iperf3 on Windows: Test Network Speed Between Two PCs

An internet speed test tells you how fast your link to the internet is. It does not tell you how fast two computers talk to each other on your own network. For that, IT engineers use iperf3: a small, free tool that measures the real bandwidth between two PCs over LAN, Wi-Fi or VPN. This guide shows how to set it up on Windows and read the results.

Table of contents

• What iperf3 is and when to use it
• How it works: server and client
• Step 1: Download iperf3 on both PCs
• Step 2: Start the server
• Step 3: Run the client test
• Step 4: Read the result
• Useful options (UDP, reverse, parallel)
• FAQ

What iperf3 is and when to use it

iperf3 is a network performance tool that sends test traffic from one PC to another and reports the throughput in megabits or gigabits per second. It is the standard way to answer questions like:

• Is my Gigabit LAN actually running at ~940 Mbps, or much lower?
• Is Wi-Fi the bottleneck, or the cable?
• Does a new switch or VPN tunnel carry the speed it should?

How it works: server and client

iperf3 needs two machines. One runs as the server (it listens), the other as the client (it sends the test). Traffic flows between them on TCP port 5201 by default.

Step 1: Download iperf3 on both PCs

1. Download the Windows build of iperf3 from the official iperf site (iperf.fr or the ESnet iperf page). You get a ZIP, not an installer.
2. Extract the ZIP to a simple folder on both PCs, for example C:\iperf3. Keep iperf3.exe and the DLL files together.
3. Open Command Prompt and move into that folder: cd C:\iperf3

Use the same major version (3.x) on both machines so the results are comparable.

Step 2: Start the server

On the PC you want to receive the test (for example the one with the file share or Tally), run:

iperf3.exe -s

It will say it is listening on port 5201. Allow it through the firewall: the first time, Windows shows a Windows Defender Firewall prompt; tick Private networks and click Allow access. If no prompt appears and the test fails, add a rule for TCP and UDP port 5201.

Find this PC's IP address with:

ipconfig

Note the IPv4 Address (for example 192.168.1.50).

Step 3: Run the client test

On the second PC, run a 30-second test to the server's IP:

iperf3.exe -c 192.168.1.50 -t 30

Replace the IP with your server's address. The test runs and prints a result every second, then a summary.

Step 4: Read the result

Look at the final summary line marked [SUM] or sender/receiver. The Bitrate column is your answer.

Link type	Healthy iperf3 result
Gigabit (1 Gbps) wired LAN	~900-940 Mbps
100 Mbps (Fast Ethernet) link	~90-95 Mbps
Wi-Fi (varies a lot)	Often 100-600 Mbps depending on band and distance

If a Gigabit link shows only ~95 Mbps, the Speed & Duplex is stuck at 100 Mbps somewhere, usually a cheap cable or a mismatched port. See our guide on laptop network problems for adapter checks.

Useful options (UDP, reverse, parallel)

• Test UDP: iperf3.exe -c 192.168.1.50 -u -b 900M sends UDP at a set rate and reports jitter and packet loss. Good for VoIP and video.
• Reverse direction: add -R so the server sends to the client. Useful to test download speed to the client.
• Parallel streams: -P 4 opens four streams, which can reach full speed on high-latency or Wi-Fi links.
• Custom port: -p 5202 on both sides if 5201 is blocked.

Example full test: iperf3.exe -c 192.168.1.50 -t 20 -P 4

FAQ

Why does my test fail to connect?

Almost always the firewall. Allow iperf3 (or TCP/UDP port 5201) on the server PC, and make sure both PCs are on the same network and can ping each other.

Does iperf3 test my internet speed?

No. It tests the link between the two PCs you run it on. For internet speed, use a browser speed test instead.

Is iperf3 safe to use?

Yes. It only generates test traffic while you run it and stops when you close it. Only run it on networks you are authorized to test.

Can I test Wi-Fi with it?

Yes. Run the server on a wired PC and the client on the Wi-Fi laptop to measure real Wi-Fi throughput at different spots in the office or home.

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Based on standard iperf3 3.x behavior on Windows. Commands were checked on October 31, 2026.