There’s no better feeling than finally upgrading your PC to the latest tech, but what happens when you put a PCIE4 drive into a PCIE3 slot? How does this impact speeds? Will the newer PCIE4 devices even work in older PCIE slots?

PCIE4 M.2 SSD devices are backwards compatible with PCIE3 slots. Likewise, PCIE3 M.2 SSD devices are forwards compatible with PCIE4 slots. The PCIE standard has been designed to be forward and backward compatible with newer, and older, iterations. The speed of your PCIE devices is always capped at whatever the speed is for the oldest PCIE slot, or drive, you’re using.

Let’s take a look at what’s going on with PCIE4 backwards, and forwards, compatibility.

Can a PCIE4 M.2 Drive Fit a PCIE3 M.2 Slot?

Installing an M.2 NVMe SSD in my Asus motherboard

Your newer PCIE4 devices are designed to work with older PCI3 M.2 slots. The PCIE standard is designed to be forwards and backwards compatible with newer generations of this technology. This means you won’t have to worry about always having the optimal combination of the latest PCIE slots with the latest devices.

There also aren’t as many performance mismatches to worry about it. Your PCIE devices are still going to work just fine, but their speeds are going to be limited to the oldest PCIE device that you have in the connection.

In this case, the PCIE4 M.2 drive in a PCIE M.2 slot is going to be capped at PCIE3 slot speeds.

Can a PCIE3 M.2 Drive Work in a PCIE4 M.2 Slot?

What if you just upgraded your motherboard to the latest model that only has PCIE4 M.2 slots? Can you keep using your older PCIE3 devices until you can upgrade those as well?

Yes. Your PCIE3 M.2 drives are going to work just fine in the newer PCIE4 M.2 slots. However, you won’t be getting any performance boost from these newer slots as long as you’re still using older devices.

Just like with trying to use PCIE4 drives in PCIE3 slots, using older drives in newer slots also has a performance cap. Those older PCIE3 drives won’t be able to make as much use out of a PCIE4 slot as a more upgraded drive would.

So, whether you’re using older drives in newer slots, or older slots with newer drives, your performance will always be capped by whichever is the slowest. They will still work, and could still have blistering fast speeds, but they are always going to be locked into the top speed of the oldest device.

Are PCIE3 M.2 Drives Forward Compatible?

Here’s the big question: Are PCIE3 M.2 slots forward compatible with new PCIE4 NVMe M.2 devices? The answer is yes. Older PCIE slots can accept, and work with, newer PCIE devices.

The PCIE standard was designed to be backwards and forwards compatible. The PCIE5 standard was released in 2021 and it is also compatible with PCIE3 and PCIE4 devices.

However it’s definitely worth noting that your speeds are always locked into whatever the slowest speed of your PCIE combination is. So a PCIE3 device in a PCIE4 slot is still going to only top out at PCIE3 speeds. This might not have much “real world” impact in your experience as most PCIE3 x16 slots are so fast the difference might not be noticeable for average users.

Are PCIE4 M.2 Drives Backwards Compatible?

A GEN4 M.2 SSD means that it supports a PCIE 4.0 slot in the motherboard

The inverse of this question is also an issue: are PCIE4 drives backwards compatible with PCIE3 slots? The answer is also yes. Your PCIE4 M.2 drives are going to work in PCIE3 slots, but they will suffer a performance downgrade to the PCIE3 standard.

For example, my WD Black SN850 drive is Gen4 (aka PCIE 4.0) leading to read speeds up to 7,000 MB/s. This is super fast, but the bottom slot of my Asus B550M-Plus motherboard is only PCIE3:

Asus motherboard listing showing one PCIE4 M.2 slot and one PCIE3 M.2 slot

As a result, if I install my SN850 drive in this bottom slot, I will not get 7,000 MB/s read speeds. They will probably be closer to 3,500-4,000 MB/s read speeds instead.

The Benefits of Using a PCIE4 M.2 Slot

New tech standards always signal new benefits. Understanding what makes PCIE4 slots superior over PCIE3 M.2 slots is the key to understanding what’s really changed with this upgrade.

