Form Factors
M.2 Keys, Module Sizes and What Fits
An M.2 socket is a mechanical filter on an electrical question. The key tells you which signal sets the socket may carry, and it is only half of the answer.
A notch and four digits, and what each one decides
M.2 hands a buyer two short strings: the letter of the key cut into the edge connector, and the four digits printed on the module. The key says which signaling the socket is wired to hand over. The digits say whether the module spans the board's mounting points. This page takes the two apart and collects the numbers that settle the rest.
One string describes the wiring, the other describes a ruler. Neither describes speed.
What the four digits on a module count
The code reads width first, length second, both in millimeters. A 2280 module is 22 mm wide and 80 mm long; a 2242 is the same width held to 42 mm. The sizes the standard allows run past the codes on store shelves: widths of 12, 16, 22 and 30 mm, and lengths of 16, 26, 30, 38, 42, 60, 80 and 110 mm. The first commercially listed cards were 22 mm wide, in lengths of 30, 42, 60, 80 and 110 mm: 2230, 2242, 2260, 2280 and 22110.
Length is the dimension a board negotiates with a screw. A module seats into a mating connector and is held by a single screw, and host boards are usually drawn for several lengths at once: a socket that takes a 22110 generally takes a 2280 by giving the screw another position. Width is not negotiated that way. The common line is 22 mm wide, and a 30 mm module asks for a connector drawn for it.
The form factor this one replaced
M.2 descends from mSATA, which reused the Mini PCIe card's form factor and connector. The newer standard allows larger boards, longer modules and parts on both faces, which is how an M.2 module carries twice the storage in an mSATA footprint.
Why does a notch refuse a drive that looks identical?
Keying keeps a module out of a socket that cannot run it: the edge connector is notched at defined pin positions and the mating socket blocks the rest. The rows below are condensed from the M.2 reference page as read on September 6, 2026, which also keeps the reserved pin ranges.
| Key | Socket | Notched pin positions | Interfaces exposed | Sizes listed |
|---|---|---|---|---|
| A | Socket 1 | 8 to 15 | 2 × PCIe ×1, USB 2.0, I2C, DP ×4 | 1630, 2230, 3030 |
| E | Socket 1 | 24 to 31 | 2 × PCIe ×1, USB 2.0, I2C, SDIO, UART, PCM, CNVi | 1630, 2230, 3030 |
| A+E | Socket 1 | 8 to 15 and 24 to 31 | 2 × PCIe ×1, USB 2.0, CNVi | 1630, 2230, 3030 |
| B | Socket 2 | 12 to 19 | SATA, PCIe ×2, USB 2.0 and 3.0, audio, UIM, HSIC, SSIC, I2C, SMBus | 2230, 2242, 2260, 2280, 22110 |
| M | Socket 3 | 59 to 66 | SATA, PCIe ×4, SMBus | 2230, 2242, 2260, 2280, 22110 |
| B+M | Socket 2 | 12 to 19 and 59 to 66 | SATA, PCIe ×2, SMBus | 2230, 2242, 2260, 2280, 22110 |
Storage is where the letters stop being theoretical. A SATA drive is typically B-keyed, an NVMe drive typically M-keyed, and a module that runs either carries a B+M key. The third case rewards a close reading: the B+M row lists PCIe ×2, not the ×4 an M key alone exposes, so the double key buys compatibility and spends lanes.
Four lanes and one SATA port out of the same edge
The connector carries up to four PCI Express lanes and one logical SATA 3.0 port rated at 6 Gbit/s, so one slot accepts a drive speaking either protocol. When those lanes carry PCIe they carry it as PCIe: the material describes them as a pure connection between host and storage device, with no extra layer of bus abstraction. Two command sets sit above them: AHCI, for compatibility with SATA-based systems, and NVMe, built around multiple simultaneous I/O operations. Three interface options exist for M.2 storage; those two are the ones the material names.
What a lane carries at 8, 16 or 32 GT/s, and what the encoding takes off the top, is the subject of encoding and usable bandwidth. The PHY side of the same lanes, the digital interface a controller presents to the analog front end, is what the PIPE interface defines.
The numbers a datasheet prints and a screw decides
The edge connector has 75 positions with up to 67 pins on a 0.5 mm pitch, offset on opposing faces of the board. Each pin is rated for up to 50 V and 0.5 A, and the connector is specified to endure 60 mating cycles. Power is the other half: an M.2 slot provides 3.3 V and 1.8 V pins, while a graphics slot deals in +12 V and can supply up to 75 W after initialization, as the card slot page sets out.
Thickness is the number met by hand. Components may sit on either face of the module to a maximum of 1.5 mm per side, on a PCB 0.8 mm thick, ± 10%. Host-side connectors differ for single- and double-sided modules and leave different clearance between module and board, so a drive that fits the socket can still foul a part standing on the board beneath it.
Which limit does a module meet first?
The order usually runs: key, then length, then clearance. A notch the socket does not match stops the module before it seats. A length the standoffs do not support stops it at the screw. Clearance stops it after seating, at the expensive end.
Checks that take a minute
- Read the four digits off a module you own and write down its width and length.
- Count the notched positions in its edge: 12 to 19 is a B key, 59 to 66 is an M key, both is B+M.
- Measure the span from connector to nearest standoff and compare it with the coded length.
- Check the clearance above and below the socket if the module carries parts on both faces.
- Find the link width the host routes to that slot, since four lanes is the most it carries.
Common mistakes
- Reading 2280 as a speed or a protocol. The code states 22 mm of width and 80 mm of length and nothing else.
- Assuming a B+M key brings four lanes. The B+M row lists PCIe ×2; the M key alone lists PCIe ×4.
- Ordering a 22110 for a board whose standoffs stop at 60 mm.
- Reading the SATA entry in the key table as the fast path. It is a 6 Gbit/s port, not the PCIe route.
If a module is in hand, the check is four digits, then notch positions, then the span from connector to standoff. If the board is the unknown, its manual lists the lengths it accepts and the key of its socket.