Account
Key takeaways
- A UDIMM has no register or buffer, an RDIMM buffers the command and address lines, and an LRDIMM buffers the data lines too.
- Which type a server takes is set by its own memory guide. The memory summary in Dell's R740 Technical Guide lists 24 DDR4 RDIMM or LRDIMM slots.
- On 24 October 2026, DDR4 LRDIMM had a lower median listed price per GB than DDR4 RDIMM in our catalog, new and refurbished.
- On the same date, the DDR5 RDIMM medians per GB were more than three times the DDR4 RDIMM medians.
- Dell's R740 manual says modules of different speeds run at the speed of the slowest installed module.
Short answer: RDIMM, UDIMM and LRDIMM differ in buffering. A UDIMM has no register, an RDIMM buffers the command and address lines, and an LRDIMM buffers the data lines too. Your server's own memory guide decides which type it takes.
RDIMM, UDIMM and LRDIMM at a glance
- UDIMM (unbuffered DIMM). No register or buffer chip sits between the DRAM and the memory controller. UDIMMs come in ECC and non-ECC versions, and each listing states which.
- RDIMM (registered DIMM). A register buffers the command and address lines, so a server can populate more slots at speed.
- LRDIMM (load-reduced DIMM). It buffers the data lines as well as command and address. It is a registered module too.
Browse them by type: DDR4 registered memory, DDR4 LRDIMM, DDR4 UDIMM and DDR5 server memory.
UDIMM vs RDIMM
The difference is the register. An RDIMM buffers the command and address lines, and a UDIMM does not. The Dell EMC PowerEdge R740 and R740xd Technical Guide (Rev. A09, Dec. 2021) gives a memory summary on PDF page 6. For the R740, it lists RDIMM and LRDIMM, not UDIMM. For any other server, read its own memory guide before you choose.
On price, new DDR4 UDIMM and RDIMM were level on 24 October 2026, at a median of $17.57 per GB. Refurbished DDR4 RDIMM had a median of $12.00 per GB, against $17.94 for refurbished DDR4 UDIMM.
RDIMM vs LRDIMM
An LRDIMM adds buffering on the data lines. The same Technical Guide lists up to 3 TB of RDIMM or LRDIMM memory for the R740 (PDF page 7). The Dell PowerEdge R740 Installation and Service Manual (Rev. A24, November 2024) sets the per-channel limit on PDF page 63. Up to two RDIMMs or two LRDIMMs can go in one channel, regardless of rank count.
On 24 October 2026, DDR4 LRDIMM medians per GB were below DDR4 RDIMM in both conditions. Refurbished, it was $8.74 against $12.00; new, $10.01 against $17.57. A median per GB mixes capacities, so compare one capacity on a price page such as DDR4 RDIMM 32GB or DDR5 RDIMM 64GB.
Median listed price per GB at Memory Clearance, 2026-10-24
| DDR generation | Module type | Refurbished, median per GB | Refurbished part numbers | New, median per GB | New part numbers |
|---|---|---|---|---|---|
| DDR5 | RDIMM | $51.05 | 335 | $60.02 | 101 |
| DDR5 | UDIMM | $35.64 | 6 | $87.92 | 28 |
| DDR4 | RDIMM | $12.00 | 1,791 | $17.57 | 629 |
| DDR4 | LRDIMM | $8.74 | 370 | $10.01 | 118 |
| DDR4 | UDIMM | $17.94 | 141 | $17.57 | 96 |
| DDR3 | RDIMM | $8.04 | 1,428 | $22.89 | 129 |
| DDR3 | LRDIMM | $3.16 | 119 | $7.15 | 16 |
| DDR3 | UDIMM | $18.32 | 222 | $44.55 | 7 |
These are our listed prices, recorded daily, not sale prices and not a market index. A cell is shown only when at least 5 part numbers had a price. No DDR5 LRDIMM part number had a listed price on that date. Of the 500 UDIMM part numbers in the table, 498 are marked ECC in our parsed specification.
Specification groups by part number: M393A4K40DB3-CWE, M393A4K40CB2-CTD, P00930-B21, P00924-B21, P00922-B21, SNP8WKDYC/32G, SNPCPC7GC/32G, 835955-B21 and 815098-B21.
Mixing module types: what Dell's R740 manual says
Mixing rules belong to each platform, so this section quotes one, the Dell PowerEdge R740. The source is its Installation and Service Manual (Rev. A24, November 2024, PDF page 63).
- All DIMMs must be DDR4.
- x4 and x8 DRAM based modules can be mixed.
- Up to two RDIMMs or two LRDIMMs can go in one channel, regardless of rank count.
- A channel can hold at most two different rank counts.
- 64 GB DDP LRDIMMs must not be mixed with 128 GB TSV (3DS) LRDIMMs.
- Different capacities can be mixed, highest capacity first in the white-tab slots, but no more than two capacities in one system.
- Modules of different speeds run at the speed of the slowest installed module.
Whether a platform takes RDIMM and LRDIMM together, or takes UDIMM at all, depends on that platform's own memory guide. Read it before you order a second type. The R740 rules and the parts we match to them by specification are on the Dell PowerEdge R740 page.
How to choose
- Find the type already installed. Read an installed part number from iDRAC or iLO.
- Check your server's memory guide for the module types, speeds and mixing rules it lists.
- Compare price per GB at the capacity you need on the server RAM price tracker.
- Look up the part number's specification group, then send a quote request with the quantity you need.
Method: active Memory Clearance listings with a price shown online, parsed as single DDR modules with a DDR generation and module type. Kits, restricted brands and hidden duplicate listings are left out. Each part number takes its listed price on 24 October 2026, in its listing's own condition. Price per GB is that price divided by the module's capacity. A cell needs at least 5 part numbers. The table covers DDR3, DDR4 and DDR5 RDIMM, LRDIMM and UDIMM.