ATX is a spec released by Intel in 1995 and occasionally updated since then. It stands for Advanced Technology eXtended, and it was intended to replace the older de facto AT standard established by the IBM AT, a PC released in 1984. It’s been used as the industry-wide standard since its introduction, and has thus far resisted Intel’s attempt to replace it, like the BTX form factor for motherboards in 2004 and the new ATX12VO standard for PSUs and motherboards published in 2019. The full ATX/ATX12V spec defines many aspects of both motherboards and power supplies, not just their size, but the only thing we’re concerned with today is motherboard form factor and why EATX is a bullshit term for it.

Note: we’re going to refer to the motherboard’s dimensions based on how they’re normally oriented in a case, so the distance from the bottom to the top is height, and from the front (of the case) to the back is width. Dimensions are written as height x width.

EATX or E-ATX stands for extended ATX, creating the unfortunate acronym Extended Advanced Technology eXtended, which sounds like a bad ad for a product you’d see on a different video website. EATX is defined as 12 inches tall by 13 inches wide, if it’s even defined at all, while a standard ATX board is 12 inches tall and 9.6 inches wide. Other companies have occasionally taken it upon themselves to complicate things, as with Supermicro’s EE-ATX, which obviously means “Enhanced Extended Advanced Technology eXtended,” and then there’s XL-ATX, a name which has been used by EVGA, Gigabyte, and MSI to describe three nearly-identical motherboard sizes. Let’s not forget BTX, or “Balanced Technology eXtended,” a standard measured 10.5”x12.8” and, although some parts of it are now laughably outdated (like the recommendation to cool the CPU using only airflow from the PSU), the additional PCB space it offered may have sidestepped this whole mess if it had been more widely adopted.

As a brief aside, XL-ATX -- any of the three that exist -- is essentially the “what if we made it taller, too” alternative to ATX. Making an ATX board taller allows more room for chipset cooling and PCIe slots, which are primarily useful for multi-GPU setups, which are increasingly rare. That and the increasing popularity of PSU shrouds which put a hard limit on motherboard height mean that XL-ATX is effectively dead, even though it’s still being used. It’s been supplanted by boards that increase width instead, known as “EATX,” which isn’t real and should be regarded as a fairy tale. Extra width gives more room for RAM and larger/multiple CPU sockets, which are more desirable at this point than extra GPUs.

Gigabyte, as another aside, can't even get it right. They don't know if their product is E-ATX or XL-ATX, but they've listed it as both on Amazon and on their own product page.

Confusingly, the SSI-EEB (Server System Infrastructure Forum’s Enterprise Electronics Bay, originally known as Entry[-Level] Electronics Bay) form factor is an ATX variant that’s exactly the same dimensions as full EATX and shares most of its mounting holes. EEB is interchangeable with SSI-EEB as a name for the form factor. The original EEB spec says that “the dimensions of this E-Bay are based on the standard AT board dimensions, 12 inches x 13 inches,” and it appears that most of the mounting holes are based on the now 36-year-old form factor as well, although component layout has changed. Version 1.0 of the SSI-EEB spec is dated 1999, but the term “extended ATX” predates this. We spoke to several manufacturers while researching for this piece, and one of them remembered ASUS as the possible origin of EATX as a marketing term through their in-house brand ElanVital. We checked, and it appears that ElanVital started calling cases “EATX” that they had formerly branded “AT” sometime in 2002-2003. The lines are extremely blurred here, because AT and EEB are different names for the same size of board, as was EATX originally. Before it was ruined.

The major problem with EATX is that it isn’t defined. We’ve skirted around this so far, but EATX is a blanket term slapped on any ATX board wider than 9.6 inches, not an official spec. EATX is used as a marketing term by both motherboard and case manufacturers to signify any motherboard that’s wider than 9.6 inches and therefore won’t fit in a normal ATX case, while 12”x13” SSI-EEB boards are usually referred to with a prefix like “true EATX” or “full EATX.” Because we needed more distinctions, and motherboard makers can reap what they sow, damn it.

The SSI Forum defined multiple form factors, like the 12”x10.5” Compact Electronics Bay, and EATX is used as a generic term for that as well. This may be EVGA’s fault: as early as 2009, they were referring to the SSI-CEB X58 Classified as “EATX,” after which other companies like Gigabyte and ASUS followed suit, which has forced case manufacturers to use the phrase “EATX-compatible” to convey that they support motherboards wider than 9.6”. From EVGA’s perspective, it was necessary to have a quick way to say that a board wouldn’t fit in a normal mid-tower. Silverstone is one of the few holdouts that refuses to use EATX to mean anything other than 12”x13”.

There isn’t an official spec for EATX, just ATX, but both EATX and SSI-EEB boards simply add an additional column of screw holes to the three that are present in normal ATX boards. You can check the numbers yourself if you want, but we’ll save you some time and confirm that normal SSI-EEB hole spacing is exactly the same as normal ATX, plus some extra. Let’s repeat that, because it’s a point of confusion even among case and motherboard manufacturers: SSI-EEB is based on ATX and uses exactly the same hole placement with some alternative and extra holes thrown in.

“Full” or “true” 13-inch EATX boards are EEB boards. EEB also has options for some extra holes around the PCIe slots and potentially a secondary CPU socket, since it’s meant for server boards.

