Can a PCIe 4.0 Device Work in a PCIe 3.0 Slot?

The short and simple answer to question whether can a PCIe 4.0 device work in a PCIe 3.0 slot is a yes! Yes you can plug in a PCIe 4.0 device like a graphics card in a PCIe 3.0 slot.

Thanks to the PCIe interface being flexible, the device and the slots are cross and backwards compatible. Meaning, you can plug in a newer device in an older slot and vice versa.

However, plugging in a newer gen PCIe device in an older gen PCIe slot can present bottlenecks.

Hence, when plugging in a PCIe 4.0 device in a PCIe 3.0 slot on motherboard, you may have to sacrifice some performance. The performance drop you experience depends upon the PCIe device.

For instance, a PCIe 4.0 graphics card in PCIe 3.0 slot may experience only a marginal performance drop. A PCIe 4.0 NVMe SSD (aka Gen 4 SSDs) may see a significant performance drop when plugged into Gen 3.0 M.2 slot.

In order to understand how a PCIe 4.0 graphics card can work in PCIe 3.0 slot, a basic understanding of the PCIe interface would help.

PCIe Interface and PCIe Version

The PCIe interface consists of communication channels known as PCIe lanes.

What are PCIe Lanes
Image: PCIe Lanes illustrated. Source: HowStuffWorks.com

Different PCIe devices require a different amount of PCIe lanes depending upon how much data they generate.

Now there are two important characteristics of the PCIe lanes that determines their overall bandwidth or the amount of information they can carry:

  1. The more lanes a slot has, the higher would be its overall bandwidth or transfer rate and thus would be able to carry more data.
  2. The newer the version of the PCIe lanes, the higher is the overall bandwidth.

The version of the PCIe lanes has a significant impact on the amount of information each PCIe lane can carry.

So much as so that with each newer PCIe version, the per lane transfer rate doubles.

The following table explain this:

Versionx1
(GB/s)
x2
(GB/s)
x4
(GB/s)
x8
(GB/s)
x16
(GB/s)
1.00.2500.500 1.0002.0004.000
2.00.5001.0002.0004.0008.000
3.00.9851.9693.9387.87715.754
4.01.9693.9387.87715.75431.508
5.03.9387.87715.75431.50863.015
6.07.87715.75431.50863.015126.031

You can see here that an x16 slot with 16 PCIe lanes if conforming to PCIe version 3.0 has a max transfer rate of 15.754 GB/s.

The same slot conforming to version 4.0 has a max transfer rate of 31.508 GB/s.

In other words, a PCIe 4.0 slot can accommodate devices that generate a much higher amount of data as compared to PCIe 3.0 slots. 

Newer gen PCIe devices are made so that they can leverage the higher max bandwidth of the newer Gen PCIe slots.

Hence, if you were to install a newer gen PCIe device in an older PCIe slot, then you may NOT get to see it max potential. 

So Can a PCIe 4.0 Device Work in a PCIe 3.0 Slot?

Yes, as alluded to earlier, the PCIe interface is backwards and forward compatible.

Meaning, you can use a PCIe 4.0 device in a PCIe 3.0 slot. Similarly, you can also use a PCIe 3.0 device in a PCIe 4.0 slot.

However, the maximum performance or the transfer rate of the plugged in device will conform to slowest of the two.

Meaning, a PCIe 4.0 Device in a PCIe 3.0 slot will certainly work, however, it will be limited to operating at the max transfer rate of the PCIe 3.0 slot. A PCIe 3.0 slot, in this case, will be the bottleneck for the PCIe 4.0 device.

So what kind of performance impact can you experience if you install a PCIe 4.0 device in a 3.0 slot?

Well, for starters, there are two popular PCIe 4.0 devices that are available right now.

  1. PCIe 4.0 Graphics Cards
  2. PCIe 4.0 NVMe SSDs (aka Gen 4 SSDs)

Let us take a closer look at each of these device individually:

1. PCIe 4.0 Graphics Cards in PCIe 3.0 Slot

There are only a handful of graphics cards from the latest gen that make use of the PCIe 4.0 interface.

Currently, both the latest high end NVIDIA and AMD graphics cards such as those from RTX 30 and AMD 6000 series make use of the PCIe 4.0 interface.

