A GPU is designed to use 16 PCIe lanes. However, once you start delving into multi-GPU configuration then things start to change. With a multi-graphics card setup, the GPU may use 8 or even 4 lanes depending upon if its an AMD or an NVIDIA GPU.
Here we look in detail into how many PCIe lanes does a GPU use and how the lanes are divided among the different PCIe slots.
However, one thing that you need to understand is that the amount of PCIe lanes you have in your system are limited. Most builds have 16-20 PCIe lanes available from the CPU and 4 from the motherboard Chipset.
The 16 PCIe lanes from the CPU are generally intended for the graphics card. If you have a budget motherboard, then you may only have a single X16 slot. The 16 CPU lanes connect to this slot.
If you have mid range motherboard, you can have two x16 slots. However, due to their limited nature, the first x16 slot still connects to the CPU PCIe lanes, whereas the second PCIe x16 connects to only 4 lanes from the motherboard chipst.
Some motherboards intended for multi-GPU setup offer two x16 slots that divide the lane count between the two so that each operates at 8 lanes IF two dedicated cards are installed.
NVIDIA graphics cards require at least 8 PCIe lanes to operate. AMD Graphics cards, on the other hand, can work with with 4 lanes as well. Hence, you can install an AMD graphics card on an x4 PCIe slot as well.
Installing the graphics card on x8 and x4 slots does reduce their performance but not by a huge factor. For instance you may believe that a graphics card installed on an X8 slot would reduce its performance by 50% (as it uses half the lanes it is designed for), but that is hardly the case. The difference is generally marginal.
If all this sounds confusing to you, do not fret as I plan to demystify all this by the end of this article.
So How Many PCIe Lanes Does a GPU Use?
Essentially, majority of the GPUs are designed to utilize 16 PCIe lanes ideally. As such they are some of the biggest and the most demanding expansion cards that you can install on a motherboard.
It is always wise to install a graphics card on the primary, or the top x16 slot, to give it as much bandwidth as possible. The primary, or the top, PCIe x16 slot connects directly to the 16 PCIe lanes coming from the CPU.
However, in the case of multiple graphics cards, then as mentioned earlier, the GPUs would operate with 8 or 4 lanes bandwidth. This is the case with most commercial and gaming systems.
Workstation grade PCs, however, have a huge amount of PCIe lanes. Workstation grade processors from the AMD Threadripper series, for instance, can have upto 120 PCIe lanes and their specialized TRX40 motherboards can even support 4 graphics cards on 4 dedicated PCIe X16 slots all using 16 lanes each. For the purpose of this article, we will stick to basic home, commercial and gaming based setups.
Exceptions – Weaker Graphics Can Require Lower Lane Count
There most certainly are exceptions here. Entry-Level dedicated graphics cards like the NVIDIA GT 710 below, can utilize a lower lane count than 16.
The following NVIDIA Geforce GT 710, for instance, uses 8 PCIe lanes.
Similarly, some graphics card even as low as only a single PCIe lane. They have an x1 connector and can be connected to an x1 slot on your motherboard.
NVIDIA GT 730 below is an example of a graphics card with an x1 connector.
Again, graphics card with x1 lane requirement are very weak and only intended for enabling basic video output for your PC.
The Version of the PCIe Lanes Also Matters
Along with the number of PCIe lanes required, equally important is the PCIe version of the lanes. Every successive PCIe version doubles the per lane transfer speed compared to the previous generation.
The following table shows the speeds of different PCIe version and their corresponding lanes.
Therefore, a graphics card that is designed to utilize 16 v3.0 PCIe lanes, can get bottlenecked if you were to install it on an x16 V2.0 PCIe slot.
Newer gen graphics cards such as those from the NVIDA RTX 3000 onwards and AMD Readeon 6000 onwards are PCIe 4.0 based and thus benefit from newer systems that conform to the PCIe 4.0 protocol.
The following study from PugetSystems shows that a PCIe 4.0 RTX 3090 looses some performance when plugged into an a PCIe 3.0 slot.
PCIe Slot Size Does NOT Correspond to its Lane Count!
This is a very important point to take not off.
Many confuse the PCIe slot size with the number of lanes it has. That is hardly the case. You may believe that an x16 slot offers 16 lanes as a rule, but that is not true.
For instance, sometimes you can find a PCIe X16 full sized slot offering only 8 lanes. Sometimes it can even offer a mere 4 lanes despite having the same large size.
