How Fast is PCIe x16?

PCIe x16 slot is generally considered the fastest slot on a given motherboard. However, the actual speed of the x16 slot depends upon the version of the PCIe standard it conforms to.

The reason for this is simple. With every new iteration of the PCIe standard, the per-lane speed doubles. Meaning a PCIe x16 slot conforming to version 4.0 would be twice as fast as the same doing to version 3.0.

To give you the exact numbers, though, the PCIe x16 slot conforming to version 3.0 has speeds of about 15.75 GB/s. The same x16 space to version 4.0 or version 5.0 would have rates of 31.50 GB/s and 63.01 GB/s, respectively.

It is understandable if all this seems confusing to you. In this article, I will talk in detail about the PCIe interface and PCIe lanes in general, the different versions of the PCIe protocol, and how they would impact the x16 slot’s speed.

What are PCIe and PCIe Lanes?

PCIe is one of the two primary interfaces (the other one being SATA) used for connecting core components to your PC.

The PCIe protocol enables High-Speed Input Output (HSIO) devices to interface with your motherboard via the PCIe slots.

While the SATA interface is much slower and primarily intended for connecting hard disk drives, the PCIe interface is much faster. It allows for connecting very highly demanding devices such as graphics cards or NVMe SSDs.

The elements that determine the speed or the throughput rate at which a particular PCIe slot operates at is determined by two factors:

1. The number of PCIe lanes a slot connects to
2. The version the said PCIe lanes conform to

PCIe lanes are information pipelines that carry information to and from the motherboard/CPU to the devices connected to the PCIe slot.

The number of PCIe lanes connected to a slot can vary. The higher the number of PCIe lanes connected to a slot, the faster it is and the more demanding expansion cards it can be occupied with.

Additionally, the number of PCIe lanes connected to a slot also impacts its size in the majority of the cases such that:

1. x1 slots have a single PCIe lane and are thus the most miniature.
2. x4 slots have four PCIe lanes. They can either have the physical dimension of an x4-sized slot or that of a full x16 slot.
3. x8 slots connect to 8 lanes but have the physical dimension of an x16 slot.
4. x16 slots connect to 16 lanes, thus the fastest and the most prominent slot on a motherboard. The most demanding components, such as a dedicated graphics card, often use them.

The point to take home here is that the number after the “x” GENERALLY corresponds to the number of PCIe lanes the slot has. Hence an “x16” slot has 16 PCIe lanes.

Read in Detail: What Are PCIe Lanes?

PCIe Versions and the Impact on Speed

The second primary trait that affects the speed of a PCIe slot and its lanes is the version of the PCIe standard they correspond to.

The following table shows the throughput rate of different PCIe lanes configurations for different PCIe versions.

The most current PCIe version is PCIe 4.0, found on the more recent motherboards. Many newer motherboards, such as those with the Intel Z690 or newer chipset, also feature the newer PCIe 5.0; however, it needs to be more widespread, and its applications must be improved.

So How Fast is PCIe x16?

As can be seen from the table above, the PCIe version has a significant impact on the PCIe slot’s overall speed.

PCIe x16 slot conforming to V3.0 has a throughput rate of 15.75 GB/s; the same space working to v4.0 has double the throughput rate at 31.50 GB/s.

To determine the speed of the x16 slot on YOUR motherboard, you must first choose what PCIe version your motherboard belongs to. You can figure that out by looking at its spec sheet online:

Why the Speed of the x16 Slot is Important

While there are very few devices that can leverage the very high throughput rate of an x16 slot, there are two instances where the speed of the x16 space can matter:

1. When using a top-of-the-line graphics card
2. When using an NVMe SSD expansion card

1. Graphics Card and x16 Speed

The newer top-of-the-line graphics cards, such as the NVIDIA RTX 3000 series, the AMD Radeon 6000 series, or more contemporary ones, require PCIe 4.0 to perform optimally.

If you used these cards on an x16 slot conforming to an older version, i.e., version 3.0 or 2.0, they would work, albeit at a slightly reduced performance. This is due to the lower bandwidth carried by the older slots.

PugetSystems conducted a study on how the PCIe slot’s speed impacts the GPU’s performance.

As seen above, in specific applications like the DaVinci Resolve, the PCIe 4.0 RTX 3090 performs slightly less when inserted in PCIe 3.0 slot. The performance incrementally decreases as the card is inserted into older PCIe 2.0 or 1.0 slots due to their lower speeds.

PugetSystems concludes this study by saying that for applications where the graphics card is being worked constantly, the speed of the slot impacted due to its version does affect the overall performance of the graphics card.

Another critical inference that can be reached from the graph above is that inserting an older PCIe device (Titan RTX 24 PCIe 3.0) into the newer PCIe 4.0 slot would not result in higher performance. Hence the PCIe version of the device/card also impacts the overall speed of the interface and not just the slot itself.

Also Read: is PCIe Backward Compatible?

2. NVMe Expansion Card and PCIe x16 Speed

NVMe SSDs use the PCIe interface. While they may connect to an M.2 slot that looks different from a typical PCIe slot, it uses the same interface.

Hence, the same PCIe interface principles also apply to the NVMe SSDs. In other words, they are impacted by the slot version they go into.

An NVMe SSD occupies four PCIe lanes typically. An x16 slot can be occupied populated expansion card with FOUR M.2 NVMe slots.

Ideally, the version of the NVMe SSD, that of the expansion card, and the x16 slot on the motherboard should all match for them to work optimally.

If you insert a PCIe Version 3.0 NVMe Expansion card into a PCIe version 4.0 x16 PCIe slot, the interface will work on the speeds of the former.

Similarly, if you insert a Gen 3 NVMe SSD into a newer PCIe 4.0 slot, the speed will again conform to the former, even if you have a PCIe 4.0 expansion card.

Final Words

The short answer to the question “How fast is PCIe x16?” is that it depends upon its version. It ranges from 4.00 GB/s for PCIe v1.0 to 31.50 GB/s for PCIe v4.0.

If you recently bought a PC, there is a high chance that your x16 slot would conform to V4.0 speeds. If you have a 3-10-year-old PC, you have a v3.0 x16 slot (15.75 GB/s). Anything older, and you may have an x16 slot conforming to version 2.0 speeds.

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