Module class (UDIMM/RDIMM/LRDIMM/3DS) and DDR generation are hard UNSAFE lines per Kingston, Intel and Samsung documentation - mixing them risks a boot failure or, on Intel platforms, a documented "Fatal Error Halt." Rank, capacity and speed mismatches are more often CONDITIONAL - allowed under specific rules the OEM states, such as matching part-number quantities or running at the slowest module's speed. Where we could not source a vendor rule (AMD EPYC platform-specific mixing), this page says so rather than guessing.
Updated · vendor documents read 14-16 September 2026
Every row below is cited to the vendor document that states it. A CONDITIONAL verdict means the combination is allowed under a specific documented rule, not that it is unrestricted - read the citation, not just the badge.
| Combination | Verdict | Citation |
|---|---|---|
| UDIMM + RDIMM (same generation) | UNSAFE | Kingston: "even though UDIMM, RDIMM, and LRDIMM fit physically in the memory socket of your server, they also cannot be mixed, as doing so would lead to system boot failure." |
| RDIMM + LRDIMM | UNSAFE | Kingston: "Different memory types (UDIMM, RDIMM, LRDIMM, MRDIMM): Cannot be mixed in the same system. If mixed, the server may not boot." Intel: mixing produces a "Fatal Error Halt during memory initialization." |
| RDIMM + 3DS RDIMM (or LRDIMM + 3DS LRDIMM) | UNSAFE | Intel: "Mixing of DDR4 DIMM types (RDIMM, LRDIMM, 3DS RDIMM, 3DS LRDIMM) within or across processor sockets produces a Fatal Error Halt during memory initialization" - all four named as distinct, non-interchangeable types. |
| DDR4 + DDR5 (any classes) | UNSAFE | Samsung: "Different DDR generations, such as DDR4 and DDR5, cannot be mixed because they use different slot designs, signaling methods, and electrical characteristics, making them incompatible with each other." Also physically prevented by the notch position - see our DDR4-vs-DDR5 page. |
| Same class, x4 vs x8 DRAM organization | UNSAFE | Dell PowerEdge R660: "Mixing different capacities, x4 with x8 DRAM, or 3DS with non-3DS RDIMMs is not supported." |
| Same class, 3DS vs non-3DS RDIMM | UNSAFE | Dell PowerEdge R660, same sentence as above: "...or 3DS with non-3DS RDIMMs is not supported." |
| Same class, mismatched rank (e.g. 1Rx4 with 2Rx4) | CONDITIONAL | HPE ProLiant DL360 Gen11: "If different-rank memory is mixed, the quantity of each part number must be the same; only x4 options with Memory Fault Tolerance." A documented rule, not a blanket ban - the quantities must match and a fault-tolerance feature narrows which rank widths qualify. |
| Same class, mismatched capacity (e.g. 32GB with 64GB) | CONDITIONAL | Lenovo ThinkSystem SR650: "All memory modules must be the same type; with same-rank modules of different capacity, install the highest-capacity module first." Type must still match - this only concerns capacity within one already-matched type and rank. |
| Same class and capacity, mismatched speed rating | CONDITIONAL | Samsung: "For memory modules within the same DDR generation but with different speeds, they can be mixed, but all modules will operate at the speed of the slowest one... If it attempted to run at a higher speed than one module can handle, the system would become unstable or fail to boot." Runs, but at the slower module’s rated speed - a documented vendor behavior, not our performance claim. |
| Odd/asymmetric DIMM count across channels | CONDITIONAL | HPE ProLiant DL380 Gen11: "Memory should be installed in even quantities of DIMMs, and mixing of 3DS and non-3DS memory is not supported." Even population is the documented expectation, not an absolute prohibition on this page - consult the platform’s own population diagram. |
| AMD EPYC platforms: any DIMM-type or population mixing rule | NOT DOCUMENTED | We could not render an AMD-published or OEM-published page stating EPYC-specific mixing or population guidance this session (amd.com timed out on every attempt; Dell's EPYC technical-guide PDFs exceeded our fetch tool's size limit; Dell's Info Hub returned 403). Supermicro's own H13SSL-N product page confirms slot count and per-generation speed but not mixing rules - the full manual PDF also exceeded our fetch tool's size limit. Not documented by the vendors we could check, not ruled safe or unsafe. |
Module class and DDR generation both sit upstream of the electrical design of the channel itself - a register buffer that is present or absent, a voltage regulator input that is 12V or 5V, a notch position that either fits the slot or does not. Rank, capacity and speed differences, by contrast, are variations within an already-compatible electrical design, which is why vendor documentation more often states a specific workable rule for them (match the quantities, install the largest module first, run at the slower speed) instead of a flat prohibition.
