UDIMM, SODIMM, RDIMM and LRDIMM are the four main types of computer memory module. They all hold the same kind of memory chips, but they differ in size, in how the signals are buffered and in which systems accept them. Pick the wrong one and the computer will not start. This guide explains what each one is, how they differ, and how to tell which type your system takes.
The short answer
- UDIMM is the standard full size module for desktops, workstations and entry servers.
- SODIMM is the compact module for laptops, mini PCs and many NAS units.
- RDIMM is the registered module for servers and high-end workstations. It is ECC memory with a register chip that lets a server hold many modules.
- LRDIMM is the load-reduced version of RDIMM, built for the highest capacities.
- ECC is separate from all of these. It is error correction that you can have on UDIMM and SODIMM as well. RDIMM and LRDIMM include it.
NEMIX RAM supplies all four, in DDR3, DDR4 and DDR5, matched to the system. Start with memory by specification or browse UDIMM, SODIMM, RDIMM and LRDIMM.
UDIMM, SODIMM, RDIMM and LRDIMM side by side
| UDIMM | SODIMM | RDIMM | LRDIMM | |
|---|---|---|---|---|
| Full name | Unbuffered DIMM | Small Outline DIMM | Registered DIMM | Load-Reduced DIMM |
| Size | Full size | Compact, about half the length | Full size | Full size |
| Buffering | None | None | Register chip on commands and addresses | Memory buffer on commands, addresses and data |
| ECC | Optional (ECC UDIMM) | Optional (ECC SODIMM) | Yes, in practice always | Yes, in practice always |
| Used in | Desktops, workstations, industrial ATX boards, entry servers | Laptops, mini PCs, NUCs, all-in-ones, many NAS units | Servers and high-end workstations | Servers and workstations that need the highest capacity |
| Typical module sizes | Up to 48GB or 64GB | Up to 48GB or 64GB | 8GB to 256GB | 64GB to 256GB |
| Can mix with the others | No | No | No | No, and not with RDIMM in the same server |
What is UDIMM?
UDIMM stands for unbuffered DIMM. The memory controller talks to the memory chips directly, with nothing in between. That keeps the module simple and fast to respond, and it is why UDIMMs are the standard in desktops and workstations. It also limits how many modules a board can handle, which is why servers that need large memory move to registered modules. UDIMMs come in DDR3, DDR4 and DDR5, and in ECC and non-ECC versions. Browse DDR5 UDIMM, DDR4 UDIMM and ECC UDIMM.
What is SODIMM?
SODIMM stands for small outline DIMM. It is an unbuffered module about half the length of a UDIMM, made for compact systems: laptops, mini PCs, NUCs, all in one computers and many NAS units. It comes in DDR3, DDR4 and DDR5, with ECC versions for NAS units and industrial PCs that support them. A SODIMM and a UDIMM do not fit each other's slots. Browse DDR5 SODIMM, DDR4 SODIMM and ECC SODIMM. Our SODIMM memory guide and NUC and mini PC memory guide go deeper on compact systems.
What is RDIMM?
RDIMM stands for registered DIMM. A register chip on the module buffers the command and address signals between the memory controller and the memory chips. That reduces the electrical load on the controller, so a server can support many more modules per channel than unbuffered memory allows. The trade is a small added delay, which servers accept for the capacity. Registered memory is ECC in practice, so RDIMM is the standard server module. RDIMM is also used in high-end workstations built on Xeon, EPYC and Threadripper platforms. Browse DDR5 RDIMM and DDR4 RDIMM.
What is LRDIMM?
LRDIMM stands for load-reduced DIMM. It goes a step beyond RDIMM: a memory buffer on the module isolates the data signals as well as the commands, which cuts the electrical load further. That lets a server carry more and denser memory per channel, and it is how DDR4 servers reach very high capacities per socket. LRDIMM and RDIMM cannot be mixed in the same server, and a system that takes one will not accept the other unless the maker says so. Browse DDR4 LRDIMM. DDR5 servers generally reach high capacity with large RDIMMs instead, and newer platforms add faster types covered in our RDIMM, MRDIMM and MCRDIMM guide.
