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Does It Matter What PCIe Slot I Use? – Learn Here

Building a custom PC is not easy, neither is installing expansion cards because a lot of planning and thought needs to go into it. This will make your work a bit easier if you have been wondering, does it matter what PCIe slot I use?

The short answer is it does matter! This is because different PCIe slot have different bandwidths. While PCIe slots and cards are backward-compatible with each other and while the smaller cards can work in longer PCIe slots,  the main hindrance comes from the bandwidth of the card and the bandwidth that the slot can provide. 

PCIe expansion cards have are equipped with a connector that determines the required slot for them to work.

It is the connector that interfaces with the slot for there to be any data transfer. For this connection to work, the slot and the connector need to be compatible. In other words, you cannot put a longer connector on a smaller PCIe slot.

Does It Matter What PCIe Slot I Use?

Any PCIe slot you use will work with any PCIe device you install as long as the connector size is compatible. This is what makes PCIe so useful for installing expansion cards to a computer.

Despite the different generations and the different lane counts, everything works with each other.

As mentioned, bandwidth is the limiting factor. It determines which device will work best in which slot.

As a rule of thumb, in a PCIe connection, the channel will always run at the maximum speed of the slowest component. Thus, installing a fast card on a slow slot underutilizes it.

The speed of the slot is determined by its size i.e how many PCIe lanes it has and PCIe generation.

Let us look at an example:

Does It Matter What PCIe Slot I Use
NVMe Expansion Card

The NVMe SSD expansion card utilizes a PCIe Version 3 X4 slot.

In other words it uses 4 version 3.0 PCIe lanes. It can support fast PCIe 3.0 NVMe SSDs that can reach about 3500-4000 MB/s transfer speeds.

If you were to install this expansion card on a PCIe v2.0 X4 slot, it would half the speeds to 2000 MB/s.

You can’t install this on a PCIe V2.0 X8 or an X16 slot to improve the bandwidth either because the CONNECTOR is limited to X4 lanes.

More on PCIe lanes and slots below:

PCIe Slots

PCIe Sizes
PCIe Slots come commonly in x1, x4, x8 and x16 sizes. X8 is rarely seen as it is merely replaced by X16 size working in X8 mode.

Before answering the question, does it matter what PCIe slot I use, here are the available options. There are X1, X4, X8, and X16 slots. They make different numbers of lanes available for the user.

Furthermore, they come in different physical sizes which also determines what device can be installed.

Also Read:

X1 Slot

This one has only one PCIe lane. It offers the least bandwidth among the four for any generation of PCIe slots.

It is still fast though, and devices like sound cards can be attached here for faster performance.

X4 Slot

The X4 slot offers four times the number of lanes available on the X1 card. It is also larger physically.

This slot is ideal for more demanding cars like network cards and port expansion cards.

X8 Slot

With twice the number of lanes as the X4 slot, the PCIe X8 slot is used by cards that require a lot of bandwidth.

X16 Slot

The PCIe X16 slot is the largest one you can find on most motherboards. An X16 slot has 16 data lanes and provides about twice the data bandwidths that the X8 slot provides.

Furthermore, this is used for very demanding devices like graphics cards.

X32 Slot

This slot size is rare. It has 32 lanes, double that of PCIe X16. However, it is not available on mainstream motherboards but is supported on some server motherboards.

Slot Assignments

When it comes to determining which slots to use, users have to be a bit keen on the labeling on the board as well as its manual.

It is common to find an X16 size slot with only 4 lanes, or 8 lanes operating in X4 or X8 respectively.

If you plug your X16 device into this slot, you will only use the available number of lanes despite the actual size of the slot.

This goes to show that the physical size of the slot may differ from the actual lane count. So, an X16 slot can only have 4 lanes, and hence it’s an X4 slot with a larger length.

Also Read:

PCIe Generations

How Many PCIe Lanes Does M.2 Slot Use
Source: Wikipedia

If you’re still wondering does it matter what PCIe slot I use, you’ll need to know about the different PCIe generations.

Currently, the most commonly available PCIe generation is the third, PCIe 3.0. Most devices have and use this. However, the fourth generation is already available but it’s only supported by AMD chipsets.

Gen 1

The first generation of PCIe devices replaced the older PCI standard. They brought inter-compatibility of slots and devices and offered faster performance.

The 1-lane slot allowed 250MB/s while the 16-lane one allowed 4GB/s.

Gen 2

The second-generation theoretically doubled the lane speeds, raising it to 500MB/s and 8GB/s for the X1 and the X16 slots respectively.

Gen 3

This is the current most widely-available version of the PCIe standard. An X1 slot from this generation can transfer at 985MB/s while the largest X16 does about 15.8GB/s.

Gen 4 and 5

Here, you get even faster performance compared to the first three generations. An X1 slot for Gen 4 and 5 offers 1.97GB/s and 3.94GB/s respectively.

At the same time, the X16 slot of the fifth generation PCIe slot is four times faster than the equivalent slot of the third generation PCIe.

Why It Matters

You have seen that the performance of PCIe slots changes across the different generations, theoretically, each new generation is twice as fast as the previous one, and things are only expected to get better with the release of future generations.

Which PCIe slot you end up using matters because it will determine the performance of your device.

For example, if you install a 3rd Generation PCIe device in a 2nd Generation slot, the device will perform at half its full bandwidth.

Furthermore, installing an X16 card in an X16 slot that has only 4 operational lanes means the card theoretically uses a quarter of its allowable bandwidth which might cause performance loss.

PCIe V3.0 – What is It?


Now that you have the answer to the question, does it matter what PCIe slot I use, you are better suited off to decide on which slot to install your device in.

For the best performance, always match your device to the same slot in terms of lane count and generation. Doing so will let it perform at its best.

It is also worth noting that installing an X4 device into an X16 slot will not improve its performance, and the same goes for installing a third-generation card in a fourth-generation slot. The slowest device’s speed is always used.

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We are team of two engineers with a keen interest and a passion for PC builds and hardware.

PCGearLab.com 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.

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