USB-C

How to check a USB-C cable's real speed on your Mac

July 24, 2026 · Cablyst

USB-C cables range from 480 Mbps to 40 Gbps and beyond — an 80× spread — and every one of them has the identical connector. The cheap one that came with a phone and the one that runs an external SSD are indistinguishable by eye. Here's how to find out how fast a USB-C cable really is, on a Mac.

What the speed numbers actually mean

USB-C is just the plug. The speed depends on the protocol the cable is wired for:

• USB 2.0 — 480 Mbps. Extremely common. Most bundled and charging-only cables.
• USB 3.2 Gen 1 — 5 Gbps
• USB 3.2 Gen 2 — 10 Gbps
• USB 3.2 Gen 2×2 — 20 Gbps
• USB4 — 20 or 40 Gbps
• Thunderbolt 3 / 4 — 40 Gbps
• Thunderbolt 5 / USB4 v2 — up to 80–120 Gbps

The single biggest trap: a USB 2.0 cable physically lacks the SuperSpeed wires for anything faster. No firmware update or better port can rescue it — it will forever be 480 Mbps. That's why an external SSD sometimes crawls: the drive and the port are fast, the cable isn't.

Check the negotiated speed in System Information

macOS reports the speed a connected device actually negotiated:

1. Connect a device (an SSD or hub is ideal) through the cable.
2. Hold Option → Apple menu → System Information.
3. Open USB in the sidebar and select the device.
4. Read the Speed line: Up to 480 Mb/s, Up to 5 Gb/s, Up to 10 Gb/s, and so on.

If a fast SSD reports Up to 480 Mb/s, you've caught a USB 2.0 cable red-handed.

Important limitation: this shows the speed of this device on this cable — the lower of the two. A fast cable with a slow device still reads slow. It tells you the link, not the cable's ceiling.

Read the cable's real ceiling from its e-marker

To know a cable's maximum — not just how fast today's device happened to run — you need the e-marker: the chip inside the plug that declares the cable's rated speed and power. Any cable rated above USB 2.0 for data or above 3 A for power is required to have one.

Cablyst reads that e-marker directly and shows the cable's rated data rate per port — so you learn a cable is a 40 Gbps cable without owning a 40 Gbps device to test it against. It also flags when a capable cable is running below its rating because the device or port is the bottleneck, not the cable.

Why a "fast" cable can still test slow

If your measured speed is lower than expected, the cause is one of:

• The cable is a lower tier than you thought (most common — check the e-marker).
• The device can't go faster (a USB 3.0 drive won't hit 10 Gbps).
• The port you used is slower (not all ports on a hub or Mac are equal).
• An adapter or hub in the chain downshifted the link.

Cablyst separates these so you're not swapping cables blindly trying to find the weak link.

The 30-second version

1. Plug the cable into a device, open System Information → USB, read the Speed line.
2. A fast device stuck at 480 Mb/s = a USB 2.0 cable.
3. That's the device's speed, not the cable's ceiling — for the true maximum, read the cable's e-marker with Cablyst.

Want the real, hardware-level answer for your own cable? Install Cablyst or get Pro ($9.99 lifetime).