so use the same tool on both ends.

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Does iperf3 use more than one CPU core?

From version 3.16, iperf3 runs each stream in its own thread. Older versions use one core for all streams. Check with iperf3 --version.

VPS and dedicated server guides

Every guide in this section.

Diagnose network latency

Find where lag and loss start.

Check bandwidth usage

See live and monthly traffic.

Enable BBR

Better TCP throughput on long paths.

Benchmark your VPS

CPU, disk and network tests.
Checked against the official iperf3 documentation and FAQ on 4 November 2026; Linux commands tested on Ubuntu 24.04 LTS (iperf 3.16). Commands can differ on other distributions or releases.

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Need Extra Help?

If you encounter any issues, our support team is ready to assist:

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Last Updated: November 2026