How to Check Your Ethernet Cable's Speed and Find the Real Culprit
A 100 Mbps reading on a wired connection doesn't automatically mean the Ethernet cable is bad. Tech site Engadget explains how to check the link speed on your computer and narrow down the cause.
If a home has a gigabit internet plan but a wired PC manages only around 90 Mbps, the Ethernet (wired network) cable may be to blame, but not necessarily. Technology site Engadget explains how to check the connection speed and narrow down the possible causes. The advice also applies if file transfers between computers at home are slow.
Finding the link speed
An online speed test gives no definitive answer, because it measures more than the cable. The first step is to check the Ethernet link speed, which requires no tester.
On Windows 11, open Settings > Network & internet > Ethernet and look at the Link speed field. On Windows 10 and 11 you can also press Windows + R, type ncpa.cpl, right-click the Ethernet adapter and choose Status; the rate appears in the Speed field. On a Mac, go to System Settings > Network > Ethernet, then Details > Hardware, and read the Speed field. The configuration should stay on Automatic, since setting a speed manually can cause connection problems.
Seeing 1 Gbps there is not the same as downloading at 1 Gbps: the figure describes only the link between the computer and the router or network switch. The category printed on the cable jacket, such as Cat5e or Cat6, is not an absolute speed ceiling either. Both Cat5e and Cat6 support gigabit Ethernet.
Telling whether the cable is at fault
If the computer reports 100 Mbps when 1 Gbps is expected, check the hardware at both ends. A 100 Mbps port on a router, or a similarly limited USB Ethernet adapter, will cap the connection regardless of how expensive the cable is. Gigabit Ethernet uses all four twisted pairs of wires in the cable, while conventional 100 Mbps Ethernet uses two, so a damaged wire or loose connector can reduce the speed.
Start by reseating the connectors, then connect a short, known-good cable to the same ports. If the reading changes from 100 Mbps to 1 Gbps, the original cable or its connectors are strong suspects. If nothing changes, check the port specifications and adapter settings before buying a replacement.
For a more thorough check, the iPerf3 tool measures real throughput between two wired computers without involving the internet service provider. A multimeter or simple cable tester can reveal some wiring faults but cannot determine the maximum data rate a cable supports; that requires specialized cable qualification equipment.