In order to really wrap our heads around this change, we’re going to need to take apart the PCIE4 slot. Well, not literally, but we are going to figuratively talk about how they work on a mechanical level:

  • Each PCIE slot consists of several lanes. Each lane is a physical connection between the motherboard and the PCIE device.
  • PCIE3 devices have a transfer rate of 1 GB per second per lane. This means that your high-end consumer PCIE3 devices are going to have a maximum transfer rate of 32 GB per second.
  • PCIE4 doubles the transfer rate. This means you can get 2 GB per second per lane. This also means that your high-end PCIE4 devices are going to top off at 64 GB per second. There are PCIE4 devices that are capable of higher transfer rates, but these typically aren’t available in consumer products and are reserved for industrial uses.

Downsides of PCIE4 M.2 Slots

A PCIE M.2 drive running at a good temperature below 50 degrees celcius

These upgrades always come with a few trade-offs. Not only are PCIE4 M.2 devices more expensive, but they also generate a lot more heat than their predecessors. This creates some unique problems with the PCIE4 devices and slots.

The biggest problem is that this heat is typically being generated right on top of your graphics card. Their combined heat is often enough to cause serious problems with your computer’s hardware. You’re going to need to invest in additional heat mitigation, like a heatsink for your PCIE4 devices, in order to avoid overheating and failures.

This is often an issue for high-end gamers and people using their computers for demanding jobs. Casual users might get away without the extra heat mitigation, but if you’re pushing the limits of what your machine can handle, your tech is going to feel the burn that comes with improved performance.

Read/Write Speeds of PCIE Drive & Device Combos

Before we get into the speeds for different PCIE device and drive combos, it’s worth reiterating that speed and performance is always locked into the oldest device’s stats.

This means that upgrading to PCIE4 M.2 SSDs when you only have PCIE3 slots won’t give you a performance boost. You might still want to upgrade to get yourself ready to buy a motherboard with PCIE4 slots down the road, or to get an M.2 SSD with more storage capacity, but you won’t get a raw performance boost.

As long as the PCIE generation is mismatched, you’ll only ever get performance based on whichever the oldest generation is.

With that said, let’s take a look at some top speeds.

PCIE4 M.2 Drive in a PCIE3 M.2 Slot

Two M.2 NVMe SSDs of 2280 size installed in an Asus motherboard

Your PCIE4 M.2 drive has some impressive speeds, but when put into a PCIE3 slot, it’s going to have its overall performance capped by the PCIE3 slot’s speed.

Your exact speeds are going to depend on the exact specifications of your device and slot combination, but PCIE3 is noticeably slower than PCIE4. We mentioned their overall data transfer speeds earlier, but here’s the PCIE4 read write speed again.

  • PCIE4 M.2: read around 7,000 Mb/s
  • PCIE4 M.2: write around 5,000 Mb/s

These factors are going to be locked into the PCIE3 slot speeds which will be noticeably slower than your PCIE4 M.2 SSD could be pulling in: you’ll maybe get the following if you’re lucky:

  • PCIE4 M.2 (in PCIE3 slot): read around 3,800 Mb/s
  • PCIE4 M.2 (in PCIE3 slot): write around 2,700 Mb/s

However, it might not matter in your day-to-day use.

Does Mixing PCIE4 M.2 Drives and PCIE3 Slots Matter?

Two M.2 SSD drives and various heat sink components

At the end of the day, it might not actually matter if you have a high performance drive in an older, and lower performance, slot. There’s a lot more factors to consider than just raw benchmarking when it comes to our day-to-day experience of how our PC performs.

Benchmarking often just looks at the numbers. This gives us a sense of the absolute maximum performance our machines can put out, but it doesn’t really tell us much about her experience of using them.

When deciding if using a PCIE4 M.2 drive in a PCIE3 slot is going to matter, it’s worth taking a moment to consider how you use your PC.

Related Reading: Is It Okay To Mix Different M.2 NVMe Brands & PCIe Types?

Dedicated gamers are going to be able to pull extra frames out of a high-end combination PCIE4 drives with PCIE4 slots. However, many games still lock their frames per second into 30, 60, or 120 for a second. This means that extra performance might not be translating it to extra frames per second.

There are other factors to consider as well. Your internet speed, PC temperature, and the state of your graphics card can all impact the performance of your PC and your experience using it.

The slight dip in performance you would take by putting a PCIE4 M.2 drive in a PCIE3 slot might not translate to any actual noticeable difference in performance.