Dimensions of Standoffs and Screws for ATX Motherboard

What are the dimensions for the standoffs and screws used for mounting an ATX motherboard to the computer casing?

It depends.

According to the ATX 2.1 standard:

• The standoff provided has to be a minimum of 6.5mm and the external cross section has to fit within a 10mm x 10mm area around the standoff hole.

• The mounting screws must (obviously) match the standoff screw wells (usually 6/32 or M3 thread size).

• the length can be whatever you want as long as the components will fit in your case. A longer standoff allows cables to be run under the motherboard and may also improve airflow.

According to the ATX 2.1 standard, the standoff provided has to be a minimum of 6.5mm and the external cross section has to fit within a 10mm x 10mm area around the standoff hole.

There's a lot of variation (as you've discovered) to how various manufacturers meet those specs, but as long as the mounting screws match the standoff screw wells (usually 6/32 or M3 thread size) it doesn't really matter what the external width is as long as it's within the 10mm x 10mm max size requirement.

Source Motherboard standoff dimensions

From experience over many years, they are all standard threads and size, but not length, that varies by case manufacturer.

Generally a computer case will come with 9 standoffs to meet ATX standards, unless the case is smaller. The majority of ATX motherboards will use 6, smaller boards even less. More expensive 'wider' ATX boards will use all 9, which is nice when they are right next to the main I/O for the disk drives.

There are many cases available that use zero or only some standoffs and use threaded and stamped raised sheet metal for the stand offs.

Source Motherboard Standoffs Sizes

You can buy a standoff anywhere between 4mm to 10mm in length as far as I know. I have standoffs measuring 4mm, 6mm, 7mm and 8mm.

Personally I like a larger standoff to allow cable routing behind the motherboard. Thin cables obviously. It also allows a better airflow behind the motherboard in my opinion.

Source Motherboard Standoff Heights?

So after a lot of deliberation here's what essentially going to boil down to opinion, but at least maybe it'll answer your question.

The bore depth on the plate can vary as much as you like, however as your image shows, the thread size is a standard which you should match.

You can get ATX standoffs in a huge assortment of sizes and shapes as well as materials (by sizes I mean the length of the thread as well as the 'nut' can vary how ever the width).

As long as your motherboard + any expansion cards which are sitting perpendicular to it fit within your case, you can have any sized stand offs you like (they obviously need to be big enough to stand off from the back plate, and for some cpus they'll require a little room for holding plates which fit to the back side of the cpu - this varies too much to easily answer, check for the specific CPU model you want).

You may want longer stand offs if you want to run cabling between the motherboard and the back plate, especially if you're using a case which doesn't have much room behind the back plate for cable management.