PCIe Slots Explained: x1, x4, x8 and x16, and What Fits Where

The number after the x counts lanes, and the physical connector grows with it, so an x16 connector is the longest. A shorter card can sit in a longer slot, which is why an x1 Wi-Fi card works in an x16 slot. A longer card does not go into a shorter slot unless the slot is open-ended, which some boards use to accept longer cards at reduced width.

The electrical width is separate. Boards often include a second full-length slot that is wired at x4 through the chipset, and short slots that are x1. The manual lists each slot as, for example, PCIe 4.0 x16 or PCIe 3.0 x1, and indicates which ones share lanes. A card in a slot with fewer lanes than it was designed for still runs, but at lower bandwidth.

Generation works the same way. PCIe is backwards and forwards compatible, so a card and slot negotiate to the highest generation both support. A Gen4 card in a Gen3 slot runs at Gen3 speed, and bandwidth per lane roughly doubles with each generation.

Most expansion cards need modest bandwidth. Wi-Fi cards, sound cards and basic USB or SATA controllers generally use x1 lanes, so a chipset-connected x1 slot is a good place for them. Cards that move more data, such as 10 Gigabit network cards, multi-port USB controllers or NVMe adapters, usually specify x4 or more, and the product page lists the width and generation it needs.

Your graphics card should keep the primary CPU-connected x16 slot. Placing another card in a nearby slot may disable lanes or physically block cooler airflow, and the manual's sharing notes tell you if putting a card in a particular slot reduces the graphics slot to x8. A thick graphics card can also cover the slot beneath it, so check which slots stay usable.

Capture cards and some professional cards specify the lanes they need to run at full rate. A card that asks for x4 in a chipset slot that is x1 will either not work or will be limited, which the card's maker usually states in the specification.