There is a big confusion amongst computer enthusiasts with all the different kinds of PCI Express (PCIe) slots. There are different generations, different speeds, and different sizes, but why should you care?
In this article, I am going to cover everything you need to know about the PCIe x16, from what it is, what the x16 means, and why you should care.
So, without further ado, let’s get started with the basics and talk about what the PCIe x16 is.
What is a PCIe x16?
A PCIe x16 slot is a slot on the motherboard that is used to connect a PCIe card. This could be a graphics card, an NVMe SSD, or any other expansion card.
The physical size of a PCIe x16 slot is always the same regardless of its version; however, the number of lanes is not always the same. Its length is 89mm, and it has 82 pins that serve as the connection point between the motherboard and the card.
The “x16” part of its name comes from the fact that it has 16 lanes that connect the card to the motherboard. These lanes are used to transmit data at high speeds and determine the card’s maximum speed.
But as I mentioned before, the number of lanes is not the same for all PCIe x16 slots. The most common configuration is 16x, but there are also 8x and 4x PCIe versions.
So when you are looking to buy a new motherboard and read about the number of PCIe x16 slots it has, make sure to check how many lanes each one of them has.
Related Article: What Are PCIe Slots?
What does a PCIe x16 slot look like?
As already mentioned, the PCIe x16 slot is always 89mm long and is the biggest of all the PCIe slots. It is located closest to the CPU socket on the motherboard, and as you can see in the image below, there is a big size difference between it and the other PCIe slots.
Are all PCIe x16 slots the same?
No, not all PCIe x16 slots are the same. As I said before, the difference is not only in the number of lanes that each one of them has but also in their version (generation)
Number Of PCIe Lanes
We already touched a bit on the number of lanes, but let’s go a bit more in-depth.
The PCIe lanes are data paths used to transmit data from the card to the motherboard. The total number of lanes your PC has is determined by your CPU and motherboard. An average number would be around 20, with 16 of them being offered by the CPU and the rest by the motherboard.
So what happens if you have two PCIe x16 slots but your system offers only 20 lanes? What happens, in this case, is that both slots will run in x8 mode. This is because 16 PCIe lanes are physically connected to the first real PCIe x16 slot, and 8 of them continue to the second slot. So if you activate the second slot with a card, the first slot will allocate 8 of the 16 lanes to it.
For example, as you can see in the screenshot below, the MSI MPG X570 GAMING EDGE WIFI runs in three modes depending on how many PCIe x16 slots are populated.
The x16/x0/x0 means that when one slot is populated, it will offer x16 lanes.
The x8/x0/x8 means that when two slots are populated, each one will offer 8 lanes.
The x8/x4/x4 means that when all three slots are populated, two will offer 8 lanes, and one will offer 4 lanes.
Related Article: How Many PCIe Lanes Does Ryzen Have?
PCIe Generation (Version)
The version or generation is the second thing you need to keep in mind when examining a PCIe x16 slot. The standard these days is PCIe 4.0 (with PCIe 5.0 coming soon), but there are also older PCIe generations like PCIe 3.0 and 2.0.
The main difference between them is the maximum data transfer rate they offer. For example, a PCIe 3.0 slot can offer a maximum of 8 GT/s (Giga transfers per second), while a PCIe 4.0 slot can offer a maximum of 16 GT/s.
This means that a PCIe 4.0 card will be able to transfer data twice as fast as a PCIe 3.0 card.
* Something to keep in mind is that at the time I am writing this, most graphic cards are not able to take full advantage of the higher data transfer rates offered by PCIe 4.0.
To better understand the speed difference, let’s take a look at the table below, which shows the maximum data transfer rate for each PCIe version and the number of lanes.
| Generation | x16 GB/s | x8 GB/s | x4 GB/s | x2 GB/s | x1 GB/s |
|---|---|---|---|---|---|
| 5.0 | 63.015 | 31.508 | 15.754 | 7.877 | 3.938 |
| 4.0 | 31.508 | 15.754 | 7.877 | 3.938 | 1.969 |
| 3.0 | 15.754 | 7.877 | 3.938 | 1.969 | 0.985 |
| 2.0 | 8.000 | 4.000 | 2.000 | 1.000 | 0.500 |
| 1.0 | 4.000 | 2.000 | 1.000 | 0.500 | 0.250 |
Related Content: PCIe 2.0 VS 3.0 – What Is The Difference?