Note that not all of the SSI-EEB holes are required, and the ones marked with an apostrophe are alternatives to the “legacy” ATX locations to allow motherboard manufacturers more freedom in placing components. The alternate holes weren’t present in the original spec, and they do NOT line up with the ATX standard, so if a board manufacturer chooses to use any of them, it limits compatibility with ATX/EATX cases. This is why there’s so much conflicting information online about whether “EATX” and EEB are interchangeable: if a manufacturer just uses the primary ATX-compatible holes, the board will probably be referred to as EATX and everything is hunky-dory. If they use any of the alternate EEB hole placements and call the board SSI-EEB, the logical conclusion for a user trying to install the board is that EEB hole spacing is different from EATX.

Even boards that refer to themselves as E-ATX may make use of the alternate EEB hole placements: for example, EVGA’s SR-3 uses the Y’ [Y Prime] mounting hole. The SSI Forum took care to point out that motherboard manufacturers can skip any mounting holes they don’t want, but cannot add any additional ones. It’s interesting to see that as of at least 2011, the SSI-EEB spec name-drops EATX and acknowledges that it’s a common name for 12”x13” boards.

This problem has expanded beyond motherboards to affect the products that are related to them, like cases. Here are the ten most recent regular consumer cases we’ve reviewed, and how they describe their maximum motherboard size compatibility, verbatim:

• Fractal Define 7: “E-ATX (max 285mm)”
• Antec P120 Crystal: “E-ATX” [in downloadable flyer: “up to 12” x 11”]
• Lian Li Lancool II: “E-ATX/ATX (width: under 280mm)”
• Bitfenix Nova Mesh TG: “E-ATX up to 272mm(10.7 inch)”
• NZXT H710: “EATX (Up to 272mm or 10.7-inches)”
• Phanteks P400A: “E-ATX *(up to 272mm wide, cannot use rubber grommets)”
• Lian Li O11 XL: “E-ATX (need to purchase an extension panel for EEB motherboards)”
• Corsair 465X: “ATX”
• Fractal Define S2 Vision: “EATX (up to 285 mm wide)”
• Corsair 220T: “full-sized ATX”

Out of those ten cases, eight of them claim to support EATX, and not a single one of them other than the O11 XL can fit the full 13in/330mm width of the EVGA SR-3. Even the XL officially requires a separately-purchased part for 13-inch wide boards to fit. We can’t get too mad at these particular case manufacturers though, because (other than Antec) they do all include specific notes about what size of EATX boards they can fit--there are “EATX” boards that are between 9.6 and 13 inches wide, like EVGA’s 10.9 inch wide X299 Dark. We can get mad about the fact that they have to include these notes at all, rather than just saying the name of a form factor. This is stupid. It actually takes more space to type out “EATX up to 11 inches” than it does to write 12”x11”.

The reason these cases aren’t compatible with wider boards isn’t consistent, either. Sometimes, cases like the Fractal Define 7 seem like they could easily support much wider boards with some minor adjustments to the rim of the motherboard tray. There are also plenty of cases that have relatively empty space in front of the motherboard tray that’s blocked by a cable management channel, like the P120 Crystal. Even if there isn’t room for a full set of SSI-EEB standoffs, it would be easy for so many cases to allow wider boards to fit and overhang at the front of the case. The elimination of optical and hard drive cages has left this area of cases a no man’s land, and wider motherboards would be a great way to fill it. EATX support is clearly an afterthought in most cases, where chassis are fully designed and then measured after the fact to see if any EATX boards will fit, rather than being designed around 13” wide boards from the start.

The reason we’ve complained about this repeatedly is because we’ve run into three separate instances where EVGA’s boards require a larger-than-ATX case, and we then have to hunt through our inventory for an enclosure that will actually support the correct size of EATX and check it with a ruler to find out if it will fit. The SR-X doesn’t really count, since it uses the unique and ridiculously large 13.6x15 inch HPTX form factor, but the X299 Dark and SR3 are both considered EATX and were both troublesome, especially the SR3. We eventually gave up and tossed it in the gigantic case Intel shipped us to house the oversized Dominus Extreme. Speaking of which, the Dominus Extreme is advertised as being “ 14”x14” EEB/ATX Form Factor.” 14”x14” is neither EEB nor ATX, ASUS. Please stop. The one thing everyone can agree on, even EVGA, is that ATX, EATX, and EEB have a height of 12 inches.

The name “EATX” implies a standard, but it’s not a standard, it’s a free-for-all. Things would be a lot easier for everyone if motherboard manufacturers stuck to the dimensions of SSI-EEB without trying to wedge custom form factors in between, or correctly referred to 12”x10.5” boards as SSI-CEB. Then case manufacturers would have no reason to write “EATX (up to 11 inches)” in every single spec sheet for normal-sized mid towers, and customers would know at a glance exactly what they were getting. We use open benches for testing, so we’d prefer it if manufacturers made the leap and committed to true SSI-EEB for anything larger than ATX, but at the end of the day it’s the consumer’s opinion that matters. Let us know your thoughts on EATX and if you’ve found it to be as much of a pain in the ass as we have.

Editorial: Patrick Lathan
Host, Editorial: Steve Burke
Video: Keegan Gallick