19" is the standard. I was a cabinet maker before opening my room, built my own cases and bought prefab rack ears from a local supplier. Everything fits just fine. :)

BUT just in case (and this depends on what type of rails you buy) it would behoove you to measure twice once you get your gear... you can just rack up both units to the bare rails (one at the very top, one at the bottom) and measure things out from there. 19" is the standard yes, but in my particular case there are 19 1/8th inches between the two sheets of MDF. Any more or less and I would have had some problems already with some things.

I have always received a decent deal on railing from this guy:
Sidekick

Good luck, and way to DIY!

Thanks all, I was just doing some rough sketches. I'll wait until the gear gets here before I cut any lumber :)

I just made an 18 space rack out of 3/4 MDF but you can use 3/4 ply (Baltic Birch is best but very pricy). make your inside cabinet width 19 1/8" wide and the inside height 1/4" greater than the length of your rails splitting the difference when installing the rails. I set back my rails 1" from the front. If you use MDF make sure you use coarse thread screws and drill a pilot hole otherwise the screw acts like a drill and has no gripping power, it just digs out a hole. Cabinet is 18" deep. I finished it with flat black latex.

When making audio racks, I usually use 3/4" plywood, with a smooth finish on one side, and allow for 1/8" on both sides of the equipment. This gives 19 1/4" between the inside walls. This just gives a little extra room when installing the equipment. Parts Express sells pre-cut rack rails and other items for building audio racks.

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http://audiomusicalresources.com

Using ISO 9001 standard rack rails:
Rack rail holes must be 18 5/16" center to center, allowing 17 13/16" from inside rail edge to inside rail edge.