A network drops out for the third time in a week, and the reflex is to buy a new router. Ninety percent of the time the router is fine and the villain is a flat CAT5e cable someone ordered on impulse in 2019. Flat cables use thinner conductors and worse shielding than round ones, and they show it under load. Diagnosing this is not hard; you just need the right small tools to prove which cable is the problem before you replace hardware you did not need to touch.
We tested seven apps for checking Ethernet cable quality on a home network. Some measure raw throughput between two machines, some inspect the link that Windows or Linux negotiated, and one is a certified capture tool that shows retransmissions the moment a cable is marginal. Below is the sequence we run on any complaint about slow WiFi that turns out to be slow Ethernet.
What to look for in a cable quality tool
- Measures actual link speed, not marketing speed. The port might negotiate 100 Mbps on a CAT6 cable when the ports are gigabit. That is a cable problem.
- Runs both endpoints. Speed tests over the internet mask cable problems. The test needs to be host-to-host on the LAN.
- Reports retransmissions and errors. Marginal cables pass ping and fail on sustained load. Retry counters are the giveaway.
- Cross-platform. Windows on one end, macOS or Linux on the other, is normal.
- Free. Nothing on this list needs a paid license.
- Quick to run. A test that takes 60 seconds gets used; a test that takes an evening does not.
Quick comparison
| App | Best for | Platforms | Free plan | Starting price | Rating |
|---|---|---|---|---|---|
| iperf3 | Raw host-to-host throughput | Cross-platform | Fully free | Free | 4.8 |
| Wireshark | Retransmit and error inspection | Cross-platform | Fully free | Free | 4.7 |
| LAN Speed Test | Simple Windows LAN throughput | Windows | Free lite | About $10 one-time Pro | 4.4 |
| NetIO-GUI | Windows GUI for netio benchmark | Windows | Fully free | Free | 4.2 |
| OpenSpeedTest | Browser LAN speed test | Cross-platform | Fully free | Free | 4.6 |
| TamoSoft Throughput Test | UDP and TCP LAN benchmark | Windows | Fully free | Free | 4.5 |
| Test-NetConnection | Link speed and diagnostics | Windows built-in | Fully free | Free | 4.4 |
The apps
1. iperf3 — Best for raw host-to-host throughput
iperf3 is the reference LAN benchmarking tool. Install on both endpoints (a desktop and a NAS, or two laptops), run one as a server (iperf3 -s) and one as a client (iperf3 -c <server-ip>), and read the number in Gbps. A gigabit link on a good cable reports around 940 Mbps sustained; a bad cable reports 100 Mbps or a flapping number.
The -P 4 parallel-streams flag catches CPU-bound bottlenecks. The -t 60 extended run reveals cables that pass the first ten seconds and fail as they warm up.
Where it falls short: Command-line only. Firewall on the server side needs port 5201 opened. Some Wi-Fi drivers throttle iperf traffic differently than real workloads.
Pricing:
- Free: BSD license
- Paid: None
Platforms: Windows, macOS, Linux, ARM
Download: iperf3
Bottom line: The first tool to run when a cable is suspected. Two commands, one number.
2. Wireshark — Best for retransmit and error inspection
Wireshark is the packet capture tool network professionals use for everything. For cable diagnostics, filter on tcp.analysis.retransmission while running an iperf3 or a large SMB copy across the suspect cable. A healthy cable shows zero retransmissions per gigabyte. A marginal cable shows dozens.
The Statistics → TCP Stream Graphs → Throughput view visualizes speed drops in real time, and the Expert Info panel flags anomalies without knowing packet formats.
Where it falls short: Learning curve is real. Capturing on Windows sometimes needs elevated permissions and an npcap driver reinstall.
Pricing:
- Free: GPL
- Paid: None
Platforms: Windows, macOS, Linux
Download: Wireshark
Bottom line: The tool that proves the cable is the problem when everything else says the network is fine.
3. LAN Speed Test — Best simple Windows LAN throughput
LAN Speed Test by TotuSoft runs a file-based benchmark against a shared folder or another running LAN Speed Test instance. Point it at a folder on a NAS, choose a file size, and it reports write and read speeds in Mbps.
The Lite version is free and enough for cable diagnostics. Pro adds report exports and repeat scheduling.
Where it falls short: File-based tests conflate cable, protocol overhead (SMB), and disk. Not as clean as iperf3 for isolating the cable, but faster to run for non-technical users.
Pricing:
- Free: Lite version
- Paid: About $10 one-time Pro
Platforms: Windows
Download: LAN Speed Test
Bottom line: The right pick when a non-technical family member needs to check the cable to grandma’s room.
4. NetIO-GUI — Best Windows GUI for netio benchmark
NetIO-GUI wraps the classic netio benchmark tool in a Windows GUI. Two machines run it, one as server, one as client, and it reports TCP and UDP throughput per packet size. Small-packet performance is often what cable defects show first.
