You open a memory page for your Dell server and see a dozen 16GB modules. Some say 2666. Some say 2933. Some say 3200. Which one fits?

Pick wrong and a module may not work in your server. Or it may run slower than its label says. Dell server memory comes in many types and speeds, so a quick check pays off.

This guide covers Dell PowerEdge memory compatibility for the R640, R740, and R750. It draws on Dell's own manuals and spec sheets, plus Intel's CPU pages, all checked in October 2026. Read it before any Dell PowerEdge RAM upgrade.

All three servers take DDR4 ECC registered memory, which means RDIMM or LRDIMM. The R640 and R740 have 24 slots. The R750 has 32. Check your CPU first, because it sets the top speed. Never mix RDIMM with LRDIMM. Use no more than two sizes. And set up both CPUs the same way.

Dell PowerEdge memory compatibility: R640 vs R740 vs R750

On memory, the R640 and R740 are nearly the same. PowerEdge R740 memory works like R640 memory, with the same module types, slot count, and speed limit. The R750 is a newer platform with more slots and more speed.

MT/s means million transfers per second. A bigger number means faster memory.

Dell launched the R640 and R740 in July 2017 with the first Intel Xeon Scalable CPUs. Its spec sheets now list 2nd Gen CPUs. So both generations are out there.

Dell also lists LRDIMMs for bigger builds. A two-CPU R740 reaches 3 TB with 128GB LRDIMMs. An R750 reaches 8 TB with 256GB LRDIMMs. Dell lists NVDIMM-N and Intel persistent memory for some of these models too. They need extra planning, so this guide sticks to standard RDIMM and LRDIMM.

Step 1: Check your CPU first

The CPU holds the memory controller, so it sets how fast your RAM can run. Dell's R740 guide names three things that set memory speed. They are the system profile, the CPU's top speed, and the module's top speed. Memory can't run faster than the lowest limit.

Intel puts the generation in the CPU number. Its rule is short: "The second digit indicates the processor generation." You can read your CPU model in iDRAC or in your operating system. Our part number guide explains the rest of the number.

Watch for two things. First, lower-tier chips have lower limits. The Silver 4210 stops at DDR4-2400.

Second, Dell warns about some 3200 MT/s modules. Its 64GB 3200 MT/s RDIMM listing says "Not Compatible with Skylake CPU." It names the R640, R740, and R750 as fits. So a 1st Gen R740 can't use that module. A 2nd Gen R740 can, but Dell lists it at speeds up to 2933 MT/s. The extra speed goes unused.

Match the module's rating to your CPU. Look up your exact CPU on Intel's site and note its DDR4 speed. Then pick a module rated for that speed. A faster module won't speed up the server, and as the Skylake warning shows, it can cause trouble. For the R750, Dell lists both 3200 and 2933 MT/s, so your CPU decides which one applies.

Step 2: Choose RDIMM or LRDIMM

These servers use registered memory only. Dell's R740 spec sheet lists "registered ECC DDR4 DIMMs only." Desktop and unbuffered memory won't work. ECC means the module fixes small memory errors on its own. Our ECC vs non-ECC RAM guide has more on that.

Then pick the type:

• RDIMM is the usual choice. Dell lists RDIMMs from 8GB to 64GB.
• LRDIMM is for very large builds. On the R740, Dell lists 64GB and 128GB LRDIMMs. The 128GB size is what takes it past 1.5 TB.

Here is the rule that trips people up. Dell's R740 guide says "RDIMMs and LRDIMMs must not be mixed." Dell's support article agrees: an upgrade must be all RDIMMs or all LRDIMMs. If your server has RDIMMs today, stay with RDIMMs.

Step 3: Read the memory label

A label packs a lot into a few characters. Here is a real one from CPU Republic: "Dell 8GB PC4-23400 DDR4-2933MHz 1Rx8 ECC Memory."

Know these speed names. PC4-21300 is 2666 MT/s. PC4-23400 is 2933 MT/s. PC4-25600 is 3200 MT/s. Some labels write PC4-23466 for 2933. It is the same speed.

A rank is a set of memory chips the server reads at once. In 1Rx8, the "1R" means one rank. The "x8" is the width of each chip. Dell lists 8GB RDIMMs as single rank and 16GB to 64GB RDIMMs as dual rank. So a Dell 8GB DDR4 ECC memory module is single rank. A Dell 16GB DDR4 ECC RDIMM is dual rank. Dell's guides also say x4 and x8 modules can share a channel. You don't need to match chip width.

Step 4: Pick the module size

Dell lists four RDIMM sizes for these servers. This table shows the most RAM each size gives with every slot filled and two CPUs.

