Expansion cards may be added to your motherboard through PCIe slots, allowing you to expand the capabilities of your computer. Various sizes are available, each tailored to a particular kind of expansion card. PCIe slots may be used to install a game graphics card, a WiFi network card, or a sound card.

SATA and PCIe are two of the most common ways to expand a computer’s capabilities. SATA is a slower interface that is used largely for spinning hard drives and CD/DVD drives these days. When it comes to data transfer rates and high-speed components, PCIe outperforms NVMe SSDs all the way to graphics cards.

There are many sizes of PCIe slots, and understanding how they work and what distinguishes one from the other is critical for PC builders. In this section, we’ll go over PCIe and PCIe slots in great detail.


In order to understand what PCIe slots look like, a picture would be the best guide.


As you can see in the image above, PCIe slots come in different sizes. They are distinguished mainly by their PCIe lane count.

What is PCIe?

Internal high-speed components are connected to your desktop motherboard through the PCIe bus, which is known as Peripheral Component Interconnect Express (PCIe). Things that can be attached to the PCIe slots may be found on this page. Here is a list of them:

PCIe Lanes in Brief

A thorough grasp of the workings of PCIe lanes is critical to comprehending PCIe slots. To put it another way, a PCIe lane is just the conduit via which data moves between a slot and the CPU. Different PCIe lanes may be found in different slot sizes. The greater the slot’s throughput rate (speed) and the more demanding expansion cards it can accommodate are directly proportional to the number of PCIe lanes it has.

Slots with PCIe lanes of type x1, x4, x8, or even x16 may have any one of these four PCIe lane configurations. You can usually identify how many PCIe lanes a slot has by the number that follows “x”. In terms of the x16 slot, the number 16 here following “x” does not necessarily represent 16 lanes; this will be explained further down.

This means that PCIe x4 slots have 4 PCIe lanes, which means that an NVMe SSD expansion card, for example, may be installed in one of these slots. The overall number of PCIe lanes in a PC is usually determined by the CPU and the motherboard. It is not possible for a PC to have limitless lane capacity. The CPU spec sheet and the motherboard CHIPSET spec sheet may both tell you how many PCIe lanes a PC has.


Only the most extreme and high-end CPUs have dedicated PCIe lanes of 44 or more, which are the norm for most mainstream processors.

Although motherboards also include PCIe lanes, not all of them may be used by the user. You may only have 4 to 6 PCIe lanes available for usage on an Intel Z490 motherboard, while the remainder of the board’s internal components (SATA ports, USB ports, etc.) would use the balance of the available lanes.

So, with a total of 20 PCIe lanes available, you’ll be able to utilise 16 CPU lanes plus four offered by the motherboard chipset. A single x16 graphics card and an NVMe SSD Card may be added with this. A single graphics card (x16), one WiFi card (x1), and one FHD video capture card (x1) may all be added while still maintaining two PCIe lanes available. All of this hinges on the motherboard’s configuration and the slots it offers..

Both the CPU and motherboard data sheets include the number of PCIe lanes. PCIe lanes are covered extensively in the following manuals.



At the very least, current desktop PC motherboards come with one PCIe slot, which lets you connect additional peripherals like video cards, sound cards, and RAID controllers to your computer. The main distinction is that not all motherboards have the same number, kind, or even version of PCIe slots.

Motherboard Form Factor Matters in PCIe Slot Configuration

In addition to the overall number of PCIe Lanes, the motherboard form factor has a significant impact on the number of PCIe slots you have.

For example, an ATX motherboard often has a large number of PCIe slots of various sizes. In contrast, although supporting more than 16 PCIe lanes, the smallest desktop motherboard, the Mini-ITX, only has a single PCIe x16 slot!

Different PCIe Slot Sizes and Their Uses

There are typically two PCIe slot sizes found on almost all commercial motherboards:

  • x1: This offers only a single PCIe lane. It is suitable for weaker and smaller cards like a WiFi card or a sound card.
  • x16: This is the largest slot found on a motherboard and also the most confusing to understand. More on this below.

The Curious Case of the x16 Slot


Many times, an x16 slot will only have x8 or even x4 lanes wired into it! The real lane count of a slot can only be determined by consulting the specifications page.

If your motherboard only has one x16 slot, then your CPU is almost certainly driving 16 PCIe lanes.

Only one of the two possible configurations is possible when you have more than one 16-slot PCIe adapter. This is because your PC only has a restricted number of PCIe lanes. A common PC with 20 to 24 PCIe lanes can’t have two full x16 slots since it would bring the total lane count to 32!

