Yes, a PCIe X8 card can most certainly fit in an x16 slot. However, the bandwidth of the card will only be limited to 8 PCIe lanes. In other words, the installed card will not utilize the entire X16 lanes the slot has to offer.
Many things can be added or upgraded on the motherboard thanks to the two important user accessible interfaces i.e PCIe and SATA. While SATA is used mostly for storage drives, it is the PCIe interface that does the heavy lifting with the HSIO (High Speed Input Output) devices.
PCIe Slots come in many sizes i.e X1, X4, X8 and X16 and each has different specifications in terms of speed and bandwidth. Key to PCIe interface’s success is its ease of use with its modular and cross compatibility aspect. Meaning a smaller card can fit on a larger slot and vice versa.
Hence as far as the question “can PCIe X8 card fit in X16 slot?”, the answer is affirmative.
However, whether you SHOULD install a smaller card or a larger slot is another question. We will explore the entire theme of this topic in the text below.
PCIe x8 and PCIe x16 Slots
The PCIe X8 slot is a peculiar one because sometimes you might not be able to tell it from an X16 slot when you see it on a motherboard. In fact on consumer grade motherboard, the X8 and the X16 slots will have the same physical size. Therefore, just checking the physical profile won’t be enough to determine what slot it is.
Instead, you are encouraged to check out your motherboard spec sheet to find out the right details regarding your PCIe slots.
An X8 slot that has the same size as an X16 slot will have fewer terminals on it. This also means that there will be fewer lanes to transport data with; so as expected, you’ll have less bandwidth.
Also Read: What are PCIe Lanes?
Take the following motherboard for instance, this has two PCIe x16 slots, however one operates at x8 bandwidth only.
Can PCIe X8 Fit in X16?
If you have a peripheral device that has a PCIe X8 connector, the obvious slot to look at would be an X8 slot. Unfortunately, most, if not all, commercial based motherboard do not what feature this slot.
Instead you will perhaps have an x16 slot. And if you are wondering whether an x8 card will fit in x16 and work then the answer is yes.
Well, the beauty of the PCIe standard is that most of the devices and the slots are cross compatible. We’ll take a look at the compatibility topic more below.
However, installing an X8 card on an X16 slot will let you utilize the card to its max potential. However there are a few caveats here:
1. The PCIe Version Matters
It would be wise to make sure that both the add-in card and the slot are of the same PCIe generation like PCIe 3.0 or PCIe 4.0 etc.
This is because different generations, although cross compatible too, have a large performance differences so slotting an X8 PCIe 3.0 add-in card into an X16 PCIe 2.0 Slot may not perform as well because the speed of the PCIe interface works at the slowest of the two.
So in this case where you have a PCIe v4.0 device, it will dial its performance down to that of the PCIe v 2.0.
It is worth noting that each successive PCIe generation doubles the throughput rate (speed) compared to the previous generation. So a single lane on PCIe version 2.0 has a throughput rate on 0.5 GB/s the same on version 3.0 and 4.0 have 0.985 GB/s and 1.969 GB/s rate respectively.
Also Read: PCIe 3.0 vs 2.0
2. The Card will NOT Utilize all 16 Lanes
While you can certainly install an x8 device in the x16 slot, the device will only utilize 8 lanes of the slot. Meaning the rest of the 8 lanes will go wasted.
So the point here is that you will have potentially wasted a larger slot which could have been instead used by more demanding cards like a graphics card.
3. No Performance Gains
Since only 8 lanes of the x16 slot will be utilizes, there will be no performance gains. In other words, the throughput rate of the card will only be limited to 8 lanes and not 16.
If You Have a Dedicated X8 Slot – What to Do?
Many high performance motherboards often have two PCIe x16 slots. The primary or the top one operates at x16 bandwidth and the secondary works at x8 bandwidth.
While it is a good idea to install your x8 device in this slot, know that when the secondary x16 slot is populated, then the primary slot also clocks down its performance to 8 lanes.
Why is that? This has to do with the amount of PCIe lanes you have. The amount of lanes you have are limited. Hence motherboards generally share their PCIe express lanes across slots and ports.
Therefore, if you populate the X8 slot, it will automatically dial down the performance of the primary X16 slot to X8 as well – this is just how consumer grade motherboards are designed. This is an important consideration when installing add-in cards like graphics card.
A graphic card requires a full x16 bandwidth to work optimally. It is hence installed in the primary x16 slot. However, as soon as you install a device in the x8 slot, the performance of the GPU will also be affected (albeit marginally)
It is also worth noting again that both the x16 and the x8 slot have the same size.
Also Read: How to Check How Many PCIe Lanes Do I Have?
Motherboard with Only One x16 Slot – What to Do?
When you have a motherboard that has no X8 slot but only has a single large X16 slot like the Asus Prime B460M-A board, you will need to decide whether you’ll use your x8 add-in card on the slower X4 slots available or use the much faster X16 slot if it’s empty.
There are add-in cards that are a bit flexible in terms of how many PCIe lanes they need. AMD Radeon Graphics Cards, for instance, ideally require an x16 slot, but they can also work on x8 (at a marginally lower performance) and even on x4 (at a much lower performance).
So if you have an x8 device and you decide to use the X16 slot the card will operate normally as long as the slot conforms to ATLEAST the same PCIe version of the add-in card if not better.
While the X16 slot has more lanes than needed by an X8 card, you won’t necessarily benefit from the extra lanes, but the card will work at its full performance.
However, if you install the same device on an x4 slot, then it may work but at a much lower performance. Plus, the smaller x4 slot will need to be open ended so that it can fit connector.
PCIe Version, Card Size and Type
There are many factors that affect the amount of PCIe lanes an add-in card would require. An X8 card card can even have an x2 connector if it conform to the newer PCIe protocol.
For instance, let us look at the network interface cards (NIC) or network adapters.
Here, the following factors determine the PCIe bandwidth requirements
- Transfer Rate i.e 1Gbps, 10Gbps of the card
- Amount of ethernet / RJ45 ports it has i.e 1 or 2
- PCIe version intended for
The 10G Network card above requires an x8 PCIe slot as it has dual ethernet ports, as well as is compatible with the slower PCIe 2.0 protocol.
Since each consecutive PCIe version doubles the transfer speed per lane, a similar network cards built for PCIe V3.0 would require far fewer lanes as compared to a network card built for PCIe 2.0 to work at the same performance level.
Table: X4 PCIe v4.0 lanes have the same performance as an X8 PCIe v2.0 lanes.
So if you want to save up on the precious PCIe lanes available to you, consider investing in cards conforming to the newer PCIe protocol. However, you will need to have a motherboard that supports the newer PCIe protocol as well though.
What are Some of the Devices that Have an x8 Lane Requirements?
There are plenty of add-in cards that require an x8 slot:
- Graphics Cards – These have an x16 connector, but they are also essentially an x8 device too since they can work on x8 slots (at a marginally lower performance).
- Certain 10G Ethernet Cards as we saw above.
- SATA Expansion and RAID Controllers
- Dual NVMe M.2 SSD Expansion Cards – Each NVMe M.2 slot require 4 lanes. Dual NVMe M.2 SSD expansion card would require 8 lanes and thus would have an x8 connector
Now then, can a PCIe x8 card fit in x16 slot and work without any issues? Of course, it can. The cross-compatibility aspect of PCIe allows an x8 card to work on an x16 slot.
However, you have to take note of the PCIe version of the slot and card to make sure that the slot, particularly, does not bottleneck the device.
Also, know that adding an x8 card onto an x16 slot will not render any kind of performance benefits to the card.
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