One row above - AMD EPYC platform-specific mixing guidance - renders NOT DOCUMENTED rather than a verdict. That is a statement about what we could confirm this session, not a claim that no rule exists: AMD's own site timed out on every attempt, Dell's EPYC-platform technical guides were too large for our fetch tool to render, and Supermicro's H13SSL-N product page (which we could source) covers slot count and per-generation speed but not an explicit mixing statement - its full installation manual also exceeded our tool's size limit. We would rather leave a cell honestly blank than fill it from an Intel-platform rule that may not transfer.
This matrix generalizes across the vendor documents we could source - it is not a substitute for your specific platform's own supported-memory list. See our OEM-verified platform pages for exact, per-model rules, or the manufacturer's own documentation for anything we have not yet built.
Cheapest in-stock modules across the catalogue right now, computed live.
| Brand | Module | Capacity | Type | $/GB | |
|---|---|---|---|---|---|
| SK Hynix | HMAA8GL7MMR4N | 64GB DDR4 | RDIMM | $4.61 | Buy Now |
| A-Tech | M393A2G40DB0-CPB | 16GB DDR4 | RDIMM | $5.44 | Buy Now |
| Samsung | M393A8K40B22-CWD | 64GB DDR4 | RDIMM | $6.23 | Buy Now |
| Supermicro | B07DFLTWN3 | 64GB DDR4 | LRDIMM | $6.23 | Buy Now |
| Samsung | M393A2G40EB1-CPB | 16GB DDR4 | RDIMM | $7.12 | Buy Now |
| MICRON | MTA36ASF4G72PZ | 32GB DDR4 | RDIMM | $7.19 | Buy Now |
| Samsung | M393A2K43CB2-CTD8Q | 16GB DDR4 | RDIMM | $8.75 | Buy Now |
| SK Hynix | HMA84GR7CJR4N-VK | 32GB DDR4 | RDIMM | $8.84 | Buy Now |
| A-Tech | B083Q3F5DX | 32GB DDR4 | RDIMM | $8.91 | Buy Now |
| A-Tech | B0BPN4L74H | 32GB DDR4 | RDIMM | $8.95 | Buy Now |
Brand alone is not the question the vendor documentation asks - type, capacity, rank and speed are. Two different brands of the same class, capacity, rank and speed generally follow the same rule as two modules of the same brand; see the matrix above for the actual variables that matter.
Vendor documentation describes outcomes ranging from a slowed-down but functional system (mismatched speed) to a server that will not boot at all (mismatched module class). Intel's own language for the worst case is a "Fatal Error Halt during memory initialization" - see the UNSAFE rows above for which combinations that applies to.
Not always - Dell's own PowerEdge R660 documentation explicitly disallows mixing x4 with x8 DRAM organization even within a matched type, and Lenovo's SR650 documentation requires installing the highest-capacity module first when capacities differ. Check the CONDITIONAL rows above rather than assuming capacity is a free variable.
Because we could not render an AMD or AMD-platform OEM source stating one this session - every attempt either timed out, exceeded our fetch tool's size limit, or returned a block. We would rather say we don't have a sourced answer than guess at a rule this page exists specifically to get right.
No - Kingston's own documentation makes this point directly for DDR4: mismatched types "fit physically in the memory socket of your server" but still cannot be mixed without risking boot failure. Fit and support are different questions; only DDR5's module keying ties the two together more tightly.
Your platform's own supported-memory documentation is the final authority - not this general matrix. See our OEM-verified /server-ram/for pages for models we've checked directly, and use the citations on this page as the general pattern those platform-specific rules tend to follow.
Specification claims on this page were checked against the pages below on 16 September 2026. Where a widely-repeated figure did not survive that check, we report what the source says now and say so in the text rather than repeating the familiar number.