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The quickest way to check an Ethernet cable’s speed is to read the negotiated link rate between your computer and the router or switch. On Windows, open the Ethernet adapter’s Status window and read Speed. On macOS, open System Settings > Network > Ethernet > Details > Hardware and read Speed and Duplex. On Linux, query the active wired interface with ethtool. These readings show the local Ethernet link—not the speed supplied by your internet provider.
What the displayed Ethernet speed actually means
A value such as 100 Mbps or 1.0 Gbps is the rate negotiated between your computer’s Ethernet adapter and the device at the other end of that cable, usually a router or switch. It is not an internet speed-test result. A cable can negotiate at 1 Gbps while your internet plan, router, or ISP delivers less; conversely, a 100 Mbps negotiation can limit a faster internet connection.
The operating-system check needs only one connected device. To measure how quickly data actually moves across your local network, use two devices and a tool such as iPerf3.
Check the negotiated link rate
Windows 10 and 11
- Connect the computer to a LAN port on the router or switch. Turn off Wi-Fi if you want to isolate the wired connection.
- Press Windows key + R, type
ncpa.cpl, and press Enter. - Right-click Ethernet (or Local Area Connection) and choose Status.
- Read the Speed field. Record the value, such as 10 Mbps, 100 Mbps, or 1.0 Gbps.
This is the negotiated speed for that adapter-to-port link. It does not measure your WAN or internet throughput.
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macOS
- Open Apple menu > System Settings > Network.
- Select the active Ethernet service and click Details.
- Open Hardware.
- Read Speed and Duplex.
If several network services are listed, select the Ethernet service that shows as connected. Wi-Fi can remain configured, but disabling it makes it easier to ensure your test traffic uses the cable.
Linux
Linux reports the negotiated rate with ethtool. First identify the active wired interface; names vary by distribution and hardware (for example, a system may use a name other than the traditional eth0). Then run:
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ethtool <interface>In the output, check the current speed and duplex fields. Replace <interface> with the actual wired interface name; do not treat the placeholder as a command you can run unchanged.
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| Method | What it measures | ISP involved? | Devices required | What it can isolate |
|---|---|---|---|---|
| Windows, macOS or Linux link-rate reading | Negotiated rate between one adapter and the router or switch port | No | One | Whether the link has negotiated at the expected rate; it does not separate cable, port and adapter performance by itself |
| iPerf3 | End-to-end throughput on your local IP network, with loss and related measurements | No, when both endpoints are on the same LAN | Two capable devices | Practical LAN performance and differences between endpoints, ports, cables and software conditions |
| Public internet speed test | Throughput over the path to an ISP or remote test server | Yes | One device and internet access | WAN/internet performance, not a cable-only result |
Measure local-network throughput with iPerf3
iPerf3 performs active measurements to determine achievable bandwidth on IP networks and reports throughput, loss and other parameters. Because the test runs between two devices on your LAN, it avoids confusing the cable with your ISP connection.
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- Place two capable devices on the same LAN, preferably with both connected by Ethernet. Note each device’s local IP address.
- On device A, start the iPerf3 server:
iperf3 -s- On device B, run a 20-second client test, replacing the placeholder with device A’s local IP address:
iperf3 -c <device-A-IP> -t 20- If direction matters, repeat with the roles reversed. Keep both devices awake, note each adapter’s negotiated rate and CPU load, and record whether either endpoint uses Wi-Fi.
Use compatible iPerf3 versions on both devices. A version mismatch can prevent the client and server from working together. Repeat tests under the same conditions rather than applying a universal pass/fail number: results depend on the hosts, protocol, CPU load, software filters and link rate.
What to do when the result is lower than expected
It shows 100 Mbps instead of 1 Gbps
Inspect every part of the path: the computer’s adapter, the cable, wall jacks or patch panels, and the router or switch port. Replace the cable with a known-good Cat6 Ethernet cable (Cat6 or better is a sensible candidate for 1 Gbps troubleshooting), then check the negotiated rate again. Cat6 guidance does not prove that the original cable was the only fault; every endpoint and port must also support the target rate.
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It shows 1 Gbps, but internet downloads are slow
The cable has negotiated at gigabit speed, so do not assume it is defective. Run an iPerf3 test between two local wired devices first. If local throughput is healthy, investigate the WAN path, router, internet service or the remote server instead.
iPerf3 throughput is unexpectedly low
- Confirm both hosts and their adapters support the intended rate.
- Check negotiated speed and duplex on both endpoints.
- Inspect or replace the cable and try another router or switch port.
- Check CPU load and temporarily account for security software or other traffic that may filter or consume bandwidth.
- Ensure no Wi-Fi hop is included when you intend to test an all-wired path.
Change one variable at a time and repeat the same test. A low iPerf3 result alone cannot identify whether the cable, adapter, port, host or software is responsible.
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When a cable tester helps
An Ethernet cable tester (also called an RJ45 cable tester) can be useful for intermittent links or suspected wiring faults, especially in installed runs with wall jacks. Choose a tester that matches the connector and wiring you need to check. A tester complements, rather than replaces, the link-rate and iPerf3 checks: it can reveal wiring continuity problems, while the other methods show negotiated rate and usable network performance.
Quick Recap
How to record a meaningful result
- Write down the negotiated rate, duplex setting and which router or switch port you used.
- Record cable category, endpoint models or capabilities when known, and whether each device was wired or wireless.
- For iPerf3, save the command, test duration, direction, CPU conditions and measured throughput.
- Repeat under the same setup after each change so you can identify which variable affected the result.
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