How to Choose RAM: What Fits Your PC and What Actually Matters
Memory (RAM) is one of the easiest upgrades to make, and one of the easiest to get wrong. A kit can look perfect and still not fit, or fit but run slower than the label says. This guide covers what works in your PC first, then how to choose between kits that all work.
The short answer
- Match the type. Your board takes either DDR4 or DDR5, never both. They are not interchangeable.
- Buy a kit of two modules. Two matching sticks give you “dual channel,” which is how desktop CPUs are built to use memory.
- Pick the size by use. 16 GB for everyday use, 32 GB for gaming, 64 GB for video editing, 3D and AI work.
- Start from your CPU’s official speed. Faster kits work, but only reach their label speed once you turn on XMP (Intel) or EXPO (AMD) in the BIOS.
- Compare latency in nanoseconds, not just the speed or the CL number on their own.
Will it work? The compatibility checks, in order
Go through these checks in order. The first three decide whether a kit works at all. The last two decide how fast it runs.
1. DDR4 or DDR5
DDR stands for “double data rate.” DDR4 and DDR5 are two generations of desktop memory. They have the same number of pins, but the notch in the edge of the module sits in a different place, so a DDR5 stick will not go into a DDR4 slot. That is on purpose. They also run at different voltages.
Which one you need depends on your motherboard and CPU:
| Socket | CPUs (examples) | Memory type |
|---|---|---|
| AM4 | Ryzen 3000 and 5000 series | DDR4 only |
| AM5 | Ryzen 7000 and 9000 series | DDR5 only |
| LGA 1700 | Intel Core 12th, 13th and 14th gen | DDR4 or DDR5, depending on the board |
| LGA 1851 | Intel Core Ultra 200S | DDR5 only |
LGA 1700 is the tricky one. The CPUs can use either type, but each board is built for one. Board makers often put “DDR4” or “D4” in the model name of the DDR4 versions, but not always. If you are not sure, check the spec page for your exact board model before you buy.
2. DIMM or SO-DIMM
A DIMM is the full-length memory stick used in desktops. An SO-DIMM (“small outline” DIMM) is the short version made for laptops and some mini PCs. They do not fit each other’s slots. Listings mix them together all the time, so check the module shape in the specs. In our catalog of 1,885 memory products, 253 are SO-DIMMs.
(Many recent laptops have soldered memory that cannot be upgraded at all. This guide is about desktops.)
3. Slots, chipset limits and server memory
Your motherboard has a fixed number of memory slots, usually two or four. Smaller boards (Mini-ITX) and many budget boards have two. A four-module kit needs four slots.
The chipset (the support chip on the motherboard that sets the board’s features) and the CPU together set the most memory the board can hold. For home use these limits are rarely a problem, because they sit far above what most people buy:
| Chipset | Socket | Most memory (as listed) | Slots at most |
|---|---|---|---|
| H610 | LGA 1700 | Up to 64/128 GB | 2 |
| B660, H670, Z690, B760, H770, Z790 | LGA 1700 | Up to 128/256 GB | 4 |
| H810 | LGA 1851 | Up to 128 GB | 2 |
| B860, Z890 | LGA 1851 | Up to 256 GB | 4 |
Where two figures are listed, the limit depends on which CPU you use.
One more trap: registered memory (RDIMM). This is server memory with an extra chip on each module. It looks like a normal stick but does not work in regular desktop boards, which take “unbuffered” memory. Of the memory products in our catalog, 343 are registered. Some listings say “ECC” too. ECC (error-correcting) memory is a separate feature: some desktop platforms accept unbuffered ECC sticks, many do not. If you do not know you need ECC, you do not need it.
