Computer Memory by Specification | RDIMM, LRDIMM, ECC UDIMM, SODIMM

COMPUTER MEMORY BY SPECIFICATION | EXPLAINED

Six form factors, 4,300+ modules, DDR2 through DDR5. If you already know what your board takes, pick it below. If you don't, this is the short version — and it's the question we're asked most.

Start here: does your machine need ECC?

ECC memory detects and corrects single-bit errors before they corrupt data. Servers, workstations and NAS units generally require it — many won't boot without it. Desktops and laptops generally can't use it. This is not a preference. Your board decides, and the wrong choice usually means the machine won't post.

The Six Computer Memory Form Factors, in plain English

RDIMM — ECC Registered DIMM
The workhorse of server memory. A register buffers the address and command lines, so the board can drive far more modules than it otherwise could. What most Xeon and EPYC servers take. If you have a rack server, this is almost certainly your answer.

LRDIMM — ECC Load-Reduced DIMM
Buffers the data lines as well as the address lines, which cuts electrical load further and lets you reach the highest total capacities a platform supports. Where you go when RDIMM can't get you enough memory — the largest kits here run to 1.5TB. Costs more per GB; only use it if your board and your capacity target call for it.

ECC UDIMM — Unbuffered ECC UDIMM
Error correction without the register. Entry-level servers, workstation boards, and most NAS units. Common on Supermicro, ASRock, ASRock Rack, MSI, GIGABYTE, ASUS, Gaming/AI Motherboards and similar single-socket boards.

ECC SODIMM — Unbuffered ECC SODIMM
The laptop-sized module in an ECC version. Mobile workstations, compact NAS units, edge and embedded systems.

UDIMM — Unbuffered, NON-ECC UDIMM
Standard Desktop Memory. No register, no error correction.

SODIMM — Unbuffered, NON-ECC SODIMM
Standard Laptop Memory. mini-PC and all-in-one, Small form factor SFF

Some of the Rules to know about when Selecting Memory for your System

You cannot mix registered and unbuffered memory in the same system — not in different channels, not in different slots.

Rank matters. A board that takes 2Rx4 may not run 4Rx4 at all, or may run it at a lower speed. Every module here lists its rank.
Faster memory is not always better. Populate more slots and many platforms automatically drop the speed. The stated rating is a maximum, not a guarantee.
Physical fit is not compatibility. DDR4 and DDR5 modules are keyed differently and won't cross-fit, but two DDR4 modules that both fit the slot can still be wrong for your board.

Which of the six, by what you are upgrading

Machine	Form factor	Where it goes
Dual-socket rack or tower server — PowerEdge R-series, ProLiant DL, ThinkSystem SR, UCS C, SuperServer	RDIMM (LRDIMM only for the largest capacities)	DDR5 RDIMM · DDR4 LRDIMM
Xeon W / Threadripper PRO workstation — Precision 7875 / 7960, ThinkStation PX, Z8 G5, Mac Pro 7,1	RDIMM	DDR5 RDIMM
Single-socket entry server — PowerEdge T160 / R360, ProLiant DL20 / ML30, ThinkSystem ST250, Xeon E boards	ECC UDIMM	DDR5 ECC UDIMM
Workstation-class desktop board — W680 / W880, AM5 boards that list ECC, Ryzen PRO OEM towers	ECC UDIMM	DDR5 ECC UDIMM
Consumer or business desktop — OptiPlex, ThinkCentre tower, EliteDesk tower, any gaming board	UDIMM (non-ECC)	Non-ECC UDIMM
Tower or rack NAS on Xeon / Ryzen PRO / Atom C — Synology xs and xs+, QNAP TVS-h, ASUSTOR Lockerstor Pro	ECC UDIMM or RDIMM — the vendor's part code says which	Manufacturer pages
Desktop NAS on Ryzen Embedded — Synology Plus, ASUSTOR Gen3, QNAP TS-x73A / h973AX	ECC SODIMM	DDR5 ECC SODIMM · DDR4 ECC SODIMM by manufacturer
Desktop NAS on Celeron / Atom / Core — Synology Value Plus, QNAP TS-x64 / x53D, UGREEN, TerraMaster	SODIMM (non-ECC)	Non-ECC SODIMM
Mobile workstation with ECC configured — Precision 7680 / 7780, ZBook Fury, ThinkPad P16	ECC SODIMM	DDR5 ECC SODIMM
Laptop, all-in-one, mini PC, NUC, Intel iMac / Mac mini / MacBook Pro	SODIMM (non-ECC)	Non-ECC SODIMM

Two rows need the vendor's part code rather than the machine type, because the same chassis class ships both ways — and that is the case the manufacturer pages exist for. If you have the part number that came out of the slot, it settles the form factor in one field.

