What size case? How ATX, mATX and ITX differ

The five figures a case imposes on everything else

A case does not merely hold the parts; it is the ceiling for the rest of the machine. Every one of these is a hard limit rather than a suggestion.

1. 1The board sizes it takes — ATX / mATX / ITX
2. 2Maximum graphics card length — see Will the graphics card fit the case?
3. 3Maximum cooler height — this decides which air coolers are available to you
4. 4The PSU size it takes — ATX / SFX / SFX-L
5. 5How many drive bays — 3.5" and 2.5" counted separately

Four cases, by their real figures

Read that table from the bottom up and the price of smallness becomes clearer — the smaller the case, the narrower your choices elsewhere, and they narrow in several directions at once.

What a small build costs you

An ITX machine looks good and takes far less desk space, but it is worth knowing what you are trading away.

• ITX boards have 2 memory slots, not 4 — so you have to buy all the capacity you want in one kit.
• You need an SFX power supply, which costs more than an ATX unit of the same wattage and comes in fewer varieties.
• Large air coolers will not fit. Cooler Master MasterBox NR200P takes 153mm, and a popular cooler like DeepCool AK400 is 155mm.
• Few drive bays, so you lean on M.2 — see M.2 NVMe vs SATA SSD.
• It is much harder to assemble. There is almost no room to route cables, and installing things in the wrong order means taking them out again.

The good news is that the graphics card is less restricted than you might expect. Cooler Master MasterBox NR200P takes a 330mm card, the same as the mATX Montech AIR 100 ARGB — the cost of ITX falls on cooler height and the power supply far more than on card length.

mATX is the sweet spot for most people

Without a specific reason to go smaller or larger, mATX is usually the answer — 4 memory slots like ATX, an ordinary standard-price PSU, room for a large air cooler, and appreciably smaller than ATX.

The reason to step up to ATX is wanting more PCIe slots for expansion cards, or wanting many drives — Lian Li Lancool 216 takes two 3.5" and six 2.5" drives, which no small case can do.

The reason people overlook is cooling and noise. A bigger case has more room for air to move, so its fans spin slower for the same result. A large machine under the desk is usually quieter than a small one on top of it.

When to choose the case

After the motherboard, before the graphics card. By then you know the board size, and the case’s figures immediately become the filter for the card and the cooler.

On the build page, choosing the case first makes the parts that will not fit disappear from the list — and you can show them again with the reason, down to how many millimetres over they are.