PC Gear Lab

Can PCIe X4 Fit in X16? – Learn Here

When it comes down to motherboards, everyone hopes that they have enough PCIe slots as well as the lane capacity to handle all the cards they need.

If you’re here then you’re probably wondering, can PCIe X4 fit in X16, and it’s important to ask this question. In this article, we go over what happens when you chose this configuration.

If there are any drawbacks or advantages, we will be sure to mention them as well as help alleviate the uncertainty. To kick this off: simply put, a PCIe X4 CAN fit in a PCIe X16 slot.

The main reason this is possible lies within the PCIe standard itself. However, the point you need to note is that PCIe devices need a slot that is at least the same size as itself or more for it to perform its best.

PCIe X4 and X16

Before we dive deeper into the subject, let’s first get acquainted with the two PCIe configurations that we will be discussing.

A PCIe X4 slot comes with 4 data 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.

A PCIe X4 card, on the other hand, has terminals that match the connectors on the X4 slot. Pairing an X4 card and an X4 slot in the same PCIe version will allow the card and the slot to operate at their maximum capacity.

An 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 on an X4 slot and as such we see that PCIe X16 cards are bandwidth-intensive and require the extra lanes.

Also Read: What are PCIe Lanes?

Different PCIe sizes on a motherboard.
Here we see different PCIe Slots on a motherboard. Source: ccboot.com

Also Read: What does PCIe x16 Mean

Can PCIe X4 Fit in X16?

Although we gave a short answer previously, there’s a lot to the compatibility between different cards and slot sizes.

Smaller cards will fit in larger slots because the slots have the required connections as well as the size to accommodate the cards.

Additionally, using a larger slot for a card (when the required slot is unavailable) is often better than using a smaller slot which may sometimes be physically impossible.

We’ll look at this and several aspects of the PCIe standard such as physical size and bandwidth. This will give us a better understanding of why this works.

Also Read: Best MSI Motherboards for Gaming

Physical Sizes

There is no question that the X4 and X16 PCIe slots are not equal in performance. However, they might be equal in size!

This may sound confusing but just follow along.

Some manufacturers install an X4 slot that’s similar in size to the X16 slot. You may see this on motherboards such as the Gigabyte GA-Z170M-D3H which has an X16 slot working as an X4 slot.

I brought this up because before you go ahead and install a device on your slot, you should first ensure that it’s the correct one regardless of its size.

Taking this into perspective, we see that the X16 slot is working as an X4 slot in this motherboard and is expected to take in X4 cards. So this configuration goes to show that an X4 card can fit into an X16 slot.

Because the larger slot has more lanes, you may be forgiven to assume that the card will perform better in the X16 slot. This is not the case though.

In standard PCIe configurations, the peripheral-slot pair will always operate at the speed of the slowest device. With this logic, the X4 device in the X16 slot that has four-times more lanes will still perform at X4 capacity.

Can PCIe X4 Fit in X16
PCIe Slots come commonly in x1, x4, x8, and x16 sizes. Sometimes a full-size slot can be used but with fewer terminals to operate in X8 or smaller modes. Source: tomshardware.com

Operational Bandwidth

At the same PCIe Version, the speed of a channel will remain the same across the configurations from X1 to X16. This means that a single lane in an X4 slot and an X16 slot will operate at the same base speed.

What changes is the bandwidth and this is a function of the product of the lane speed and the number of lanes. As such, more lanes will give you more bandwidth. This explains why the X16 slot has a higher data-carrying capacity than the X4 slot.

The difference in bandwidth doesn’t affect whether or not an X4 device will work on an X16 slot because the device will only connect to the lanes it needs. The remaining lanes in the slot will remain unused.

The X4 device in the higher lane-count slot will have a few benefits. The biggest of all is that there will be no compromise in the bandwidth department. The device will be able to use all the lanes it needs to work and hence its performance will remain as expected.

Ideal Situations for this Configuration

You will not always need to use an X4 card in an X16 slot; but, certain situations call for this kind of configuration. The most important of which is when you have no other option.

When the only usable slot you have is an X16 slot, then you have no other choice than to plug your device into it.

This should be the worst-case scenario.

A PCIe X4 USB Expansion Card
This PCIe X4 Expansion card can be used to upgrade the USB ports on your computer. It has 2 USB 3.1 Slots, one type-A and one type-C. Source: lindy.co.uk


We have seen that a PCIe X4 can fit in X16. However, what do you sacrifice for this configuration and is it important to consider other alternatives?

When you use a device in a slot with more lanes than the device has, you end up sacrificing the difference in the number of lanes.

In this case, 12 lanes go unused and if you didn’t know this, the lanes on a motherboard are precious resources.

Devices like a graphics card require 16 lanes to function optimally and if you need to install one then you can see the problem with installing an X4 card in your X16 slot. Many motherboards come with just one X16 slot and so having it occupied with an X4 card will mean you won’t get to use the remaining lanes.

Granted, this is a drawback when you only have one X16 slot on your motherboard. More premium ones may have more slots.

Also Read: What are PCIe Slots?


You should consider other alternatives before going ahead and plugging your X4 device into an X16 slot.

The best way to go about this is to settle for an option that offers the best performance for your card but minimizes the number of lanes that are wasted.

A better alternative where you don’t have an X4 slot is to use the next best thing, an X8 slot if it’s available.

It’s up to the user to determine whether the cost is really worth the benefit.


In this article, we have looked at the question, can PCIe X4 fit in X16 and found that it is possible. The card will fit and work well in the slot without much of a hassle.

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.

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