I've got a couple of shelves that were manufactured to 120cm in length, which is slightly less than 48" (47.25"). Unfortunately, I discovered that the desired location has stud spacing of 24" rather than 16". This means that if I hang the shelf using two brackets placed on the studs, I'm going to have an overhang of around 11.625" on either side.
Is that too much overhang? Would it be better to use drywall anchors to space one or both (or more?) of the brackets out farther?
The recommendations I've found online vary as to how much overhang is safe, most suggesting no more than 6" but I've also seen 12". My gut feeling is it would be ok as long as I didn't put heavy objects on the ends of the shelves, but as a DIY noob I don't trust my gut.
Edit: The shelves are 1" particle board and weigh 10 pounds. They will be used for a reasonably light load – display objects, small potted plants and a few books.
Best Answer
The issues with shelves are sag and safety.
Given 11" overhang and the bracket spacing you are at extremely (maybe none) low risk for sag. Two feet isn't really a wide enough space for a board to sag unless it is really thin.
The 11" on each side, its nothing. The only issue there is tipping but that would be you adding a 40 pound anvil on the very very edge (I don't think a 40 pound anvil would but just for the sake of arguing). Also that is the kind of weight that would be needed to sag the outside and that would be based on the anvil's center of mass - if it were 10" wide your weight would only be about 5-6" outside the bracket.
So you have no issues. Also I would not worry about adding more support given you have screws into studs. If you add more than 2 brackets on a system - one of the 3 brackets ends up with the most weight on it. And if it is a bracket not on a stud it will sag carrying most weight.
Most important thing on the install is shelf is snug on the wall. The shelf being flush will keep it from bending out - which happens to some mdf shelves over time.