About three years ago, I showed you a great $8 cable tester that quickly tells you if your USB-C cable is potentially fast, slow, powerful, or weak. Sadly, that gadget got discontinued, and I’ve never found anything quite as intuitive or affordable since. But if you have a Mac with Apple Silicon chips, you can download an even more impressive tester for free.

It’s called WhatCable.and it works by reading the data your Mac collects about previously connected USB devices, data that Apple doesn’t normally pass on to you. Just click a small widget that sits in the menu bar at the top of your Mac, and you can see every USB-C cable and device connected to your computer.

Here’s how creator Daryl Morley explained it to me:

Every Apple Silicon Mac has a port controller chip that handles USB power delivery negotiations. When you connect a cable with an e-marker, the port controller sends a “Discover Identity” message to the chip in the cable and gets back a structured message: vendor ID, speed rating, current rating, voltage limits, whether it’s enabled or disabled, etc.

macOS writes this response to the IOKit registry. WhatCable reads it using Apple’s public APIs. No root access, no private privileges. The data is not hidden, Apple’s firmware does the negotiation and publishes the result. It is not exposed anywhere in the standard macOS tooling. WhatCable reads what’s already there.

E-Marker is a resource. Watt also reads from the cable Mac’s own hardware, the actual negotiated connection speed, the Thunderbolt link speed, and the live voltage and current on each port. An attached device tells us what it is, who made it, and what it supports. Put the three together, the cable, the device, and the Mac, and WhatCable can tell you not only what each item claims to support, but what’s actually happening on the connection at that moment, and which part is the bottleneck if something isn’t performing as expected.

Want to see it in action? I took pictures this week testing some of my favorite cables. It’s not a perfect solution, as cables can lie about their capabilities, but WhatCable helped me find a bad cable along the way.

When I plugged the short, lightweight Satachi cable you see above into the two ports on my MacBook Pro, I got this:

I know from experience that this information is correct, and that means it’s still a valuable cable. 480Mbps USB 2.0 is pretty slow, but the cable itself is reporting that it can charge at 100 watts, about as fast as my Mac can charge.

This is slightly more useful information than my $8 tester provided. It also shows that the cable only offers USB 2.0 speeds and maybe 60W or better charging because there is an E-marker. But it cannot read the e-marker data to indicate that the cable supports 100W charging speed.

Sure enough, I’m getting more than 60W when I put a 140W battery in my Mac:

WhatCable can detect that I’m connected to a 100W charger too:

Now, let’s try one of my five favorite USB-C cables yet — my 10Gbps, 100W Supercalla cable with magnetic winding beads:

It’s weird: the cable’s e-marker claims it’s 10Gbps and 100W, but the Mac isn’t treating it that way!

When I plug in a faster 10Gbps SSD I’m not getting the same speed with this cord:

And it looks like that’s because my daily driver cable is finally running out. Guess it’s time to retire it!

Now let’s try the theoretically latest and greatest cable in your drawer: a 240W USB4 40Gbps cable.

Again, EMarker seems to validate these speeds, even if the Mac isn’t connecting at that rate itself.

Once I plug in the drive, WhatCable detects that the Mac has a 10Gbps link:

Here’s what it looks like: This 25GB transfer is measured in seconds instead of minutes:

This is a cable that just arrived at my house the other day specifically for 100W charging. I’m not expecting more than USB 2.0 480Mbps data. On Amazon, the company only advertised USB 2.0 speeds:

But WhatCable says its own e-marker advertises 10Gbps USB 3 data… could it be?

I’m not afraid: This cable’s e-marker has written a check that his body can’t cash. Minutes, not seconds, for the same 25GB transfer:

Here, my $8 tester did a better job, quickly detecting that the cable doesn’t support SS (SuperSpeed, aka USB 3).

It provides at 5MP charging speed, though:

Next, I thought I’d plug in my magnetic accordion USB-A to USB-C cable, which is definitely only capable of 480Mbps USB 2.0 speeds:

Strangely, the Mac insists it’s running at 10Gbps… while connected to my external battery. It sounds wrong!

Last but not least, this is the old faithful cable that came with a LaCie drive I bought back in 2019, which I’ve always turned to for stability and speed:

It’s reporting as a 20Gbps Thunderbolt cable, even though it says 10Gbps at the end. I don’t have one working, but I’ll have to try it with a Thunderbolt drive to check!

Morley isn’t the first to realize that the MacBook could be a USB-C cable tester. USB connection information There is a similar paid app that came out a year ago. But Morley’s version is free, and he tells me it will “always be free at its core,” though you can pay £9.99 to get in. Pro version which offers real-time power monitor, diagnostics, and terminal view.

He has now created an even simpler version of the idea. Called WhatPort which easily monitors what each of your Mac’s USB-C ports is doing at the moment, including power, data and video.

Morley tells me he won’t be able to build a version of WhatCable for Windows because “there’s too much hardware variation and the Windows APIs don’t reflect what WhatCable needs,” and says Android and iOS don’t provide the same low-level access.

“If anyone has a solution, I’d love to hear it,” he says.

But he is already working on a Linux port and continues to update the Mac version. You can follow along with updates. on its GitHub page.

Photos by Sean Hollister/The Verge