How much weight can that shelf actually hold?

Shelving fails at the wall, almost never in the shelf. The board holds. The bracket holds. What lets go is the connection between the bracket and whatever is behind the drywall.

Every shelf is a lever. A bracket that sticks twelve inches off the wall multiplies the load at the top fastener, which is trying to pull straight out of the wall while the bottom fastener pushes in. That pull-out force is the number that matters, and drywall on its own resists almost none of it.

In a garage, overhead and wall racks go into studs, full stop. Manufacturer ratings assume framing. A rack rated for 600 pounds anchored into drywall is rated for nothing.

In a closet, the standard rail systems are designed to hang from one continuous top rail lagged into studs, with everything below hanging off it. Skip a stud on that rail and the whole system sags in the middle once the clothes go on.

Floating shelves are the least forgiving, because the entire load is cantilevered on hidden rods. They need the rods in framing or a proper blocking layout behind the drywall. Anything else and the shelf slowly tips down over a few months.

We locate framing, use lag bolts sized to the load, and tell you honestly when a layout needs to shift a few inches to catch a stud. A shelf three inches off center that holds beats one perfectly placed that does not.