Simply put, a PCIe x4 card CAN fit in a PCIe x16 slot. The main reason this is possible lies within the fact that PCIe standard is cross-compatible as well as backward compatible.
Meaning, PCIe devices can work in different slots as long as they are provided the required PCIe lanes. They can also work across different generation provided that they get enough bandwidth.
So Can PCIe x4 Card Fit in x16 slot? Yes, it can. However, whether this is a good idea or not is something we explore further in this article.
The key point to note here is that the PCIe device and slot work at the speeds of the slowest component. So putting the x4 device in an x16 slot will not result in any performance boost.
Let’s explore this topic a bit in depth below.
What are PCIe x4 and x16 Slots?
Image: NVMe SSD Expansion Cards Require x4 PCIe Lanes
Before we dive deeper into the subject, let’s first get acquainted with the two PCIe configurations that we will be discussing.
For starters, different PCIe cards have different PCIe lane requirement. An NVMe SSD Expansion Card, for instance, has a requirement of four PCIe lanes. The more demanding a card and the more data it generates, the more PCIe lanes it will require.
A PCIe x4 slot comes with 4 PCIe lanes. What this means is that the slot has 4 channels through which data is transmitted between the PCIe device and the computer’s processor.
PCIe lanes are scalable. Meaning if a single PCIe lane has speeds of 1.0 GB/s, 4 PCIe lanes will have four times the speed.
A true x16 slot is the largest PCIe slot on most motherboards, both in terms of connectors and the number of lanes it has to transport the data with.
It has quadruple the lanes compared to an x4 slot and are idea for demanding expansion card like graphics cards.
x4 Slots Look the Same as x16 slots
In most cases on modern motherboards, an x4 slot has the physical look of an x16 slot.
Image: Gigabyte GA-P67A-UD3, has two x16 slots (Blue) but the bottom one only has 4 lanes, hence it is an x4 slot in terms of performance.
Gigabyte GA-P67A-UD3 above has an x16 slot but connected only to 4 PCIe lanes.
Hence, when looking at the PCIe slots, particularly the x16 slots, it always important to consult your motherboard specifications to learn about its true PCIe lane count.
Also Read: Does it Matter Which PCIe x16 Slot I Use?
PCIe Version Matters
Another very important point to note is that every new generation of PCIe doubles the throughput rate (speed) of lanes.
The most current PCIe generation available in the market is PCIe v4.0. This generation promises 1.969 GB/s transfer speed per lane.
Therefore, an x4 slot here has throughput rate of 7.877 GB/s and an a true x16 slot has a rate of 31.508 GB/s.
This is important to note here, because if you were to install a PCIe 3.0 device in a PCIe 2.0 slot, it will conform to the speeds of PCIe 2.0 only.
Also Read: PCIe 3.0 vs 2.0
So Can PCIe x4 Card Fit and Work in an x16 Slot?
Although the short answer to the question is yes, there’s a lot to the compatibility between different cards and slot sizes.
Smaller x4 cards WILL fit in larger x16 slots because the slots have the required connections as well as the size to accommodate the cards.
This is possible due to the fact that PCIe interface is Cross and Backward Compatible. Meaning devices can work across different slots as well as across different generations.
Therefore, if you have a PCIe v3.0 device, you can install it in PCIe v4.0 slots and vice versa.
Similarly, if you have a PCIe x4 card, you can fit it on an x16 slot. The reverse is also possible. For instance, you can put an AMD graphics card, that requires 16 lanes ideally, on an x4 slot – however, the performance of the graphics card will be greatly reduced.
Should You Plug a PCIe x4 Card in an x16 Slot?
No, in ideal case this arrangement must be avoided if possible because of the following reasons:
1. You Have Limited True x16 Slots – Don’t Waste Them
Image: Top PCIe x16 has 16 lanes, use it for the graphics card or more demanding devices.
As mentioned earlier, sometimes x4 slots are disguised as x16 slots. Meaning a slot may have the full x16 size but only have 4 PCIe lanes connected to it.
So there is a difference between TRUE x16 slots that have full 16 lanes and x16(x4) slots.
True x16 slots are very limited. In fact, a commercial motherboard only has a SINGLE true x16 slot, mostly intended for graphics card.
As a rule of thumb, if your motherboard has two or more x16 slots, the top one is always the TRUE x16 slot. Hence, if you have a high speed device like a graphics card, always occupy it on the top x16 slot.
The bottom x16 slot can be used to install x4 cards like M.2 NVMe expansion card, 4k video capture card etc.
The point to note here is that while you can still put the x4 card in the true x16 slot, you will have wasted the potential of the latter.
2. You Don’t Get Any Performance Boost
Putting an x4 card in an x16 slot does not give any kind of performance boost to the installed card.
Again, you waste the potential of the larger slot here.
PCIe card and slot operate on the throughput rate of the weakest of the two. This applies to both lane count as well as the PCIe version.
So if you were to install an x4 PCIe v2.0 device on an x16 PCIe v3.0 slot, the slot being the more powerful of the two, will clock down to 4 lanes and V.2.0 speeds.
Of course there are exception cases where this arrangement of putting an x4 device in an x16 slot may be warranted.
For instance, if you want to add an extra NVME SSD expansion card, but you have no x4 slot on your motherboard, then x16 slot would be your only option to go with.
This would of course mean sacrificing your graphics card, but if you are not a professional or a gamer, then you wouldn’t really have a need for a graphics card anyways.
In this article, we have looked at the question, can PCIe X4 card fit in X16 slot and found that it is possible. The card will fit and work well in the slot without much of a hassle.
However, this it not ideal to waste the potential of the x16 slot on a mere x4 device. Because the X4 card has only four data lanes, we find that the entire link will work at the speed of the X4 card and not at the speed of the X16 slot.
While you don’t get an increase in bandwidth, you end up losing the chance to use some of the available lanes and this can cost you somewhere along the road.
The only workaround this is be mindful your requirements and what devices you want to install when procuring a motherboard.