Board size does not change how fast a PC is. The same chipset and processor perform the same on a 170 mm square as on a full-size board. What size changes is how many things you can plug in, which cases you can use and what you pay. For most builds ATX is the easy default, Micro-ATX is the value choice if you need one graphics card and a couple of SSDs, and Mini-ITX is for people who want a small PC enough to pay more and plan more carefully.
The sizes side by side
| ATX | Micro-ATX | Mini-ITX | |
|---|---|---|---|
| Size | 305 × 244 mm | Up to 244 × 244 mm | 170 × 170 mm |
| Expansion slot positions | Up to 7 | Up to 4 | 1 |
| Memory slots | Normally 4 | Usually 4, 2 on the cheapest | 2 |
| M.2 slots | Most, varies by board | Fewer, varies by board | Fewest, often one on the back |
| Fits in | ATX cases | Micro-ATX and ATX cases | Almost any case |
| Price for the same chipset | Middle, widest choice | Often the cheapest | Often the dearest |
The three standards share mounting-hole positions, so a smaller board always fits a larger case. The reverse is never true. E-ATX sits above all three: wider than ATX, used loosely as a label, and only worth considering for flagship and workstation boards. Our E-ATX guide covers the case-clearance problems that come with it.
ATX: the default
ATX has the room to do everything without compromise: four memory slots, the most M.2 slots a chipset allows, a full set of fan and USB headers and space between the graphics card and everything else. It also has by far the widest range of models, which matters on a deals site, because more models on sale means more of them discounted at any one time.
The seven slot positions matter less than they once did. A modern graphics card covers two to four of them on its own, and sound, networking and USB are all on the board already. Most ATX builds use exactly one slot. What you are really buying with ATX is spacing, M.2 slots and headers, not the expansion slots themselves.
Read more in the ATX form factor guide.
Micro-ATX: the value choice
Micro-ATX removes the bottom 60 mm or so of an ATX board. You keep the graphics slot, usually four memory slots and normally two M.2 slots, and you lose the lower expansion slots and some headers. Because makers aim this size at budget builds, a Micro-ATX board is often the cheapest way onto a given chipset, and the cheapest boards of any platform are nearly always this size.
That budget focus is the thing to watch. Micro-ATX ranges are thin at the top: there are few with strong power delivery, and the very cheapest drop to two memory slots, a single M.2 slot and bare power circuitry with no heatsink. If you are fitting a high-core-count processor, read VRM explained before choosing the cheapest board in any size.
A practical point: a thick graphics card on a Micro-ATX board will cover most of what lies below it, so any second slot may be unusable. If you need a capture card or a network card as well as a large graphics card, check the slot spacing on the board's photo or choose ATX. There is more in the Micro-ATX guide.
Mini-ITX: small, capable, dearer
A Mini-ITX board has one PCIe slot and two memory slots, and that is enough for a fast gaming PC: one graphics card, two modules, one or two SSDs. Two memory slots are not a performance handicap. The short, simple memory wiring often handles fast DDR5 profiles very well. The limit is capacity, since you cannot add a second pair later, so buy the amount you will want from the start.
What you give up:
- Money. Packing the same features into a third of the area needs more board layers and denser layout, and the models sell in small numbers. Expect to pay more than for the equivalent ATX board, and expect a smaller range: many chipsets have only a handful of Mini-ITX models between all the makers.
- M.2 slots. Usually two, often with one on the underside of the board, which means taking the board out of the case to change that drive.
- Headers. Fewer fan headers, fewer internal USB headers and often fewer SATA ports.
- Room to work. The socket sits close to the memory and the PCIe slot, so large air coolers can clash with tall memory or with the back of the graphics card. Check the cooler maker's compatibility notes.
The board is only part of the cost of a small build. Small-form-factor cases frequently need a compact SFX power supply, limit cooler height and graphics card length, and run warmer. None of that is a reason not to build small, but it is a reason to choose the case first and the board second. See the Mini-ITX guide for the details.
What stays the same at every size
- Processor support and performance. A B850 board takes the same Ryzen processors at any size. The exception is sustained load on a high-end chip, where a small or cheap board's power delivery may run hotter.
- The main graphics slot. Every size has one full-length x16 slot wired to the processor.
- Chipset features. Overclocking support, memory profiles and PCIe generation follow the chipset, not the size, although the number of slots and ports that expose them does shrink.
- Mounting and power. The same 24-pin and 8-pin power connectors, the same cooler mounting for the socket.
Choosing in four questions
- Do you already have a case? It sets the maximum board size. An ATX case takes all three sizes; a Micro-ATX case takes two; a Mini-ITX case takes one.
- How many SSDs? Count the M.2 drives you own or plan to buy and check the board has that many slots. This is the limit people hit first on small boards.
- Anything in a slot besides the graphics card? If yes, choose ATX, or check slot spacing very carefully on Micro-ATX.
- Is a small PC the point of the build? If yes, accept the Mini-ITX premium. If you just want a tidy desk, a compact Micro-ATX case gets most of the way for less.
Size and open-box buying
Form factor affects what you will find among Warehouse offers. ATX and Micro-ATX boards sell in the largest numbers, so they are returned in the largest numbers, and there is usually a choice. Mini-ITX offers are rarer, and because the new price is higher the saving in pounds can be worth having when one appears. Whatever the size, a returned board needs the same inspection: our used motherboard checklist sets it out step by step, and how to choose a motherboard covers the rest of the decision.
Questions people ask
Will a Micro-ATX motherboard fit in an ATX case?
Yes. Micro-ATX and Mini-ITX boards use a subset of the ATX mounting holes, so both fit any standard ATX case. You may need to move a few standoffs. The board will look small in a large case, and cables have further to travel, but nothing else changes.
Is a smaller motherboard slower?
No. Performance comes from the processor, memory and graphics card. A Mini-ITX board with the same chipset runs the same processor at the same speed. The only caveat is a high-end processor under long, heavy load, where a board with weaker or hotter power delivery can hold it back, and that depends on the board rather than its size.
Why are Mini-ITX motherboards more expensive?
Fitting the same features into a 170 mm square needs more board layers and a denser design, and Mini-ITX models sell in much smaller numbers than ATX ones, so the cost is spread over fewer boards. There are also fewer models competing on price.
Are two memory slots enough?
For most people, yes. Two modules is the configuration that runs fast memory most reliably on any board. The drawback is that you cannot add more later without replacing what you have, so buy the full capacity you expect to need.
Can I use a large graphics card on a Mini-ITX board?
The board does not limit it: the single slot is a full x16 slot. The case does. Check the maximum graphics card length and thickness your case allows, and whether the card would block anything you need on the board.