Both are DIMMs — sticks of RAM — but they’re built for different machines and are not interchangeable. Registered memory (RDIMM, and its bigger sibling LRDIMM) goes in servers and workstations; unbuffered memory (UDIMM, and the laptop form-factor SODIMM) goes in desktops and laptops. Server memory almost always resells for far more, which is why it’s worth knowing which you have.
The quick differences
| RDIMM / LRDIMM | UDIMM / SODIMM | |
|---|---|---|
| Used in | Servers & workstations | Desktops & laptops |
| Register / buffer | Yes (a register chip between DRAM and CPU) | No (unbuffered) |
| ECC | Almost always ECC | Usually non-ECC |
| Typical capacity | 16, 32, 64, 128GB per stick | 4, 8, 16, 32GB per stick |
| Label clue | …R or …L · “REG ECC” | …U or …E · no “REG” |
| Resale value | High | Lower, but still worth selling |
How to tell which you have
- Read the label. Server modules usually say REG or ECC REG, and the speed code often ends in R (RDIMM) or L (LRDIMM) — e.g. PC4-3200AA-R. Desktop sticks end in U (or E for unbuffered ECC).
- Look at the board. RDIMMs have an extra small chip (the register) in the middle of the stick that UDIMMs don’t.
- Check the machine. If it came out of a rack or tower server, it’s almost certainly RDIMM/LRDIMM. Out of a PC or laptop, it’s UDIMM/SODIMM.
- Still unsure? Send a photo of the label — see our how to identify your RAM guide.
Why server RAM is worth more
Registered memory comes in higher capacities (64GB and 128GB sticks are common), is built to run in large multi-DIMM server boards, and is in constant demand for refurbished servers and upgrades. A single 64GB RDIMM can be worth many times a 16GB desktop stick — so if you’re clearing out a server, don’t let that memory go for scrap.
Not sure what you’ve got? Drop the type and quantities into the offer calculator for an indicative value, or send a bulk list if it came from a server refresh.
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