Key takeaways

  • The memory generation is fixed by the platform: DDR4 for Xeon Scalable first to third generation and EPYC 7001 to 7003, DDR5 for fourth-generation Xeon Scalable and EPYC 9004 onward. The two are keyed differently and do not interchange.
  • In September 2026 a 64GB DDR5 RDIMM costs several times a 64GB DDR4 RDIMM: in our catalogue the medians were $3,186 and $929 on 20 September, with DDR4 available from $292.
  • Keeping a DDR4 platform in service and filling its slots is the cheapest memory decision available this year; a DDR5 refresh should be sized to the workload, not to the slot count.
  • Whatever you buy, match module type (RDIMM, LRDIMM or UDIMM), rank and speed grade to what the system already holds, and buy the whole set at once.

Start with the platform, not the price

Memory generation is not a choice you make at purchase time. Each server platform accepts one generation: DDR4 for Intel Xeon Scalable first to third generation (Skylake-SP, Cascade Lake, Ice Lake-SP) and AMD EPYC 7001 to 7003 (Naples, Rome, Milan); DDR5 for fourth-generation Xeon Scalable (Sapphire Rapids) and later, and EPYC 9004 (Genoa) and later. The modules are keyed so a DDR5 DIMM cannot be seated in a DDR4 slot. So the real question is whether to keep the DDR4 fleet running and fill it, or to move workloads to a DDR5 platform and pay DDR5 prices for its memory.

What the 2026 shortage did to the two generations

Both generations rose sharply once the three DRAM makers moved wafer capacity to high-bandwidth memory for AI accelerators. DataCenterDisk's 17 August 2026 analysis put the rise at roughly 60 to 80 percent for DDR4 ECC RDIMMs and 100 to 116 percent for DDR5 ECC RDIMMs between early 2025 and the first quarter of 2026, and the fixed contract price of a 64GB DDR5 server module reached $1,500 on 15 September 2026, 5.5 times a year earlier (Seoul Economic Daily, 17 September 2026). The gap between the generations widened rather than closed: DDR5 is where the AI build-out bids, DDR4 is the residual market.

Our own catalogue on 20 September 2026 showed the same shape. Median online prices for single registered modules priced and in stock were $3,186 for 64GB DDR5 against $929 for 64GB DDR4, and $1,792 for 32GB DDR5 against $358 for 32GB DDR4, with the lowest priced 64GB DDR4 RDIMM in stock at $292. The full table by capacity is in our server memory prices for September 2026.

When keeping DDR4 is the right call

If the workload runs on a DDR4 platform today and the constraint is memory, not CPU generation, filling the existing slots is the cheapest capacity available this year. A two-socket third-generation Xeon Scalable or EPYC Milan server takes sixteen 32GB DDR4-3200 RDIMMs or sixteen 64GB DDR4 RDIMMs for 512GB or 1TB, and both parts exist in volume in the refurbished channel because the installed base is large. Older second-generation systems take DDR4-2933; first-generation and EPYC Naples take DDR4-2666, and 3200 modules run at those speeds on those platforms.

The limit is per-socket capacity and memory bandwidth. DDR4 platforms top out at a few terabytes per socket with load-reduced or 3DS modules, depending on the platform, and a DDR5 platform moves about 50 percent more data per channel at launch speeds (DDR5-4800 against DDR4-3200). Workloads that are bandwidth-bound, in-memory databases and large model inference among them, may justify the platform move on performance alone.

When a DDR5 refresh is worth it

A refresh makes sense when the platform brings something the workload needs: more cores per socket, PCIe 5.0 for NVMe and accelerators, or DDR5 bandwidth. Size the memory to the workload rather than filling every slot. On a DDR5 platform a 64GB module costs less per gigabyte than a 32GB module in our catalogue ($49.78 against $56.00 per GB at the medians above) and the new-module tracker shows the 128GB step costing more per gigabyte than 64GB, so the economical configurations are eight or sixteen 64GB DDR5 RDIMMs at the platform's speed grade: DDR5-4800 for fourth-generation Xeon Scalable and EPYC 9004, DDR5-5600 for the platforms that list it.

Rules that hold for both generations

  1. Match the module type. Registered (RDIMM) and load-reduced (LRDIMM) modules cannot be mixed in one system; unbuffered ECC (UDIMM) belongs to single-socket and workstation boards.
  2. Match rank and speed within a channel. A channel runs at the speed of its slowest module, and mixed ranks can force a lower speed or an unsupported configuration.
  3. Buy the set at once. Prices are moving quarter to quarter and quotes are valid for days, so a set bought in two halves costs more and may not match.
  4. Check the part number against the platform's memory guide. OEM part numbers (Dell, HPE, Lenovo) map to specific manufacturer modules; the guide states which are qualified for which system and speed.

For quantities beyond a single server, request a volume quote with the platform model, module type, capacity and count, and ask for the exact part numbers, condition and a price-valid date on the quote.