Why your RAM runs slower than the box says
XMP, EXPO, MT/s, sweet spots and slots in plain words.
Why does new RAM run slower than the box says?
Every kit has two speeds. The default one is a slow, safe speed that any board can run, such as 4800 MT/s for DDR5 or 2133 MT/s for DDR4. The speed on the box is a stored overclock profile called XMP (Intel) or EXPO (AMD). The board uses the default until you switch the profile on in the BIOS.
How to turn on XMP or EXPO
Enter the BIOS (usually Delete or F2 at startup), look for XMP, EXPO, DOCP or A-XMP, choose the profile, then save and restart. The first boot can take a minute or two while the board trains the memory.
MT/s vs MHz: why CPU-Z shows half
DDR memory transfers data twice per clock, so a 6000 MT/s kit runs at a 3000 MHz clock. Task Manager shows MT/s, CPU-Z shows the clock in MHz. Both are right.
The CPU sets a limit too
Each CPU family has an official memory speed it guarantees. Anything above that is an overclock: most CPUs manage typical XMP and EXPO speeds, but not all. On AMD Ryzen there's also a sweet spot (about 6000 MT/s for Ryzen 7000 and 9000, 3600 MT/s for Ryzen 5000) above which the memory controller usually drops to half speed.
The motherboard sets another limit
Boards list a maximum supported speed. A kit faster than that still works, but at the board's speed or lower.
Four sticks are slower than two
Filling all four slots puts more load on the memory controller. With DDR5 this often means running well below the rated speed. For 64 GB, two 32 GB sticks are easier to run fast than four 16 GB sticks.
Use the right slots
With two sticks on a four-slot board, use the second and fourth slots from the CPU, usually labelled A2 and B2. Other pairs can leave the memory in single-channel mode or stop it reaching full speed.
DDR4 and DDR5 don't mix
The notch sits in a different place, so a DDR5 stick won't go into a DDR4 slot. Some boards are sold in both versions with almost the same name, so check for D4 or DDR4 in the model name.
Questions
How do I check if RAM is compatible with my motherboard?
Match the memory type first: DDR4 and DDR5 are not interchangeable. Then make sure the kit has no more sticks than the board has slots and doesn't exceed its maximum capacity. Finally compare the kit's speed with the board's rated maximum and the CPU's official memory speed. The checker above does all of this for you.
Why does my RAM run at 4800 MT/s instead of 6000?
New memory always starts at its safe default (JEDEC) speed, which is 4800 MT/s for most DDR5 kits and 2133 or 2400 MT/s for DDR4. The speed on the box is a stored overclock profile. Enable XMP or EXPO in the BIOS to use it.
What is the difference between XMP and EXPO?
Both are stored speed profiles. XMP comes from Intel and EXPO from AMD. Most AMD boards can load XMP profiles, but many Intel boards can't load EXPO-only kits. Kits that carry both profiles work on either platform.
Can I put DDR5 RAM in a DDR4 motherboard?
No. The notch sits in a different place, so the sticks physically won't fit. Some boards are sold in both versions with almost the same name, so check for D4 or DDR4 in the model name.
Which slots should I use for two RAM sticks?
On most four-slot boards, use the second and fourth slots from the CPU, usually labelled A2 and B2. This runs the memory in dual-channel mode. Check the board's manual to be sure.
Will four RAM sticks run as fast as two?
Usually not, especially with DDR5. Four sticks put more load on the memory controller, so boards rate them lower. For 64 GB, two 32 GB sticks are easier to run at full speed than four 16 GB sticks.
What RAM speed is best for Ryzen 7000 and 9000?
Around 6000 MT/s. Above roughly 6400 MT/s the memory controller usually drops to half speed, so faster kits are rarely quicker in practice than a good 6000 CL30 kit.