M.2 is an interface widely used today for high speed SSDs. It offers very small form factor and is thus considered almost a necessity for the modern setup. However, unlike the older SATA SSDs, the M.2 utilizes PCIe lanes. So how many PCIe lanes does M.2 slot use?
At the moment with the current NVMe SSD speeds and the current PCIe 3.0 version, the M.2 slot uses a maximum of 4 PCIe lanes. However, depending upon your motherboard configuration the amount of PCIe lanes the M.2 slot uses may differ.
Knowing the amount of PCIe lanes the M.2 slot uses is important in order to figure out if you have enough PCIe lanes for other peripherals. PCIe lanes are limited and they become a luxury if you have plenty of other expansion cards to install.
In the following write up we will explore this topic further and talk about why PCIe lanes matter and how M.2 lane count can differ.
So How Many PCIe Lanes Does M.2 Slot Use on an Average?
As mentioned earlier, the maximum amount of lanes an M.2 slot uses is 4 at the moment. 4x PCIe lanes are capable of supporting the fastest NVMe SSDs out there.
However, some motherboards may also features M.2 slots utilizing only 2 x PCIe lanes. Take for example the ASUS X470-F. This motherboard offers 2 x M.2 slot.
The first M.2 slot on this uses 4 lanes whereas the second uses 2 lanes. While both would be able to able to operate an NVMe SSD, the second will have the speed bottle necked significantly.
The configuration of the M.2 PCIe lane count depends upon the manufacturer and plenty more factors like the motherboard chipset as well as the PCIe generation.
Let us briefly look at how differently the M.2 slot can be configured.
Depending upon the manufacturer, the configuration of the M.2 slot can differ drastically.
For instance on some motherboards, the M.2 slot could be using the PCIe lanes of the CPU while on the other the M.2 slot would be connected to the chipset lanes.
If there are two M.2 slots, then there could be a combination of one M.2 slot using CPU lanes and the second one using chipset lanes.
The most important point to note is that since PCIe lanes are limited, using certain M.2 slots could disabled other peripheral slot such a PCIe X4 slot or some SATA slots.
Also Read: Best Motherboards with M.2 Slots
Lets us look at some examples:
Take the specifications of ASUS B550M-A Prime for instance (shown above). This motherboard features 2 x M.2 slot. One of the M.2 Slot connects to the processor PCIe lanes and the other utilizes the Chipset lanes.
Now, have a look at the following:
The specifications above show the M.2 slot configuration for ASUS ROG MAXIMUS XII HERO. This motherboard features 3 x M.2 slots. However, due to the limited PCIe lanes, there are caveats attached to using the M.2 slots here.
For instance one of the caveat is that the PCIEX16_3 shares bandwidth with the M.2_2 slot. Thus when using the M.2_2 slot, the PCIEX16_3 will be disabled.
Hence, when figuring out how many PCIe lanes does M.2 slot use on your motherboard or which PCIe lanes it occupies, you must refer to the manufacturer information and board specifications.
Different processors support different PCIe version and can thus reflect differently on the SSD performance even if it utilizes the same amount of lanes.
The following example clarifies this:
MSI B550M PRO-VDH can support 2 x M.2 slots as shown above. However, depending upon which processor you install, the PCIe version the M.2 slot conforms to will differ.
In other words, you will need atleast a 3rd gen AMD Ryzen processor to support PCIe V4.0 speeds on this motherboard. If you install any other processor, say an AMD Ryzen processor from 2nd gen or AMD Ahtlon processor, the speeds will be dialed down PCIe v3.0’s.
The PCIe V4 X4 slot can support transfer speeds of 7.877 GB/s. The PCIe V3 X3 slot can support transfer speed of only half as much at 3.938 GB/s.
This means that if you were to install the latest PCIe NVMe SSDs here that can top speeds of 7500 MB/s but you have on older PCIe V3 processor, then you would have bottlenecked your SSD significantly.
Also Read: Best SSD for NAS
The chipset of the motherboard also determines the PCIe lanes and the speed.
Take for instance the B550 chipset. While the M.2 slot connected to the CPU lanes would be able to operate at PCIe v4 speed and protocol. The M.2 slots using the chipset lanes would only operate at PCIe V3.(You can see this in the image above)
Now if you were to use a motherboard with X570 chipset. Both its processor connected M.2 slot and the Chipset connected M.2 slot would utilize the PCIe V4 protocol and speeds.
You can see above in the specifications for Gigabyte X570 Aorus Elite that both the chipset and the processor connected to M.2 slot can operate at PCIe 4.0 speeds.
Of course you do need to have a processor that can support PCIe V4 as mentioned earlier.
So in short when determining how many PCIe lanes does M.2 slot use and which PCIe version it conforms to, both the chipset and the processor matter.
Why M.2 Uses x4 Lanes? Do NVMe SSD Utilize Full X4 PCIe Bandwidth?
In order to understand if the NVMe SSD can truly utilize the full bandwidth capacity of 4 x PCIe lanes you first need to look at the theoretical max transfer speeds possible the by PCIe protocols.
The table above shows the possible or the theoretical transfer speed possible for different version of the PCIe Express and for different lane count.
We can see above that the most common version at the moment, i.e PCIe V3.0 would have a theoretical maximum of 3.938 GB/s transfer speed at x4 bandwidth/lanes.
Hence an M.2 slot which commonly utilizes the x4 lanes would theoretically be able to handle NVMe SSD rated at maximum 3938 MB/s read and write speeds.
Now, most NVMe SSD nowdays can very close to the theoretical maximum. Take Samsung 970 EVO Plus for example – a very popular NVMe SSD – this can easily reach read speeds of 3.5 GB/s.
Hence you would at least need to provide x4 lanes in order to fully utilize it.
Putting it an x2 M.2 slot, as we saw in the ASUS X470-f motherboard earlier, would drastically bottleneck the SSD. You can see in the table above that an x2 PCIe v3 would only be able to achieve 1.969 GB/s of transfer speed, which is far lower than what Samsung 970 Evo Plus is capable of.
SSDs Faster than X4 PCIe v3 Bandwidth
There most certainly are PCIe NVMe SSD already out there that can use the PCIe V4 X4 bandwidth.
You can see from the table above that an M.2 slot with PCIe v4 X4 lanes would have theoretical speeds of 7877 GB/s. One of the latest PCIe NVMe SSD, the Samsung 980 Pro, can already achieve sequential write speeds of 7000 MB/s.
Hence even the PCIe v4 X4 lanes are going to get saturated soon with faster SSDs.
Can PCIe Version Affect the Amount of Lanes for the M.2 slot?
Technically yes. If a certain NVMe SSD is has a max speed of 3500 MB/s and uses X4 M.2 PCIe v3 lanes, then it can theoretically utilize only X2 PCIe V4 lanes since the V4 doubles the bandwidth per lane.
However, at the moment, the motherboards featuring PCIe v4 still conform to X4 PCIe lanes for M.2 slot mostly to cater for the faster PCIe v4 SSDs that are already out in the market as mentioned earlier above. It is also future proof to have PCIe v4 X4 lanes for M.2 slot.
The only last comment we want to make here is that reading the motherboard specifications is almost a must when figuring out the PCIe lanes for the M.2 slot, the size of SSD it can fit, the PCIe version it uses, and the Chipset it features.
All these factors are highly related to the choice of SSD you will make when you go buy one.
But if you want the short answer to how many PCIe lanes does M.2 slot use, it is 4!