Last time, in So You Wanna Build a PC?, we talked about deciding what your computer is for. Now that you know your goal, a few more rules come into play. Think of it like baking: the ingredients have to go together in the right proportions, and in a PC build, that means checking form factors and making sure every part physically fits and works with the others.
This is where a lot of first builds go wrong. A motherboard that doesn’t fit the case, a graphics card that’s an inch too long, a cooler that won’t mount to the socket. None of it is hard to avoid if you check before you buy, so here is what to check.
The Build-Your-Own-PC series
- So You Wanna Build a PC?
- Form Factors: Making Sure Your Parts Are Compatible (you are here)
- Choosing PC Components: What to Buy and Why
Motherboard Form Factors
The form factor is the size and layout standard for your motherboard. It decides what case the board fits in, how many RAM slots you get, and how many expansion cards you can add. There used to be a long list of competing standards. Today, almost every desktop build uses one of these four:
| Form factor | Size | RAM slots | Best for |
|---|---|---|---|
| E-ATX | 12 x 13 in | 4 to 8 | High-end work PCs |
| ATX | 12 x 9.6 in | 4 | Most gaming and work builds |
| Micro-ATX | 9.6 x 9.6 in | 2 to 4 | Budget and office builds |
| Mini-ITX | 6.7 x 6.7 in | 2 | Small and living room PCs |
ATX is the safe default. It has room for a big graphics card, plenty of M.2 slots for SSDs, and good airflow. Micro-ATX gives up a few expansion slots to save space and money, and for a home office machine you will never miss them. Mini-ITX is great for a compact PC, but you get one expansion slot, two RAM slots, and less room to work, so it’s a harder first build.
Cases are sized to match. A case built for ATX will also take Micro-ATX and Mini-ITX boards, but not the other way around. Check the case spec sheet for the board sizes it supports.
What to Check Before You Buy
CPU Socket
The socket is the connection between the CPU and the motherboard, and it’s the first thing to match. Each CPU generation fits a specific socket:
- AMD Ryzen 7000 and 9000: AM5
- Intel Core Ultra 200 series: LGA 1851
- Intel 12th to 14th gen Core: LGA 1700
Older guides will tell you Intel uses LGA (pins on the motherboard) and AMD uses PGA (pins on the chip). That’s no longer true. AMD switched to LGA with AM5, so both brands now put the pins in the motherboard socket. Handle the board carefully, because bent socket pins are one of the most common build mistakes we fix.
Even with the right socket, an older board may need a BIOS update to run a newer chip. Check the motherboard’s CPU support list before you buy either part.
RAM Type and Speed
Current boards use DDR5. Older boards use DDR4. A board supports one or the other, never both, and the sticks are notched differently so you can’t install the wrong type. If you’re buying used parts on Facebook Marketplace, double check this before you pay.
RAM is also rated by speed, like DDR5-6000. Your system will only run as fast as the slowest limit between the RAM, the CPU, and the motherboard. When one part holds the others back, that’s called a bottleneck. Check the board’s supported memory list (often called the QVL), and turn on the memory profile in the BIOS (EXPO on AMD, XMP on Intel), or the RAM will run at a slower default speed.
Storage Connections
Most builds now boot from an NVMe SSD that plugs straight into an M.2 slot on the motherboard. A few things to check:
- Size: almost all drives and slots are the 2280 size (22mm wide, 80mm long)
- NVMe or SATA: some M.2 slots only accept NVMe drives, and some older M.2 drives are SATA. Match the drive to the slot
- Shared lanes: on some boards, using certain M.2 slots turns off SATA ports or slows another slot. The manual will tell you which
SATA ports are still around for hard drives and 2.5 inch SSDs. Any current board runs them at SATA 3 speed (6Gb/s), which is plenty for a hard drive.
Graphics Card Clearance
Modern graphics cards are big. Many are over 12 inches long and take up three expansion slots. Check the case’s maximum GPU length, and make sure nothing like a drive cage or front fan is in the way.
Also check power. Most cards need one or more 8-pin power connectors, and high-end NVIDIA cards use the newer 12V-2×6 connector. Make sure your power supply has the right cables.
CPU Cooler Fit
Two things to check with any cooler. First, it has to support your socket. Most coolers include mounting hardware for current sockets, but older coolers may need an adapter kit. Second, it has to fit the case. Every case lists a maximum CPU cooler height, and tall tower coolers are often the first thing that doesn’t fit.
If you’re using an all-in-one liquid cooler, check where the case can mount the radiator and what size it takes (240mm, 280mm or 360mm).
Power Supply Size
Standard cases use ATX power supplies. Many small and Mini-ITX cases need a smaller SFX unit instead. Check which one your case takes, and how long a unit it can fit, before you order. Our power supply guide covers wattage and quality.
Good to know
Free tools like PCPartPicker check most of these compatibility rules for you as you add parts. They aren’t perfect, so still read the spec sheets for anything that seems tight, like GPU length and cooler height.
Next Up: Picking Your Parts
Once you know your parts will fit together, it’s time to choose them. In Choosing PC Components: What to Buy and Why, we walk through every part and what to look for. And if you’re still on the fence about building it yourself, read Why Your Next Computer Should Be Custom Built.
Not sure your parts will fit? Our techs can check your parts list, or build it for you.
