Every power rack attachment lives or dies on the same three numbers, no matter which brand made the rack or the attachment. Manufacturers report them differently, use different terminology, and sometimes don’t report them at all. That’s why “will this fit?” is one of the most common home-gym questions with one of the most inconsistent sets of answers online.
If you own Bulletproof Fitness attachments, skip the manual work and use the Bulletproof Rack Attachment Compatibility Checker, which checks these numbers for you against Bulletproof’s published specs. Everything below applies whether you’re shopping Bulletproof or any other brand.
The three numbers that decide everything
Upright size is the outside dimension of the tube, almost always quoted in inches: 3×3, 2×3 or 2×2. Attachments that clamp or wrap around the upright care about this above all else, because the jaw either closes on the tube or it does not.
Hole diameter is the size of the holes drilled through the upright, and it is measured across the hole, not across the pin that goes through it. Common sizes are 1 inch, 5/8 inch and 1/2 inch, with 3/4 inch and 11/16 inch appearing on some imported racks. Most manufacturers publish this number somewhere in their spec sheet or manual, but not always on the main product page, so check both before assuming it isn’t listed.
Inside width is the clear distance between the inner faces of your two front uprights. This is the number most people have never measured, and it is the one that most often turns a yes into a no for wide attachments like belt squat carriages and pulley systems.
“3×3” is not a standard, and neither is the hole
It is tempting to treat 3×3 as a specification. It is not. It describes the outside of the tube and says nothing about wall thickness, hole size, hole spacing, or whether the holes are laser-cut round or slightly oval. Two racks both honestly advertised as 3×3 can take different pins.
Hole spacing is a separate axis again. Westside spacing puts holes 1 inch apart through the bench and press range and 2 inches elsewhere; standard spacing is 2 inches throughout. Spacing does not usually decide whether an attachment mounts, but it decides how finely you can position it, which is the difference between a comfortable pressing height and the nearest hole being an inch wrong forever.
Don’t infer one number from another. A rack’s upright size doesn’t tell you its hole diameter, and its hole diameter doesn’t tell you its spacing. Inferring is exactly the shortcut that produces confident wrong answers, so check each number on its own.
When you can’t find a number, here’s how to check it yourself
If a manufacturer doesn’t publish a spec, the honest move is to measure it rather than guess. Hole diameter is the easiest to settle: a caliper gives you an exact reading in about a minute, or you can work it out with a drill bit from a set where you know the sizes: find the bit that just slides through without play.
Inside width takes a tape measure across the clear distance between your front uprights, at the height the attachment will actually sit, not at the floor or the top crossmember. Some racks aren’t perfectly square, and a rack that’s been moved or assembled on an uneven floor can be off by enough to matter.
Your own measurements are a primary source. A product page is not always kept current, and a spec sheet PDF can lag behind a running production change. When the two disagree, trust the tape measure and the caliper.
Fit and safety are different questions
An attachment that physically bolts on isn’t automatically safe to load. A bolted-down 3×3 rack with heavy-gauge uprights and a free-standing 2×2 rack with thin-wall tubing can both take the same pin, but only one of them should carry a loaded carriage or a rack-mounted pulley system. Many attachments, including belt squats, pulleys and anything that puts load forward or outward on the rack, require the rack to be bolted to the floor or wall, or heavily weighed down, regardless of whether the pin fits.
Check the manufacturer’s stated mounting requirements for the specific attachment, not just whether the hardware lines up.