How Fast is PCIe x16?

PCIe x16 slot is generally considered to be 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 conforming to version 3.0.

To give you the exact numbers though, PCIe x16 slot conforming to version 3.0 has speeds of about 15.75 GB/s. The same x16 slot conforming to version 4.0 or version 5.0 would have speeds 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 PCIe interface and PCIe lanes in general and also about the different versions of the PCIe protocol and how they would impact the x16 slot’s speed.

What is 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 the interface with your motherboard via the PCIe slots.

PCIe Slot
PCIe slots of varying sizes. Notice that bottom x16 slot, while having the full x16 physical dimension actually only has 4 lanes connecting it.

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

The elements that determine the speed or the throughput rate at which a certain 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 basically 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 majority of the cases such that:

  1. X1 slots have a single PCIe lane and are thus the smallest slots.
  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 and they are thus the fastest and the largest slot available on a motherboard. They are often used by the most demanding components such as a dedicated graphics card.

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 major 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.

1.00.2500.500 1.0002.0004.000

The most current PCIe version is the PCIe 4.0 found on the more recent motherboards. Many of the newer motherboards, such as those with the Intel Z690 or newer chipset also feature the newer PCIe 5.0, however, its not as widespread and its applications are limited at the moment. 

So How Fast is PCIe x16?

As can be seen from the table above, the PCIe version has a great 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 slot conforming to v4.0 has double the throughput rate at 31.50 GB/s.

As such, if you want to figure out the speed of the x16 slot on YOUR motherboard, you will first need to determine what PCIe version your motherboard belongs to. You can figure that out by looking at its specsheet online:

How Fast is PCIe x16
Excerpt from the ASUS ROG Strix B550-F’s Spechsheet. Shows that it has PCIe 3.0 as well as PCIe 4.0 slots. Take note that the choice of the CPU also matters for enabling newer version of the PCIe protocol to work on a certain motherboard. On this motherboard for instance, you would need atleast an AMD Ryzen 3000 CPU to enable PCIe 4.0 on the x16 slot.

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 slot can actually 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 from the NVIDIA RTX 3000 series or from the AMD Radeon 6000 series or newer require PCIe 4.0 to work at optimal performance.

NVIDIA RTX 3090 PCIe 4.0
NVIDIA RTX 3090 PCIe 4.0

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

There is a study conducted by PugetSystems on how the speed of the PCIe slot impacts the performance of the GPU.

Davinci Resolve PCIe testing
This study shows that a PCIe 4.0 x16 device like the RTX 3090 suffers from performance drops when inserted into the PCIe 3.0 x16 slot. Source:

As can be seen above, in certain applications like with the DaVinci Resolve, the PCIe 4.0 RTX 3090 works at a slightly reduced performance 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 important 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 which looks different compared to a typical PCIe slot, it uses the same PCIe interface.

samsung 980 pro
Samsung 980 Pro. M.2 PCIe 4.0 NVMe SSD

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

An NVMe SSD occupies four PCIe lanes typically. As such, an x16 slot can be occupied with an expansion card that has FOUR M.2 NVMe slots.

NVMe m.2 expansion card
 Asus Hyper M.2 – NVMe M.2 expansion card. This is an x16 card with 4 x NVMe M.2 slots.

Ideally the version of the NVMe SSD, and that of the expansion card and of 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 that of 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 all the way to 31.50 GB/s for PCIe v4.0.

If you have just recently bought a PC, there are high chances that your x16 slot would conform to V4.0 speeds. If you have a 3-10 year old PC, then chances are that 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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Atif Qazi
Atif Qazi is the founder of PCGuide101. He is a digital nomad who loves everything PC. He is a PC builder, tech enthusiast, engineer, and a lover of single player lore-rich RPG games.

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