4. The CPU’s official speed
The memory controller lives inside the CPU, not on the motherboard. So the CPU maker sets the official (“supported”) memory speed. Here are some real examples from our data:
| CPU | Type | Official speed, two modules | Official speed, four modules |
|---|---|---|---|
| Ryzen 5 3600 | DDR4 | 3200 MT/s | not listed |
| Ryzen 5 5600X | DDR4 | 3200 MT/s | 2667 MT/s |
| Ryzen 7 7800X3D | DDR5 | 5200 MT/s | 3600 MT/s |
| Ryzen 7 9800X3D | DDR5 | 5600 MT/s | 3600 MT/s |
| Core i5-12400 | DDR4 / DDR5 | 3200 / 4800 MT/s | not listed |
| Core i5-13600K | DDR4 / DDR5 | 3200 / 5600 MT/s | DDR5: 4400 MT/s |
Two things stand out. First, the official speed is often lower than the speed printed on popular kits. Second, the official DDR5 speed drops a lot when you fill all four slots. Each channel then carries two modules, which makes the electrical signal harder to keep clean, so the CPU maker only promises a lower speed.
5. XMP and EXPO
Every memory module stores a few speed settings on a small chip. By default, your PC uses a safe standard setting. To run a kit at its advertised speed, you switch on its stored profile in the BIOS (the setup screen you reach when the PC starts). On Intel systems that profile is called XMP. On AMD systems it is EXPO. Many kits carry both, and many AMD boards can also read XMP profiles.
Running a profile above the CPU’s official speed is a mild form of overclocking. It usually works, but it is not guaranteed, and how well it works depends on the board, the CPU and the kit together. That is why RigFit tells you the official speed and flags any kit that is faster: it will fit, but its label speed needs the profile turned on.
Two sticks or four?
For most people, two sticks is the right answer. Desktop platforms like AM4, AM5, LGA 1700 and LGA 1851 have two memory channels. Two modules fill both channels at the highest official speed. Four modules still use two channels, just with two sticks on each, and as the table shows, DDR5 then has a much lower official speed. If you need a lot of memory, two larger modules (2 × 32 GB, for example) are usually a better choice than four smaller ones.
If your board has four slots and you use two, put them in the slots the board manual recommends (often the second and fourth from the CPU). The wrong pair can leave both sticks on one channel.
Mixing kits
Memory is sold and tested as a kit. Two kits with the same label can use different memory chips inside, especially if they were made months apart. Mixing kits often works, but nobody tested that exact combination, so it can fail to boot or need a lower speed. When kits differ in speed, all the memory runs at the slowest one. If you can, buy one kit with all the memory you want.
How much memory do you need?
These are the sizes the RigFit checker suggests, always as a kit of two equal modules:
| What you do | Size | Kit |
|---|---|---|
| Everyday use: web, office, streaming video | 16 GB | 2 × 8 GB |
| Gaming | 32 GB | 2 × 16 GB |
| Video editing and 3D | 64 GB | 2 × 32 GB |
| AI and machine learning | 64 GB | 2 × 32 GB |
Why these numbers? Windows, your browser and background apps use memory before any game or project opens. When memory runs out, the PC moves data to the much slower drive, and things stutter. 32 GB gives a modern game room next to a browser, voice chat and a launcher. Large video projects, 3D scenes and AI models fill memory quickly, so 64 GB gives them space. If your chipset’s limit is lower, the checker caps the suggestion there.
Newer DDR5 modules also come in “in-between” sizes such as 24 GB and 48 GB per stick, so a 2 × 24 GB (48 GB) kit is a real option if 32 GB feels tight but 64 GB is more than you need.
Speed and latency, explained honestly
Memory speed has two parts: how much data it moves, and how long it takes to start.
Data rate (MT/s). The number after DDR4- or DDR5- is the data rate in megatransfers per second (MT/s). DDR4-3200 moves data 3,200 million times per second. Many stores call this “MHz,” but that is not quite right. DDR memory makes two transfers per clock tick, so DDR4-3200 actually runs at a 1,600 MHz clock. When a listing says “3200 MHz,” it means 3200 MT/s.
CAS latency (CL). CL is how many clock ticks the memory waits before it starts sending data after a request. Lower is quicker, but only at the same speed, because a tick is shorter on faster memory.
