Can PCIe x8 (Or x4/x1) Fit Into x16 Slots? (And Vice Versa?)

PCI express is great because it allows us to make our computers more powerful than what our motherboard and CPU provides. For example if we want to play lots of PC games, we can buy an graphics card and install it in our motherboard’s x16 PCIe slot – giving us much better graphics than we get with our CPU.

Or if you wanted to add WiFi capability to your motherboard, you could buy a simple WiFi PCIe card and insert this into the x1 slot on your motherboard – giving instant internet access. Nice!

But what happens if you have an x8 PCIe card (for example) but your motherboard only has x1 or x16 slots available? Can you fit the x8 card into either of these slots? And even if you CAN do this… should you do it?

I explore all these topics and more in this video.

https://www.youtube.com/watch?v=VlFE3rx4va8

If you prefer text over video, please read on for the guide/transcript version of this video.

Video Transcript And Guide

Hey everyone, PCIexpress is great because it allows us to expand our computer’s capabilities BEYOND what our CPU and motherboard offers. Want to do proper PC gaming? Buy a graphics card and put it in the x16 slot… well, maybe not THIS GPU since it’s ancient, but you get my point. In my case, I wanted 10 Gigabit Ethernet, so I purchased an Intel card that “requires” a x8 slot. There’s one problem though – barely any motherboards have x8 slots.

So can you fit a “shorter” device like a x1, x4 or an x8 card into an x16 PCIexpress slot on your motherboard? And also can you fit a “larger” x8 or x16 PCIexpress device into a smaller x1 slot, for example? That’s exactly what I’ll answer in this video – firstly covering can you PHYSICALLY do this and secondly, is it a GOOD IDEA to do it?

Physically Installing PCIe Devices

Let’s start with the easiest case first: if you have a shorter PCIexpress card, like an x1 WiFi card or an x8 10 Gig card, can you fit it into one of your motherboard’s longer slots – such as the x16 slot which is often used for graphics cards?

Well yes, you can. It will physically fit fine. The PCIexpress specifications all allow for this to happen, meaning that you should never have a case where the notches on an x8 card won’t physically fit into an x16 slot – for example. Equally an x1 card can fit into ANY PCIexpress slot, and an x4 card can also fit into an x8 or x16 slot. There should never be a performance hit for doing this either, although I’ll discuss this point more in a minute.

So we know that going bigger will always work, awesome! But what about the other way around – when you have an x8 or x16 card (for example) but your motherboard only has a spare x1 or x4 slot? Well this is where it gets a bit tricky. The PCIexpress spec ALSO says that this should be allowed – meaning that you could buy some awesome new RTX 4090, and plug it into an x1 PCIexpress version 2 slot. That SHOULD work, even though it would massively limit the bandwidth of course.

Equally my x8 Ethernet card SHOULD fit into x1 and x4 slots – based on the general spec. HOWEVER some manufacturers use “closed end” PCIexpress slots. – including on my Asus motherboard. This means that I can’t physically fit an x4, x8 or x16 card into my x1 slot. The plastic at the end physically stops this from happening.

In this case you have two options: you could either buy an x1 to x16 riser cable (for example) OR you can physically cut the closed end plastic off like Ironwing Tech Haven bravely did! Fair play to them! Both of these options will then allow you to fit a LARGER PCIexpress card into a smaller slot… with one exception. Some manufacturers lay out their boards badly – for example my Asus motherboard has an NVMe slot right next to the x1 slot. So even if I hacked all the plastic off the end, I still wouldn’t be able to fit my x8 card into it and so I’d still need to buy a riser cable.

Should You Fit ‘Wrong Sized’ PCIe Devices?

Okay so we now know that we CAN fit a ‘wrong’ sized PCIexpress device into different slots, but SHOULD we do this? Well the various PCIexpress designations (like x1 or x8) refers to how many lanes of bandwidth a PCIexpress card can take up. Basically an x8 device might need double the bandwidth of an x4 card, for example:

PCIe Revision	x1	x2	x4	x8	x16
PCIe 1.0 / 1.1	250 MB/s	500 MB/s	1 GB/s	2 GB/s	4 GB/s
PCIe 2.0 / 2.1	500 MB/s	1 GB/s	2 GB/s	4 GB/s	8 GB/s
PCIe 3.0/3.1	1 GB/s	2 GB/s	4 GB/s	8 GB/s	16 GB/s
PCIe 4.0/4.1	2 GB/s	4 GB/s	8 GB/s	16 GB/s	32 GB/s
PCIe 5.0	4 GB/s	8 GB/s	16 GB/s	32 GB/s	64 GB/s
PCIe 6.0	8 GB/s	16 GB/s	32 GB/s	64 GB/s	128 GB/s

So if you were to take a powerful graphics card and plug it into an x1 slot, you’re cutting its available bandwidth down by a factor of 16. That naturally isn’t good, because you’ll get less rendering performance.

Equally the x1 to x16 riser cables that I mentioned earlier don’t magically “give” you an extra 15 lanes of PCIexpress bandwidth – you’re just gaining the ability to plug an x16 card into an x1 slot. The card WILL still be throttled down to the available bandwidth of the x1 slot. On the other hand, putting a SMALLER PCIexpress card into a larger slot ‘should’ be fine – but there’s one exception to all of this that’s worth knowing.

Some motherboards might have an x16 slot, for example, but not all the pins are connected up. So you might see a motherboard with two x16 slots like THIS motherboard, but if you look at the specs, one of the slots only runs at x4:

Basically the slot allows you to plug in all sizes of PCIexpress cards, but it’ll have its max bandwidth limited to a quarter – down to x4 speeds. This might still be fine for SOME PCIexpress cards, especially SATA expansion cards because SATA drives don’t typically run fast enough to exhaust x4 bandwidth for example, but it’s worth bearing in mind.

This brings me onto the final point of my video – PCIexpress slots supply different speeds based on the generation (basically, the version) of PCIexpress that your motherboard and CPU support. The latest gen 5 PCIe is pretty fast – with even an x1 slot giving 4 Gigabytes of bandwidth a second. This is often sufficient for many NVMe SSDs, and will certainly be fine for supporting 10 Gigabit Ethernet too.

You will, however, still be limited by the PCIexpress version. For example I can’t take my PCIe gen 2 card (that requires up to 4 Gigabytes per second of bandwidth) and stick it in a gen 5 x1 slot – it’ll still only run at gen 2 speeds, and then get throttled down because it’s in an x1 slot.

So it’s a bit of a confusing topic, but I hope that all made sense. Please drop me a comment if you had any further questions though, and I hope you found this useful.