M.2 sizes and SSD form factors

The numbers are millimetres. An M.2 drive called a 2280 is 22 mm wide and 80 mm long; a 2242 is the same width and 42 mm long. Form factor is the fit question — the shape your machine has room for, how the drive is held down, and how much space sits above it once it is in.

What the numbers on an M.2 drive mean

The first two digits are the width and the rest is the length. The standard allows widths of 12, 16, 22 and 30 mm, but essentially every M.2 SSD on sale is 22 mm, so only the length moves. The lengths you meet are 2230, 2242, 2260, 2280 and 22110. The last one has five digits because 110 is a three-digit length, not because the card is wider.

2280 is the default: desktop boards, full-size laptops and most external enclosures are built around it. 2230 is the compact size, used where there is no room for anything longer, such as handhelds and some ultraportables. 2242 sits between the two and turns up in thin laptops and mini PCs. 2260 is uncommon and mostly older. 22110 is a workstation and server length that consumer boards rarely have room for.

Length matters because of the screw. An M.2 drive is held down by one small screw at the end furthest from the connector, driven into a threaded post standing proud of the board. The post is at a fixed distance, so the drive's length has to match one that exists. Many desktop boards provide posts at several lengths, or one post you unscrew and move. Laptops usually give you one position and no choice. The screw comes with the board or the machine rather than with the drive.

What the notch in the connector tells you

The notch cut into the edge connector is a key, and its job is mechanical before it is informational. A card only seats in a socket whose key matches, so the notch is a physical bar against putting an SSD into a socket meant for a different class of card, such as a wireless card.

The standard defines several key positions. The two that matter for storage are B and M: a B-keyed socket is specified to carry SATA and up to two PCIe lanes, an M-keyed socket SATA and up to four. The other positions on a motherboard, A and E, are for wireless cards, and an SSD will not go into one.

What the key does not tell you is what the vendor wired behind it. A key states the largest set of signals a socket of that type is allowed to present; which of them are actually routed is a board decision. That is why a slot can be M-keyed and still speak only SATA. NVMe or SATA covers reading a drive's own keying and finding out which protocol a slot speaks.

Where a 2.5-inch drive fits

Nothing on a 2.5-inch drive measures 2.5 inches. The name carries over from the disk format the case was built around. The case is standardised at 69.85 mm wide and no more than 100.45 mm long, so a drive out of one machine drops into another's bay.

Height is the dimension that varies. The standard defines a series of them, and two are worth knowing: 7 mm, which is what SSDs are built to, and 9.5 mm, which is what many older bays were cut for. A 9.5 mm bay takes a 7 mm drive with room to spare, and some drives ship with a clip-on spacer to take up the slack. A 7 mm bay will not take a 9.5 mm drive.

Two connectors sit side by side on the back edge: a 7-pin data connector and a 15-pin power connector. In a desktop those are two separate leads, one from the board and one from the power supply. In a laptop the drive usually pushes straight onto a mating connector in the bay, or onto a short ribbon, so there is nothing to route. Desktop cases differ in what they will hold: some have dedicated 2.5-inch mounts, others only 3.5-inch bays, which need a tray or bracket to bridge the difference.

Whether a heatsink or a double-sided drive will fit

A drive's footprint says nothing about its height, and height is where M.2 fit goes wrong. The card is a 0.8 mm circuit board. The standard caps the components on it at around 1.5 mm on a side, so a double-sided drive — chips on both faces — is roughly that much thicker than a single-sided one.

Thickness is coupled to capacity, because the board is a fixed size. There is only so much room for flash packages on one face, so the larger sizes in a range tend to go double-sided while the smaller sizes stay single-sided. On a machine that clears single-sided drives only, fit constrains the capacity you can buy, not just the shape.

Clearance is a property of the machine rather than of the standard. A slot with a chassis panel, a thermal pad, or a board heatsink directly above it may have a millimetre to give and no more. Documentation that cares says so, usually in the words "single-sided only". Silence about height is not a promise of clearance, and a thin laptop is where that costs you.

Heat competes for the same space. An NVMe controller slows itself down when it gets too hot — thermal throttling — and a heatsink exists to delay that. Many desktop boards supply their own M.2 cover, in which case a drive that arrives with a heatsink already attached has to fit under it, instead of it, or not at all. A slot underneath a graphics card is the same problem with less room. Laptops generally have space for neither, which is why drives sold for them come bare. Whether a drive needs a heatsink depends on that drive and the air around it, not on any number in its name.

How to check what will fit before you buy

Read your machine's own documentation, and read it for three things: how many M.2 slots there are, what length each one accepts, and whether either is limited to single-sided drives. A support page that lists more than one length for a slot is telling you there are posts at each. A self-built desktop has the same table in the motherboard manual, and the board layout diagram shows where the posts sit.

The drive already in the machine is the other source, if you are willing to take a panel off. Its label states the size, and the post it is screwed into is one the board definitely has. That settles a replacement of the same length, though not whether anything longer would fit.

What will not settle it is a listing's compatibility blurb, which describes a category of machines rather than yours.

Fit is the first of the four questions in how to choose an SSD, and the only one where being wrong means the drive goes back. Settle it, then spend the decision on capacity and price. How to install an SSD picks up from here, once the right drive is in your hands. Every SSD we track is on one page.