Don't start with the total. Start with the channels. For the R640 and R740, Dell's guide says, "Populate six identical memory modules per processor." That is one module in each channel. Six 16GB modules give 96 GB per CPU. Six 8GB modules give 48 GB per CPU.

CPU Republic lists a Dell 16GB DDR4 ECC RDIMM rated 2933 MT/s, the top speed Dell lists for the R640 and R740. It also lists a Dell 8GB DDR4 ECC memory module rated 2666 MT/s, which suits a CPU like the Gold 6130. For more sizes and speeds, see our Dell server memory range.

PowerEdge R740 memory: slot map and rules

The R740 has 24 slots, 12 per CPU. Dell's slot guide names them A1 to A12 for CPU 1 and B1 to B12 for CPU 2. Each CPU has six channels with two slots each. The slots pair up as A1 and A7, A2 and A8, and so on up to A6 and A12. CPU 2 follows the same pattern.

These rules from Dell's guides apply to both the R640 and R740:

• White tabs first. Dell says, "Populate all the sockets with white release tabs first, followed by the black release tabs."
• Biggest size first. When you mix sizes, fill the highest capacity first.
• Same on both CPUs. In a two-CPU server, "the memory configuration for each processor must be identical."
• Two sizes at most. Dell does not support more than two capacities in one system.
• Slowest speed wins. Mixed speeds work, but every module runs at the slowest speed.
• DDR4 only. "All DIMMs must be DDR4."
• Balance the channels. Dell says unbalanced setups lose performance.

Here is a speed catch. Dell supports a 2933 MT/s bus with one module per channel. Fill both slots in a channel and the speed drops below 2933 MT/s. In Performance Optimized mode, four modules per CPU go in slots 1, 2, 4 and 5. Eight go in slots 1, 2, 4, 5, 7, 8, 10 and 11.

The R750 has 32 slots: A1 to A16 on CPU 1 and B1 to B16 on CPU 2. Each CPU has eight channels. Dell's R750 guide allows 1, 2, 4, 6, 8, 12, or 16 modules per CPU. It says CPU 1 and CPU 2 should match.

Dell PowerEdge RAM upgrade: step by step

1. Find your server's guide. Enter your Service Tag at Dell Support. Open Documentation, then Manuals and Documents. Pick the Installation and Service Manual.
2. List what you have now. Note the type, size, and speed of each module and the slot it sits in.
3. Pick matching modules. Use the same type, a speed that fits your CPU, and no more than two sizes in all.
4. Power down. Dell says the memory slots are not hot-pluggable. Follow Dell's safety steps first.
5. Seat the module. Open the socket ejectors. Line up the module with the key in the socket. Press both ends until the levers click. Dell says to hold each module "only by the card edges" and never press on its center.
6. Check the result. Put the air shroud back, close the server, and power on. Press F2 at startup to open System Setup, then go to System BIOS and Memory Settings. The System Memory Size should show your new total. If it doesn't, reseat the module. Then run the memory test in system diagnostics.

Dell PowerEdge memory compatibility FAQ

What RAM does a PowerEdge R740 use?

DDR4 ECC registered memory, as RDIMM or LRDIMM. It has 24 slots. Dell lists RDIMMs from 8GB to 64GB and LRDIMMs of 64GB and 128GB. Speeds go up to 2933 MT/s.

Can I mix 8GB and 16GB modules in an R740?

Yes, with limits. Dell allows two capacities in one system. Fill the larger size first. Match the setup on both CPUs. A third size isn't supported.

Can I use 3200 MT/s RAM in an R740?

Yes, if the server has a 2nd Gen CPU. Dell marks some 3200 MT/s RDIMMs "Not Compatible with Skylake CPU," so a 1st Gen server can't use them. The R740's top listed speed is 2933 MT/s. So the module runs at that speed at best.

How much PowerEdge R740 memory can the server hold?

With two CPUs, Dell's table shows 1,536 GB using 64GB RDIMMs. It shows 3 TB using 128GB LRDIMMs. With one CPU, the figures are half.

Is R640 memory the same as R740 memory?

For the module type, yes. Both use DDR4 ECC RDIMMs and LRDIMMs in 24 slots, with a top speed of 2933 MT/s. Dell's 64GB 3200 MT/s RDIMM listing names both servers as fits. Very large LRDIMM sizes can differ, so check your manual.

Not sure which module fits?

These servers all run DDR4. If you are weighing a move to DDR5, read our DDR4 vs DDR5 server memory guide. If you are not sure which module fits your Dell, contact CPU Republic. We carry ECC registered DDR4 server memory for Dell PowerEdge systems.