SO-DIMM vs DIMM: the memory a laptop takes

Desktop memory and laptop memory are the same technology in two different shapes, and the shape is not a detail. Before speed, latency or capacity comes a question with only one right answer: what physically goes into the machine in front of you. Get that wrong and the rest never gets tested, because the module will not seat.

The module is a different shape, not a smaller version

A desktop stick is a DIMM — a dual in-line memory module — and it stands roughly 133 mm long, dropped vertically into a slot that clips shut at both ends. A laptop stick is a SO-DIMM, small outline, at roughly 67.6 mm: a little over half the length. It goes in at an angle and presses flat into the chassis, held by a metal clip on each side, because a laptop has no room for anything standing up.

The pin counts differ too, and that is what makes the incompatibility absolute rather than a matter of clearance. A DDR4 SO-DIMM has 260 pins against the desktop DDR4 DIMM's 288; a DDR5 SO-DIMM has 262 against the desktop module's 288. Different lengths, different pin counts, notches in different places. A module of one kind cannot be persuaded into a socket of the other, and nothing sold as an adapter changes that — the electrical routing, not just the plastic, is different.

Vendors sell the same line in both shapes and treat them as separate products, which is why a laptop module's part number is its own. Every module on our laptop page is a SO-DIMM, and the recorded form factor is the only thing that puts it there: a kit is laptop memory here if that field says so, and desktop memory otherwise. There is no other test.

DDR4 and DDR5 are keyed apart

Within the SO-DIMM shape, the generations are incompatible in the same physical way the two form factors are. The notch in the bottom edge sits at a different position on DDR4 and DDR5, and a socket is cut for one of them. A DDR5 SO-DIMM will not seat in a DDR4 laptop, and the reverse is just as true.

So the generation is not something you choose. The machine was built around a memory controller that speaks one of them, and that settles it before you start reading grades. What is left to pick is the speed and latency inside that generation, and the capacity — and those, unlike the generation, are real choices.

The two generations sit in separate bands on every number we record for them. The ranges below count each SKU's own grade, not just the headline one a line is named for:

The CAS numbers look alarming next to each other and are not comparable that way: a DDR5 cycle is far shorter, so a much larger CL can be the same wait in real time. Speed and latency goes through the arithmetic that makes the two figures one figure.

Some laptops take no module at all

This is the part that decides whether any of the above applies. Three arrangements are common, and the machine's own service manual is the only place the answer is reliable:

• Two sockets. Both can be reached, and memory is a component you replace like any other.
• One socket. Often paired with memory soldered alongside it, so an upgrade means replacing the one module you can reach and living with the fixed half.
• Nothing socketed. The memory is soldered directly to the mainboard. It cannot be removed, added to, or upgraded. Whatever the machine shipped with is what it has for life.

Soldered memory is common in thin machines and in most of the ones built around low-power LPDDR packages, which are not modules at all. Newer compression-attached designs — CAMM2 and LPCAMM2 — are a third thing again: a flat board screwed down rather than a stick in a socket, removable, but a different part from a SO-DIMM in every way.

We do not carry soldered memory or compression-attached modules on the laptop page, and that is a consequence of what they are rather than a decision about them. A soldered configuration has no part number a buyer can order and no price of its own; it is an attribute of a laptop, and tracking it would mean tracking laptops. Everything we list is a module you can hold, order by part number, and put in a socket.

One module or two

Where two sockets exist, two modules are usually better than one, for the same reason they are on a desktop: a controller with two channels can read from two modules at once. Machines leaning on integrated graphics — which is to say most laptops, since the graphics share system memory — feel that most.

The kits here reflect it. Almost every laptop SKU we list ships as a single module or as a matched pair, with a small number of four-module kits for the rare machine that has four sockets. Buying the capacity you want as a matched kit is the safer route: two modules bought apart may refuse to agree on a speed and drop both to a slower setting. How much RAM you actually need covers sizing that capacity from what the machine is actually doing.

Speed and latency in a laptop

The grades are tighter than a desktop builder is used to, and the ceiling is lower. The fastest laptop module we list runs at 6400 MT/s; the desktop kits in the same catalog run past 8000. Laptops also mostly take the speed written into the module's own default profile rather than an overclocking profile, so the grade you buy is usually the grade you get, without a setting to change.

That limits how much speed is worth chasing. Within one generation the distance between the slowest and fastest grade is real but small, and it matters most on machines whose graphics borrow system memory — which is nearly all of them. Capacity moves more, for more people, than the grade does.

What the datasheets leave blank

We record what a manufacturer publishes and leave the rest alone, and on laptop memory the blanks are consistent. Not one laptop module here states whether it carries error correction: the field is left empty rather than filled in from an assumption, while roughly a third of the desktop kits beside them do state it. None of these lines carries a release date either, because the vendors do not publish one for a memory line the way they do for a processor. And not one of them is lit — every laptop module here records its lighting as absent, which follows from where the module lives: inside a closed chassis, under a keyboard.

What we compare on

Memory here is compared on three figures: capacity, speed in MT/s, and CAS latency. Higher is better on the first two, lower on the last, and that is the whole of what the comparison knows. Voltage, the full timing tuple behind CL, and the stick count are recorded where a datasheet states them, but they are not what two modules are placed side by side on.

Every laptop memory module in our catalog is on one page, with specs from the datasheet and prices tracked over time. The desktop kits are on their own.