Where ECC fits
ECC (error-correcting code) memory detects and corrects single-bit errors. It is not a fifth module type. It is a feature that sits across them. ECC UDIMM and ECC SODIMM exist for workstations, NAS units and industrial PCs, and RDIMM and LRDIMM are ECC by design. A board must support ECC for the extra bits to be used. For the full picture, see our ECC vs non-ECC RAM guide.
Pin counts by generation
The generation is the other thing that has to match. DDR3, DDR4 and DDR5 are not interchangeable, and the notch is cut in a different place on each so a module cannot be seated in the wrong generation. Pin counts also differ by type:
| Pins by generation | DDR3 | DDR4 | DDR5 |
|---|---|---|---|
| UDIMM | 240 | 288 | 288 |
| SODIMM | 204 | 260 | 262 |
| RDIMM | 240 | 288 | 288 |
| LRDIMM | 240 | 288 | Check the platform |
Which type does your system take?
| System | Module type | Note |
|---|---|---|
| Laptop, NUC or mini PC | SODIMM | Confirm DDR4 or DDR5 first. |
| NAS | SODIMM, ECC SODIMM, ECC UDIMM or RDIMM | Depends on the model. Use the type the maker lists. |
| Gaming or home desktop | UDIMM | Non-ECC. Match the speed your processor supports. |
| Workstation with ECC | ECC UDIMM or RDIMM | Depends on the platform. Threadripper and Xeon W platforms typically take RDIMM. |
| Entry server (tower or 1U) | ECC UDIMM | Registered modules will not start. |
| Mainstream server | RDIMM | The standard choice. |
| High-capacity server | LRDIMM, or large RDIMM on DDR5 | Used where capacity per socket matters most. |
Two rules cover most mistakes. First, no module type mixes with another. A desktop will not take RDIMM, a server that wants RDIMM will not take UDIMM, and a laptop slot takes only SODIMM. Second, match the generation. Memory for a DDR4 system will not fit a DDR5 slot.
How to find out what your system takes
- Find the exact model of the computer, server or motherboard.
- Read the maker's memory specification. It names the type: UDIMM, SODIMM, RDIMM or LRDIMM, the generation and whether ECC is supported.
- Look at what is installed. The label on a module shows its type, generation and speed. On a server, the management controller lists the installed modules.
- Check slots and maximum capacity for the model.
- Send us the model and we will confirm the module. For server examples, see the Dell PowerEdge memory guide, and for workstations the workstation memory guide.
Frequently asked questions
What is the difference between RDIMM and UDIMM?
RDIMM has a register chip that buffers commands and addresses, which lets a server hold many modules, and it is ECC. UDIMM is unbuffered, goes straight to the memory controller, and is used in desktops, workstations and entry servers. They are not interchangeable.
What is the difference between RDIMM and LRDIMM?
LRDIMM adds a buffer on the data signals as well as the commands, which lets a server hold denser modules and reach higher capacity per socket. RDIMM and LRDIMM cannot be mixed in the same server.
What is SODIMM?
SODIMM is the compact memory module used in laptops, mini PCs, NUCs, all in one computers and many NAS units. It is about half the length of a desktop UDIMM and the two do not fit each other's slots.
Can I use UDIMM in a server?
Only if the server takes unbuffered memory, as entry servers do. Most servers take RDIMM or LRDIMM and will not start with UDIMM. Check the maker's specification for your exact model.
Can I put SODIMM in a desktop?
Not directly. The slots are different sizes. Some small desktops and mini PCs use SODIMM, so check what your system takes.
Is ECC the same as RDIMM?
No. ECC is error correction. RDIMM is a registered module type that includes ECC, but ECC also comes on unbuffered modules such as ECC UDIMM and ECC SODIMM.
Are DDR4 and DDR5 modules interchangeable?
No. The generations use different pin layouts and the notch is in a different position, so a module will not seat in the wrong generation of slot.
Related guide: Server will not start after adding the wrong memory type? Causes and fixes by vendor
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