First-word latency (in nanoseconds). To compare kits fairly, turn CL into real time:
latency in nanoseconds = CL × 2000 ÷ data rate (MT/s)
Here are common kits from our catalog:
| Kit | Latency | Products in our catalog |
|---|---|---|
| DDR4-3200 CL16 | 10.0 ns | 149 |
| DDR4-3600 CL18 | 10.0 ns | 55 |
| DDR4-3200 CL22 | 13.8 ns | 70 |
| DDR5-4800 CL40 | 16.7 ns | 56 |
| DDR5-5600 CL46 | 16.4 ns | 73 |
| DDR5-5600 CL36 | 12.9 ns | 40 |
| DDR5-6000 CL30 | 10.0 ns | 51 |
| DDR5-6400 CL32 | 10.0 ns | 71 |
So a DDR5 kit with a “bad-looking” CL of 30 can respond just as fast as a DDR4 kit with CL16, while moving far more data. And two DDR5-5600 kits can differ a lot: at CL36 the first word arrives in 12.9 ns, at CL46 in 16.4 ns.
Does it matter? Some programs and games are sensitive to memory speed and latency, others barely notice. How much a faster kit helps depends on the CPU, the game and the settings. RigFit does not run speed tests, so we will not give you a percentage. What we show you are the specs and whether the kit fits your PC. A sensible rule: get enough memory first, in two sticks, at or near your CPU’s official speed. Pay extra for faster kits only if the price gap is small.
How to compare two kits: a checklist
- Type: DDR4 or DDR5, matching your board.
- Shape: DIMM for desktops, not SO-DIMM.
- Unbuffered, not registered: skip RDIMM unless you have a workstation board that asks for it.
- Total size and module count: for example, 32 GB as 2 × 16 GB.
- Speed vs your CPU’s official speed: is it at, below or above? Above means you will need XMP or EXPO.
- Profile type: XMP, EXPO or both. On AMD, EXPO is the native one.
- First-word latency: CL × 2000 ÷ MT/s. Lower is better.
- Height: tall heat spreaders can hit large air coolers. Check your cooler’s memory clearance.
- Board support list: your board maker’s tested memory list (often called the QVL, “qualified vendor list”) is useful, though it is never complete.
- Warranty and price per GB.
Adding memory or replacing it?
If you already have 2 × 8 GB and want 32 GB, you can either add another 2 × 8 GB or replace everything with 2 × 16 GB.
Adding costs less up front, but it means four sticks, mixed kits and, on DDR5, a lower official speed. It needs four slots too.
Replacing with one kit of two larger sticks is cleaner. You keep the higher two-module speed, get a tested matching pair, and keep two slots free. If you do add, match the speed and size of your current sticks.
Common mistakes
- Buying DDR5 for a DDR4 board, or the other way around.
- Buying a laptop SO-DIMM for a desktop.
- Buying registered server memory because it was cheap.
- Buying one big stick instead of two, which leaves one channel empty.
- Filling four slots with DDR5 and expecting the label speed.
- Forgetting to turn on XMP or EXPO, so the kit runs at its slower default speed.
- Judging kits by CL alone, or by speed alone.
- Mixing kits and expecting them to behave like one.
FAQ
How much RAM do I need for gaming? We suggest 32 GB as a kit of 2 × 16 GB. It leaves room for the game plus the apps that run next to it. 16 GB still works for lighter games and everyday use.
Can I use DDR5 in a DDR4 motherboard? No. The slots are keyed differently and the two types run at different voltages. You would need a new board, and possibly a new CPU.
Is 2 × 16 GB better than 1 × 32 GB? Yes, for desktops. Two modules use both memory channels. One module uses only one.
Should I turn on XMP or EXPO? If you bought a kit rated above your CPU’s official speed, yes, or it will not run at its rated speed. It is one setting in the BIOS. If the PC is unstable afterwards, turn it off again, or pick a lower profile.
Can I mix different RAM brands or kits? Often, but it is not guaranteed. All the memory will run at the slowest kit’s settings. One matched kit is safer.
How do I know what RAM my PC has? In Windows, Task Manager > Performance > Memory shows the size, speed and slots used. Your motherboard model is listed in the System Information app.
Find the kit that fits your PC
Memory makers’ tools show you their own kits. RigFit starts from your PC instead: tell us your motherboard and CPU, what you use your PC for, and DDR4 or DDR5 if your board could take either. We tell you the kit to look for in plain terms: the size, two modules, your CPU’s official speed and your chipset’s limit, plus a note when a faster kit needs XMP or EXPO.