Reading a specification line

Every listing on this site is titled the same way, and every field is a decision you have already made or one the machine has made for you:

64GB  DDR5  5600MHz  PC5-44800  2Rx4  1.1V  CL46  288-PIN  ECC RDIMM
 |     |      |         |         |     |     |      |        |
 |     |      |         |         |     |     |      |        └─ ECC and form factor  (the row above)
 |     |      |         |         |     |     |      └────────── pin count  (288 DIMM · 262 DDR5 SODIMM · 260 DDR4 SODIMM · 240 DDR3 · 204 DDR3 SODIMM)
 |     |      |         |         |     |     └───────────────── CAS latency in clocks  (lower is faster at the same speed)
 |     |      |         |         |     └─────────────────────── voltage  (1.1V DDR5 · 1.2V DDR4 · 1.35V DDR3L · 1.5V DDR3 · 1.8V DDR2)
 |     |      |         |         └───────────────────────────── rank and DRAM width  (1R / 2R / 4R / 8R × x4 / x8 / x16)
 |     |      |         └─────────────────────────────────────── JEDEC bandwidth code  (MT/s × 8 = MB/s)
 |     |      └───────────────────────────────────────────────── transfer rate in MT/s  ("MHz" by convention)
 |     └──────────────────────────────────────────────────────── generation
 └────────────────────────────────────────────────────────────── capacity per module

Generation	Speeds (MT/s)	PC codes	Voltage	Module capacities
DDR5	4800 · 5200 · 5600 · 6000 · 6400	PC5-38400 · 41600 · 44800 · 48000 · 51200	1.1V	8GB – 48GB UDIMM / SODIMM · 16GB – 256GB RDIMM
DDR4	2133 · 2400 · 2666 · 2933 · 3200	PC4-17000 · 19200 · 21300 · 23400 · 25600	1.2V	4GB – 32GB UDIMM / SODIMM · 8GB – 128GB RDIMM · 32GB – 256GB LRDIMM
DDR3 / DDR3L	1066 · 1333 · 1600 · 1866	PC3-8500 · 10600 · 12800 · 14900	1.5V / 1.35V	2GB – 8GB UDIMM / SODIMM · 4GB – 32GB RDIMM · 32GB – 64GB LRDIMM
DDR2	533 · 667 · 800	PC2-4200 · 5300 · 6400	1.8V	1GB – 4GB

Rank, in enough depth to order correctly

A rank is one set of DRAM chips the controller addresses as a unit — 64 bits wide on non-ECC, 72 on DDR4 ECC, 80 on DDR5 ECC. 1R is one set, 2R two, 4R and 8R four and eight (the latter only on LRDIMM and 3DS RDIMM). The x4 / x8 / x16 is how many data bits each chip contributes; x4 means more chips per rank, which is what servers want for full chipkill-class correction, and x8 or x16 means fewer, which is what desktops and laptops use.

Three things rank decides. Whether the module fits the platform at all — a server that lists 2Rx4 may not train 4Rx4, and a Celeron NAS that lists 1Rx8 may not see a 2Rx8 at full capacity. What speed a loaded channel runs — total ranks per channel is what drives the drop at two DIMMs per channel. And whether two modules match — a 32GB 2Rx8 and a 32GB 1Rx8 are the same capacity and different modules, and mixing them in a channel costs speed or, on some platforms, stability. Match the rank of the installed module when adding; buy identical modules when replacing.

Speed: the module's rating is a ceiling, the platform sets the floor

The processor's memory controller runs at the speed it was designed for and no faster. A DDR5-6400 module in a 4800 server runs at 4800; a DDR4-3200 in a 2400 laptop runs at 2400. Going the other way is where trouble lives — a module slower than the platform's rating may post and run slow, or may not post. The economical buy is the speed the platform runs, which our listings state per system. Populating a second DIMM per channel lowers the platform's speed on almost every DDR4 and DDR5 platform, so the trained speed after an upgrade is a function of how many slots you filled as much as what you filled them with.

