Memory modules: how to choose the right RAM for a laptop, desktop or server

What a memory module does

Memory, or RAM, holds the programs and data the computer is working on at that moment. It is not storage: everything in it is lost when the machine is switched off, and files stay on the drive. More memory lets more programs and browser tabs run at once without the system slowing down to swap data out to the drive. A module is a small circuit board carrying memory chips, with a row of gold contacts along one edge that plugs into a slot on the system board.

Signs you need more memory, or a replacement module

Too little memory and faulty memory show different symptoms. The machine probably needs more memory when it slows down badly with several programs open and the drive is busy all the time. A module may be faulty when one of these happens:

• The machine crashes, freezes or restarts at random, sometimes with a blue screen or a memory error.
• It will not start, and beeps or flashes a light in a pattern instead. The manual explains what the pattern means.
• The operating system reports less memory than is installed.
• A memory test, such as the one built into Windows or the manufacturer's diagnostics, reports errors.
• The problem moves or stops when the modules are swapped between slots or taken out one at a time.

Shape and generation must match

Each generation of memory has its notch in a different place, so a module of the wrong generation will not go into the slot. Never force one.

• SODIMM modules are short, about 68 to 70 mm long, and are used in laptops, mini PCs and many all-in-one computers.
• DIMM modules are long, about 133 mm, and are used in desktops, workstations and servers.
• DDR3 has 240 pins on a DIMM and 204 on a SODIMM.
• DDR4 has 288 pins on a DIMM and 260 on a SODIMM.
• DDR5 has 288 pins on a DIMM and 262 on a SODIMM, with the notch in a different place from DDR4.
• DDR3L is low-voltage DDR3 that runs at 1.35 volts. Many later DDR3 laptops need DDR3L, so check before fitting standard 1.5 volt DDR3.
• Many thin laptops have some or all of their memory soldered to the system board. Soldered memory cannot be replaced or added to, so check how many slots the machine has before ordering.
• A few recent laptops use a flat CAMM2 or LPCAMM2 module instead of SODIMMs. These are not interchangeable with SODIMMs.

Server and workstation memory

Servers and many workstations use error-correcting memory, shown as ECC, which detects and corrects small memory errors. They also need the right kind of buffering, which the label states.

• Unbuffered memory is what desktops and laptops use: UDIMMs in desktops, SODIMMs in laptops. Some entry servers take unbuffered ECC modules.
• Registered memory (RDIMM) has a register chip that lets a server carry many more modules. It is the usual choice in servers.
• Load-reduced memory (LRDIMM) allows larger total capacities again. Registered and load-reduced modules cannot be mixed in one server.
• Ranks and chip width appear on the label as, for example, 2Rx4 or 1Rx8. Servers have rules for how many ranks each channel takes, so match the old module or follow the server's memory population guide.
• HPE, Dell and Lenovo each publish the memory modules they have qualified for each server model, and some servers show a warning at start-up for modules that are not on that list. Matching the maker's spare part number is the simplest way to stay within it.

How to read a memory label

A label such as 16GB 2Rx8 PC4-2666V-SA1 carries most of what you need. 16GB is the capacity of that one module. 2Rx8 is two ranks of chips eight bits wide. PC4 means DDR4 (PC3 is DDR3, PC5 is DDR5), and an L after it, as in PC3L, means low voltage. 2666 is the speed in millions of transfers per second. The letter straight after the speed, such as the V, is the timing grade. On DDR4 and DDR5 labels, the first letter after the second dash gives the module type (the S in -SA1): S for SODIMM, U for unbuffered DIMM, E for unbuffered ECC, R for registered and L for load-reduced. Older DDR3 labels give a bandwidth figure instead of the speed: PC3-10600 is DDR3-1333 and PC3-12800 is DDR3-1600.

Capacity, speed and mixing modules

• Check the most memory the machine supports in total and per slot. The manual or the manufacturer's specification page gives both.
• A faster module than the machine needs usually works, running at the slower speed the system supports. A slower module may hold the whole system back, or not work at all.
• Modules of different sizes can usually be mixed in a desktop or laptop. Matched pairs in the right slots let the system use dual-channel mode, which is faster.
• Never mix unbuffered and registered modules in one machine.
• Mixing ECC and non-ECC modules usually turns error correction off or stops the machine starting.
• In a server, fit identical modules in each channel as the population guide shows.

Fitting memory safely

• Shut the machine down fully and unplug it. In a laptop, remove the battery if it is removable. If it is internal, disconnect it as soon as the bottom cover is off, before touching anything else.
• Touch a bare metal part of the case, or wear an antistatic wrist strap, before handling modules, and hold them by the edges.
• Line the notch up with the key in the slot.
• A SODIMM goes in at an angle and is pressed down until the side clips click.
• A DIMM goes straight down until the clip or clips close.
• Start the machine and check that it reports the full amount of memory, then run a memory test.