There are three ways to configure an x16 slot:

  • x16(x4): This slot has the physical size of an x16 slot but is hardwired to only 4 lanes. This slot is used for devices like an NVMe M.2 SSD expansion slot or 4k video capture card, that require a higher bandwidth than an x1 slot an offer.
  • x16(x8): These are found on premium motherboards for dual graphics card builds. Again, this slot has the physical size of an x16 slot, but only has four lanes connected to it.
  • x16(x16): A true x16 slot that connects to x16 lanes is found on almost all motherboards. They have the highest throughput rate and thus these are typically occupied with graphics cards. 


Among the finest features of the PCIe standard is its cross and backward compatibility across various PCIe devices and generations. There’s no need for additional software to get past the x1 limitation if you have an x4 slot and the device you wish to install is x1.

Plugging in an extension card with a lower lane count into an expansion slot with a greater lane count will allow the device to function as intended, although at the combined bandwidth of the slowest component. It is possible to put a weaker x1 device into a stronger x4 slot, but the x1 device will not benefit from this.

The same holds true for expansion cards with a larger lane count, as long as they fit. Even while your graphics card will still operate, it will only run at the x8 slot’s speed, which means that you may see a decrease in performance when using an x16 card in an x8 slot. As a result of this compatibility, a PCIe 3.0 device may be used in a PCIe 2.0 slot and vice versa.

The sole restriction is that the configuration must operate at the lowest speed possible between the slot and the component.




A typical mid-range full ATX motherboard’s PCIe slots are laid out in the manner shown above. The motherboard shown above has three PCIe x16 slots (two in silver colour and there is one black coloured x16 slot at the very bottom that may be hard to see).

It has been indicated that this motherboard may only be utilised in x16/0/4 or x8/x8/4 modes because to the Lane constraint.

Mode 1: ( x16/x0/x4)

  • Top slot working in x16 mode i.e with a single Graphics Card.
  • Middle slot empty
  • Bottom slot working in x4 mode i.e for an NVMe SSD expansion card for instance.

Mode 2: (x8/x8/x4)

  • Top slot working in x8 mode i.e with a Graphic Card
  • Middle slot working in x8 mode i.e with another Graphics Card
  • Bottom slot working in x4 mode i.e for an NVMe SSD expansion card for instance.

Mode 2 is ideal for SLI or Crossfire configurations that use two graphics cards. The main drawback is that only half of the available bandwidth will allow both graphics cards to run at their optimal levels. However, in our testing, the difference between using the card at x16 and x8 was negligible. Additionally, this motherboard features two x1 slots that may be utilised for smaller cards as well.

Two full x16 slots will need a workstation-grade CPU (such as an Intel Xeon or AMD Threadripper) and a workstation-grade motherboard (which has additional PCIe lanes) to support the increased bandwidth demands of dual x16 slots at full speed.

Difference Between PCI vs PCIe


PCIe replaces the shared bus of the earlier PCI standard with a system of separate transfer lanes. An increase in bandwidth may be achieved by the use of several lanes.

The primary distinction between PCI and PCIe was the former’s use of parallel architecture, while the latter made use of serial architecture. With the PCI parallel design, there was a single bus for all of the linked devices.

As each device connected to the network shared the same bus, further devices would have to wait for the bus to clear up before they could transmit and receive data. There are now separate buses for each and every device in PCIe, a serial design that radically transformed the topology.

When comparing the two user interfaces, the disparity in responsiveness was staggering. Whereas the bidirectional bandwidth of PCI-X 2.0’s final PCI-X 2.0 version peaked in 2002 at 2.13 gigabits per second (GB/s), PCI Express v1.0’s debut in 2002 boasted an astounding bidirectional bandwidth of 8 gigabits per second (GB/s). This is a massive improvement over the previous version.

When it comes to bi-directional bandwidth, it’s now at roughly 64 GB/s with the current v4.0.


Using a PCIe slot to attach high-speed expansion cards to your PC is the most common method of enhancing its capabilities. PCIe lanes, slot design and size, and PCIe version were all examined in detail in this section to help clarify what PCIe slots are. Of course, it’s possible that your system may never need the usage of PCIe slots as all.

Even if your CPU has an integrated graphics card and your motherboard has display output ports, a soundcard, and a network card, you may never need any expansion card since these features are already built into the motherboard.

When it comes to typical home and business users, this is especially true. The motherboard and CPU of a basic home PC are pre-installed with everything you’ll need. PCIe slots are often only needed by professionals and serious gamers that want to expand their systems’ capabilities.

Bryan V. Root
Bryan V. Root
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