The following images shows the specs for ASUS ROG RTX 3090 using the PCIe 4.0 interface.

Will RTX 30 and 40 Work in PCIe 3.0
NVIDIA RTX 3090 PCIe 4.0. Source: ASUS

As such, ideally the newer graphics should be plugged into motherboard that have the PCIe 4.0 slots.

However, if you only have a PCIe 3.0 slot, you can most certainly use these cards. And fortunately, as it stands, the performance drop from using the newer PCIe 4.0 GPUs in PCIe 3.0 slots is quite marginal.

I would like to reference two studies here.

Puget Systems Study on PCIe 4.0 Device in 3.0 Slot

Puget Systems carried out a fairly comprehensive study on the impact of using a newer gen device in an older gen slot. In the end, they concluded the test with the following words:

For applications where data is constantly traveling across the PCI-Express bus, we can see that the generational bandwidth differences do have a very measurable impact on real-world performance. 

In other words, the performance impact was dependent on the application you use. 

For instance, in application like DaVinci Resolve, the impact was noticeable.

Can a PCIe 4.0 Device Work in a PCIe 3.0 Slot

Image: PugetSystems.com. Comparison of how RTX 3090 and Titan RTX perform on different PCIe generations. A significant impact can be seen on performance in DaVinci Resolve (video editing software) when using the cards in older PCIe slots.

In other instances, there was no performance difference at all.

Read in Detail: Will RTX 30 Work in PCIe 3.0?

As far as gaming is concerned, there wasn’t a major impact of using PCIe 4.0 GPU in PCIe 3.0 slot either. Gamer Nexus explains this:

So in short as far as GPUs are concerned, there isn’t a significant performance bottleneck when using a newer device in an older PCIe slot.

2. PCIe 4.0 NVMe SSD in PCIe 3.0 M.2 Slot

The M.2 slot where the NVMe SSDs plugs into also use PCIe lanes. As such, the performance of an NVMe SSD is heavily impacted by the amount of PCIe lanes or by the version of the M.2 slot.

Primer on SSDs

There are two common types of SSDs:

  1. SATA SSDs, they use the SATA interface. They have a max transfer rate of 550 MB/s.
  2. NVMe SSDs, they use the PCIe interface.

The transfer rate of the NVMe SSDs depends upon their PCIe version or generation

  • Gen 3 (PCIe 3.0) NVMe SSDs have a nominal transfer rate of 3500 MB/s
  • Gen 4 (PCIe 4.0) NVMe SSDs have a nominal transfer rate of 5500 MB/s.

While GPUs see only a marginal performance impact, with NVMe SSDs, there is a huge performance difference when using a Gen 4 NVMe SSD in an Gen 3 M.2 slot.

Lets take two very popular gen 3 and gen 4 SSDs and see how the gen 4 SSD performs in the older PCIe 3.0 slot.

  • Gen 3 Samsung 970 Pro
  • Gen 4 Samsung 980 Pro

The following table summarizes the results as tested on CrystalDiskMark.

 Gen 3
Samsung 970 Pro
in PCIe 3.0

MB/s
Gen 4 Samsung 980 Pro in PCIe 3.0
MB/s
Gen 4 Samsung 980 Pro in PCIe 4.0
MB/s
SEQ1M
Q8T1
Read: 3086
Write: 2863
Read: 3093
Write: 2858
Read: 6875
Write: 4951
SEQ128K
Q32T1
Read: 3073
Write: 2945
Read: 3258
Write: 2995
Read: 4158
Write: 4262
RND4k
Q1T1
Read: 41.04
Write: 135.74
Read: 60
Write: 133
Read: 74
Write: 148

You can see here that putting a Gen 4 Samsung 980 Pro in an older M.2 slot conforming to PCIe 3.0 can result in significant bottlenecks. 

You can see the performance impact across all parameters including sequential and random read and write values.

Hence, again, while a PCIe 4.0 NVMe SSDs WILL Work in an older PCIe 3.0 M.2 slot, you may experience significant performance drops.

Final Words

So the gist of the discussion above is simple. Yes, a PCIe 4.0 device CAN work in a PCIe 3.0 slot.

However, it is always advisable that you match a PCIe 4.0 device with a PCIe 4.0 slot in order to get the best performance.

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Author:

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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