ECC — three different things that share a name

Side-band ECC is the one this page's first section is about: extra data bits on the module, checked by the controller, corrected and logged. It is what RDIMM, LRDIMM, ECC UDIMM, and ECC SODIMM carry and what servers, workstations, and ECC NAS require.
On-die ECC is inside every DDR5 chip, on every module including non-ECC consumer parts. It corrects errors within the DRAM array, is invisible to the system, and is not a substitute. A DDR5 non-ECC UDIMM is a non-ECC module.
Registered is not ECC. The register on an RDIMM buffers the command bus; the ECC is the extra data width. All RDIMMs in practice carry ECC, which is why the terms travel together, but an ECC UDIMM has the width and no register, and the two do not interchange.

The pin-count trap and the key

DDR4 UDIMM, ECC UDIMM, RDIMM, and LRDIMM are all 288-pin, and so are all four DDR5 versions — with the DDR5 key notch in a different position from DDR4. Within a generation, UDIMM and ECC UDIMM share a key and physically interchange; RDIMM and LRDIMM share a different key and physically interchange with each other; the unbuffered and registered keys are offset so a UDIMM does not seat in an RDIMM slot or the reverse. So a module that fits is either the right type or its unbuffered / registered sibling — which is why the ECC question and the registered question are the two the intro tells you to settle before anything else.

Finding what is installed, on anything

Server management controller — iDRAC, iLO, XCC, CIMC, IPMI → Memory — shows every field of the spec line per slot, plus the trained speed.
Windows: Task Manager → Performance → Memory for form factor and slots; wmic memorychip get PartNumber,Capacity,Speed,ConfiguredClockSpeed,FormFactor for the part per slot. Linux: sudo dmidecode -t memory. macOS: About This Mac → Memory.
NAS: DSM Info Center · QTS System Status → Hardware Information · ADM System Information · TOS System Information · UGOS Device Info.
The module label carries the manufacturer part number, which decodes to every field above on the memory-brand cross-reference pages.

What "compatible" means here

A NEMIX RAM module listed by specification is detailed to that specification — every field on the line, on Micron, Samsung, or SK hynix DRAM — and run in the platform class it is listed for before it is offered. Where it is listed against a system or an OEM part, it matches that part's specification field for field. Some vendors' firmware shows an informational notice for memory outside their own catalog; it does not affect operation and each manufacturer page says where it appears. Lifetime replacement warranty and free US-based technical support on every module, since 1993.

Fleets and contracts

NEMIX RAM is a registered federal supplier (UEI GV8MMYNDJ5J1, CAGE 59AB2) and accepts purchase orders with net terms for business, government, and education buyers. For a mixed fleet, send the models or service tags and target capacity per system to our sales team and we return one quote with the specification and count per line. Volume and reseller pricing is available on request.

Frequently asked

Is there any case where a faster module helps? When the platform runs faster than what is installed — a 2400 module in a 3200 platform is leaving speed on the table, and replacing it with 3200 helps. A module faster than the platform does not.

CL46 versus CL40 at the same speed? Lower CAS is lower latency; at server and NAS workloads the difference is not measurable and the platform's JEDEC profile decides anyway. Buy the CL the platform lists.

What is PC4-25600? DDR4-3200: 3200 MT/s × 8 bytes = 25,600 MB/s per channel. The PC code and the MT/s figure are the same specification written two ways.

2Rx8 or 1Rx8 at 16GB? Match what is installed if adding; if replacing, 2Rx8 is marginally faster in a single-DIMM channel and 1Rx8 trains more easily with two per channel. Either is correct where the platform lists both.

Can I use 1.35V DDR3L in a 1.5V board? Yes; DDR3L is specified to run at either. The reverse — 1.5V in a board that requires 1.35V — is the one that fails.

The module fit but the machine will not post. Almost always the unbuffered / registered sibling, or the wrong generation for a chipset that shipped in two versions. Check the board's manual for the form factor, then the part on the label.

Not sure which you need? Search your machine or its part number directly — PowerEdge R750, MBD-X13SRN-H, DS923+ — and we'll show only what fits it. Or contact our team; we've been matching memory to machines since 1993.

NEMIX RAM can deliver any Configuration your Fleet Requires

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