If you want to upgrade your PC, you need to make sure you have room to plug them in. Many upgrades, including GPUs, plug into PCI Express slots. Here’s how to tell which PCI Express slot you have, so you know your options.
You can identify your PCI Express cards visually (x1 slots are 1 inch long and x16 are 3.5 inches long), or using the command line from within Windows or Linux. The command line is more reliable and helps you identify an x4 or x8 slot with an x16 connector.
What PCI Express is
Most computers made after 2005 have PCI Express (PCIe) slots. This means that PCI Express is older now than its predecessor was when PCIe arrived on the scene, and older than PCI’s predecessors were when PCI came to be. Not that that’s a huge problem.
The varying length of the slot can make them hard to identify, but the mere presence of slots of widely varying lengths is a good indication you have PCIe. And it helps that consumer motherboards often only have two sizes, the small x1 and the larger x16.
The variations are called “lanes”, and are referred to by the letter x followed by a number (x1, x4, x8, or x16). All of them start with the same small segment. More lanes means more connectors, and therefore a larger second segment. Generally speaking, more lanes translates into greater interface speed.
An x1 PCIe expansion card will fit into a larger slot, even an x16 slot, and work flawlessly. The physical size difference will leave the rest of the slot unoccupied, but this doesn’t cause any problems. Even an x16 card doesn’t necessarily use all of its lanes all of the time.
Most non-video PCIe cards are x1 or x4. GPUs are almost always x16 cards.
PCIe slots can be differentiated from others (particularly 32 bit PCI) by their physical size. All connectors on a PCIe slot are noticeably smaller, and the connector is set farther into the motherboard than other PCI slots.
Identify PCI Express slots by measuring
If you have your computer open, one way to identify PCI express slots is to just measure them. A x1 slot is around 25mm long, or 1 inch.
An x4 slot is 39mm long, or about 1.5 inches.
An x8 slot is 56mm long, or just over 2 inches.
Finally, an x16 slot is 89mm long, or about 3.5 inches.
When x16 isn’t x16
Many motherboards have two x16-sized slots on them, but the second slot doesn’t always have all 16 lanes connected. Sometimes the second x16 slot is a x8 or even x4 slot with an x16 connector. You can still use a GPU in this slot, but you will get somewhat lower performance.
And for that matter, if you want another GPU and don’t have an x16-sized slot available for one, with an adapter you can use a x1 slot.
Identify PCI Express slots with software
You can identify what PCI Express slots you have and whether they are occupied or not from inside the operating system as well.
Click the Windows icon in the lower left or hit the Windows key, then type cmd and hit enter to open a command prompt, then issue the following command:
WMIC SYSTEMSLOT GET /VALUE | MORE
The output is rather cryptic but will at least tell you what slots you have.
If you have PCIe x16 slots that are actually x4 or x8 slots, this command can tip you off, as they will show up as x4 or x8 slots. On consumer boards, x4 and x8 slots are fairly rare, so there’s a good chance any x4 or x8 slots have an x16 connector.
Open a terminal window, then issue the following command:
sudo dmidecode -t slot | more
The output will tell you what slots you have and if they are occupied or not. This is a case of Linux being less cryptic than Windows.
Like the Windows counterpart, if you have PCIe x16 slots that are actually x4 or x8 slots, this command can help identify those.
If you’re exploring upgrade options, you can use a similar methodology to identify what motherboard you have.