So What Does The x16 Really Mean?
While the number after the “x” usually denotes the number of lanes that a slot has, this is not always the case with the x16 slots.
This is because, as I already said, a PCIe x16 slot’s number of lanes is not always 16. It can be 8, 4, or even 1 in some very rare cases.
The “x16” simply means that the slot can physically accommodate a PCIe x16 card.
While the x16 slots are made for x16 cards, you can also plug smaller PCIe cards into these slots, but the card will run at a reduced speed.
For example, if you insert a PCIe x1 card into a PCIe x16 slot, it will work, but it will only be able to transfer data at x1 speed.
Related Article: Which PCIe Slot For GPU Is The Best?
Other PCIe slots and their uses
More often than not, your motherboard will have other PCIe slots, so let’s talk about them briefly.
PCIe x8 Slot
The PCIe x8 slot is very similar to the PCIe x16 slot. The main difference between them is that the x8 slot has only 8 lanes while the x16 has 16.
This slot is mostly used for a second GPU, although you will also find RAID controllers and other high-end expansion cards that require more than x4 lanes and can be plugged in this slot.
PCIe x4 Slot
The PCIe x4 slot has only 4 lanes, and it’s a really common slot.
This slot is mostly used for SSDs that use the NVMe protocol since they don’t need more than x4 lanes to reach their full potential.
You will almost never see a graphics card in this kind of slot because even the slowest ones need at least x8 lanes to work properly.
PCIe x1 Slot
This is the smallest and slowest PCIe slot. It has only one PCIe lane, and it’s mostly used for sound cards, WiFi adapters, and other low bandwidth devices. If you want to know about the PCIe x1 slots, read our complete guide, “What Are PCIe x1 Slots And What They Are Used For.“
What are the PCIe x16 slots used for?
You probably guessed it by now, but the PCIe x16 slots are used for graphics cards. The GPUs are the devices that require the most bandwidth, so they need a slot that can offer them a lot of lanes.
What you should pay attention to when plugging the graphics card into the PCIe x16 slot is to choose the PCIe x16 slot with the highest number of lanes.
For example, if your motherboard has two PCIe x16 slots, make sure to plug your GPU into the one with the most lanes, which is usually the top closest to the CPU.
If you plan to add more than one graphics card to your computer and use SLI or Crossfire, keep in mind that both cards will run at x8 speed in most cases.
Related Article: How To Check If PCIe Slot Is Working? [Solved]
Where are the PCIe x16 slots located?
The PCIe x16 slots are located on the bottom part of your motherboard, close to the I/O ports. This is because the PCIe devices you are going to plug into these slots will have some ports that need to be accessible from the back of your computer case.
What is the difference between PCIe and PCIe x16?
There is no difference between PCIe and PCIe x16. The PCIe x16 slot uses the Peripheral Component Interconnect express interface, and the x16 it’s just a physical specification that denotes how many lanes the slot has.
Do I have a PCIe x16 Slot?
To see if you have a PCIe x16 slot on your motherboard, you will need to find its manual and look for it there.
If you don’t have the manual, the best way to find out is to visually inspect the motherboard. The PCIe x16 slots are longer than other slots (89mm) and are located close to the I/O ports.
Is it significant which PCIe x16 slot I use for my GPU?
Yes, which PCIe x16 slot you use for the graphics card is significant. You should always use the PCIe x16 slot that has the most lanes because it will offer better performance. It is the one that is located closest to the CPU.
Related Content: Does It Matter Which PCI-e x16 Slot I Use?
Conclusion
I think I have covered everything you need to know about the PCIe x16 slots. If you have any questions feel free to ask them in the comments section below, and I will try to answer them as soon as possible.
Nick Ryley is a computer architect and the owner of The Pc Geekz.
His love for playing games got him interested in computers in the first place. He wanted to be able to build a gaming PC that could run all the latest games at max settings, and this has him pursuing a major in computer architecture!
He started this blog to help people out and answer some of the most common questions about computer building