Uses a small executable with no install required. Portable and quick to run from a USB drive.
Where it falls short: UI dates from the mid-2010s. Not maintained frequently but still works because the protocol is stable.
Pricing:
- Free: Freeware
- Paid: None
Platforms: Windows
Download: NetIO-GUI
Bottom line: Grab this when iperf3’s command line is a barrier for the person running the test.
5. OpenSpeedTest — Best browser LAN speed test
OpenSpeedTest is a self-hosted speed test server you run in Docker on any machine on the LAN. Any device on the network opens a browser tab, points at the server’s IP, and gets a Fast.com-style speed test that stays inside the LAN.
Great for testing multiple client devices (phones, tablets, TVs) without installing anything on them. The result exposes both cable and Wi-Fi issues.
Where it falls short: Requires Docker or a container-capable host. Results are less precise than iperf3 because they include browser overhead.
Pricing:
- Free: Self-hostable
- Paid: None; a hosted version at openspeedtest.com is also free
Platforms: Docker on Windows, macOS, Linux; browser clients anywhere
Download: OpenSpeedTest
Bottom line: The right pick when every device on the LAN needs a quick test with zero install.
6. TamoSoft Throughput Test — Best UDP and TCP LAN benchmark
TamoSoft Throughput Test is a free Windows tool with a friendly GUI that runs simultaneous TCP and UDP tests in both directions between two hosts. UDP loss counters are its differentiator; a marginal cable that hides errors under TCP retransmissions shows them cleanly here.
The chart view over 60 seconds reveals cables that pass and fail in cycles, usually due to thermal issues on the crimped end.
Where it falls short: Windows-only on both endpoints. Mac and Linux users bounce.
Pricing:
- Free: Full features
- Paid: None; TamoSoft’s other tools are commercial
Platforms: Windows
Download: TamoSoft Throughput Test
Bottom line: The right pick when UDP loss on the cable is the actual symptom.
7. Test-NetConnection — Best link speed and diagnostics
Test-NetConnection is the built-in Windows PowerShell cmdlet that reports IP reachability plus a lot more. Combined with Get-NetAdapter | Format-List Name,LinkSpeed,MediaType, it takes ten seconds to prove that Windows negotiated 100 Mbps on a port that should be a gigabit.
For scripted diagnostics across many hosts, PowerShell wins because you already have it installed.
Where it falls short: Not a throughput test by itself. Combine with iperf3 or LAN Speed Test to measure sustained performance.
Pricing:
- Free: Bundled with Windows
- Paid: None
Platforms: Windows 10, Windows 11, Windows Server
Download: Built into PowerShell 5.1+ and PowerShell 7
Bottom line: The first line of defense. Confirms the link speed before you plug anything else in.
How to pick the right one
- If you have shell access on both ends: iperf3. The fastest number-in-hand test on this list.
- If iperf3’s numbers look good but the network still feels bad: Wireshark. Retransmissions or errors mean the cable.
- If a non-technical user is doing the test: LAN Speed Test or NetIO-GUI. GUI, portable, one button.
- If you need to test many client devices at once: OpenSpeedTest. Any browser, any device.
- If UDP loss is the symptom: TamoSoft Throughput Test. Bidirectional and cycle-visible.
- If you just need to confirm negotiated link speed: Test-NetConnection with Get-NetAdapter. No install.
FAQ
Are flat Ethernet cables slower than round ones?
Usually. Flat cables use ribbon-style conductors with thinner shielding, which under load makes them more susceptible to interference and running slower than the marketed category. A gigabit-rated flat cable often negotiates gigabit but hits sustained speeds well below round CAT6.
How do I test if my Ethernet cable is bad?
Run iperf3 or LAN Speed Test between two machines connected by the suspect cable. Compare against a known good round CAT6 cable. If the numbers differ significantly, or if Wireshark shows retransmissions on the suspect cable but not the good one, the cable is bad.
What speed should I expect on a gigabit LAN?
A healthy gigabit link reports around 940 to 950 Mbps in iperf3 after protocol overhead. Anything under 800 Mbps points at the cable, the port, or the driver. Sustained speed dropping over time (from 940 to 400 as the test runs) usually points at a cable.
Do I need special hardware to test cables?
No. Every tool in this list uses software on machines you already own. A physical cable certifier (Fluke, Klein) is only necessary when the cable is inside a wall.
Can Wireshark tell me the cable is bad?
Not directly. Wireshark shows the symptoms (retransmissions, out-of-order packets, small window sizes) that a bad cable produces. Correlating a spike with a specific cable is on you.
Are these all free?
Every tool on this list is free for the features required. LAN Speed Test has a paid Pro tier for reports; the free Lite is enough for diagnostics.