Take for instance the following ASUS Strix Gaming motherboard, it offers 3 PCIe x16 slots, however, the second and third slot offer only 8 and 4 lanes respectively despite having the full x16 physical size.
Hence, size of the slot does not correspond to the PCIe lanes it has to offer! You must always consult with the technical specs of your motherboard to check how many PCIe lanes a slot offers.
Also Read: Can You SLI Two Different GPUs?
Which Slot to Install the Graphics Card On?
Most Modern GPUs have an x16 bus interface and hence, the most ideal slot is the true x16 slot.
On most motherboards, the first x16 slot offers full 16 lanes and hence that is the most ideal slot to plug your graphics card in.
Of course, the number of lanes that a GPU takes will also depend upon the number of graphics cards you will install.
If you have a single graphics card, things are easy – just plug it into the first PCIe x16 slot. If you have two graphics cards or more then there are few things you need to understand:
- The motherboard must have the required amount of PCIe slots for multiple GPUs.
- On commercial PC, the PCIe lanes are limited. A system often has 20-24 PCIe lanes. Hence you cannot have 2 full x16 slot for 2 graphics card as that would amount of supplying 32 PCIe lanes – more than the total available. As a result, the PCIe lanes between the x16 slots are divided.
A Practical Example
Let us look at the how the lane configuration changes depending upon what motherboard you have and which slots you occupy.
The following motherboard has two PCIe x16 slots. These two slots on the motherboard work in either 16/0 or 8/8 configuration. What does that mean? Basically, it points to three things:
- If you put a single graphics card in the top slot, it will utilize all 16 PCIe lanes. The second, bottom PCIe slot, as a result, will not utilize any PCIe lanes.
- If you occupy both slots with a PCIe card, then each of them will operate at x8 bandwidth max.
- If you occupy ONLY the second PCIe x16 slot, both of them will still operate at x8/x8 bandwidth. For this reason, if you have a single graphics card, always use the top slot.
So the top slot works in either 16 or 8 lanes configuration and the bottom slot works in either 0 or 8 lanes configuration.
Do note that many budget motherboards only have a SINGLE PCIE x16 slot.
Also Read: Can a Motherboard Bottleneck a GPU?
x16 PCIe Slot with 16 Lanes
In the following image you can see that if the graphics card occupies the first PCIe x16 slot, it will receive complete 16 lanes bandwidth.
With this slot, you get a total of 16 lanes. This slot offers the highest bandwidth on a given commercial motherboard.
If you only have a single graphics card, this is the slot you will want to use.
Since it provides the highest bandwidth, you can expect to get the best performance from your graphics card when you use a x16 slot.
X16 PCIe Slot with 8 Lanes
If you occupy the second x16 PCIe slot with you GPU, both slots will divide their lanes and operate at the bandwidth of 8 lanes each as shown in the picture below.
This slot looks similar in size to the X16 slot on many boards, however, it has half the lanes of the latter and half the bandwidth.
Nevertheless, this slot will run a graphics card too but only at x8 bandwidth. For this reason, you may end up underutilizing the GPU. However, the performance difference is only marginal.
Also Read: Which PCIe Slot for GPU is Ideal?
X16 PCIe Slot with 4 Lanes – A Peculiar Case
This slot also looks like a full x16 sized slot but only offers 4 lanes. These are found on a lot of motherboard.
These can also be used for graphics in two instances:
- For weak entry level graphics card designed for x4 lanes.
- For a multi-GPU setup FOR AMD CROSSFIRE ONLY.
You see, AMD graphics cards can operate on 4 lanes. NVIDIA graphics cards cannot. NVIDIA GPU’s need at least 8 lanes to run.
In other words, if you have two x16 slot – one being the primary x16 (16 lanes) slot and the other being the x16 (4 lanes) slot, you can have dual GPU AMD CROSSFIRE setup, albeit the performance of the second graphics installed in the x4 slot can get clocked down due to the slot’s lower lane count.
Also Read: How Many PCIe Cables Do I Need?
With GPUs, you always want to ensure that you’re getting the maximum power you can from the card.
Knowing the amount of PCIe lanes a graphics card needs and how the PCIe lanes are distributed among the slots will make sure you do not underutilize your card or procure components that go beyond the PCIe lane count that you PC can support.
So the simple answer to the question: how many PCIe lanes does a GPU use is 16. However, when you have multiple graphics card then that is another story. Also, there are exceptions as some entry-level GPUs are designed to utilize 8 or even 4 PCIe lanes only.