Can PCIe X8 Fit in X16? – Learn here

When it comes to electronics, not many things can beat a modern motherboard where modularity and customizations are considered.

Many things can be added or upgraded on the board, therefore, PCIe slots are crucial because they hold some useful equipment.  PCIe slots come in a variety of shapes and sizes that all together let you add different components to your computer.

Slots come in X1, X4, X8 and X16 configurations and each has different specifications in terms of speed and bandwidth. If you have a board and are wondering, can PCIe X8 fit in X16, then you have come to the right place!

The short answer is yes. You can use a PCIe X8 card in an X16 slot and vice versa. You may notice a few things if you choose to do so and we will discuss them in a bit.

PCIe X8 and PCIe X16

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. A lot of the time, the X8 and the X16 slots will look similar in size so just checking the physical profile won’t be enough to determine what slot it is.

Instead, you are encouraged to go deeper and see if you can find the board’s spec sheet to confirm which slot it is.

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?

A PCIe X16 slot is the largest slot you can find on consumer grade motherboards. It has the most lanes as well as the highest bandwidths.

Devices attached to this slot are high-speed devices that have high bandwidth demands for optimal performance. An example is a graphics card.

PCIe X16 Slot in X8 Mode
The Full-size X16 slot in the middle works in the X8 configuration. Notice that only half the slots have terminals. Source:

Also Read: Best Motherboards for SLI

Can PCIe X8 Fit in X16?

If you have a peripheral device that has a PCIe X8 connector, you will often be looking for an X8 slot to connect it to fulfill its requirements.

However, if you have an X16 slot available and are wondering whether it will fit then the answer is yes.

The beauty of the PCIe standard is that most of the devices and the slots are very compatible. We’ll take a look at the compatibility topic more.

However, installing an X8 card on an X16 slot will let you utilize the card fully. It even gets better if both the card and the slot are in the same PCIe generation like PCIe 3.0 etc.

This is because different generations, although compatible, have large performance differences so slotting an X8 PCIe 3.0 card into an X16 PCIe 2.0 Slot may not perform as well as they would if they were both in the same PCIe version.

Also Read: What are PCIe Slots?

Compatibility Concerns

When it comes to compatibility, PCIe offers some of the best support. When you have a motherboard that has no X8 slot but has the larger X16 slot like the B450 Aorus Elite board, you will need to decide whether you’ll use your X8 card on the slower X4 or X1 slots available or use the much faster X16 slot if it’s empty.

If you decide to use the X16 slot and you plug the device in, the card will operate normally. While the X16 slot can accommodate more bandwidth than a regular X8 slot in the same generation, you won’t necessarily benefit from the extra lanes.

Instead, half the lanes in the X16 slot you’re now using for your X8 expansion card will be left unused. The slot will operate at the speed of the device which is the slowest component between the two.

What this means is that you do not get any performance boost for using a faster, higher-bandwidth slot on a device that operates at a slower speed.

However, the device will perform better than it would have done when in a smaller slot and that’s a plus.

Different PCIe Sizes
The different PCIe card lengths are shown here. They each support different maximum bandwidths. Source:

Performance Negotiations

We have seen that using an X8 card on an X16 Slot is possible. This is because the standard allows the use of PCIe devices on any slot as long as it can fit and is supported.

For a regular PCIe 3.0 X16, the speed is twice as fast as a PCIe 3.0 X8 device. This can be attributed to the fact that there are twice as many lanes on the X16 as there are on the X8. Where on an X8 card you get 7880 MB/s in transfer rate, an X16 card does about 15760 MBs per second.

PCIe device controllers are smart and thus they allow you to use different speed devices on your motherboard without jamming or having to carry out extra configurations. This means you won’t need to do any extra work in any software to get things to work.

Performance is negotiated quite simply. The link will operate at the speed of the slowest device. So as stated, if you plug in your X8 device into an X16 slot, the connection will operate at the highest speed that the X8 device can handle.

It would be easier to underutilize a part of a link than to push another part to perform beyond its specified parameters.

Also Read: How Many PCIe Lanes do I Have?

Things to Consider

While manufacturers do make it possible to use different PCIe slots and connector sizes, there are certain situations where this mix is called for and others where mixing sizes should not be considered.

Wobbly Cards

Using an X8 device on an X16 slot is relatively safe. However, some users point out that in some instances, the slot’s locking mechanism for the card may not be capable of securing the X8 card firmly due to its smaller size.

Though this is not such a widespread issue, you should still need to countercheck to ensure that the card is firmly fixed and held in the motherboard. A wobbly card is the last thing you need on your computer.

Cross Compatibility

While you can use an X8 device on an X16 slot, it’s also possible to use an X16 device on an X8 slot.

The whole connection will operate at the speed of the X8 slot and so you get less performance from your card. Nevertheless, it’ll still work just fine. Also, the X8 card will be able to operate at its maximum capacity when needed.

In this case, however, if using the regular X8 slot, ensure that it’s open-ended. Some manufacturers put in slots with blocked rear ends to prevent users from fitting in larger cards into the slot.

Can PCIe X8 Fit in X16
On a motherboard without an X8 slot, the next best alternative is a free X16 slot. Source:


Now then, can PCIe X8 Fit in X16 and work without any issues? Of course, it can. The only setup you’ll need to do on your part is to plug the device into the slot and once it has been detected by the computer, you’ll be good to go.

Plugging a “smaller” card into a larger slot is great when all you have is a large slot to spare and don’t have the exact slot for the card you have.

With X8 and X16 cards and slots respectively, installing the card should be easy and straightforward for most users.

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We are team of two engineers with a keen interest and a passion for PC builds and hardware. is essentially the culmination of our enthusiasm towards this subject. We review PC peripherals and hardware, talk about custom builds and informative topics regarding troubleshooting issues, understanding a component better and general tips for DIY PC builders.