Mantels come in a variety of shapes and sizes.There are industry standard sizes. Standard mantels are made to fit most fireplaces and have preset sizes.
Mantel Shelf Depth
Standard mantel shelf depth is eight inches from the wall to the front edge. This is just deep enough to provide a display area for decorations while not intruding too far into the room.
Mantel Shelf Height
Mantel shelf height depends on the amount of decorative trim used. A simple unadorned mantel shelf can be as small as four inches, while a highly ornate mantel shelf can be as tall as 10 inches.
Mantel Shelf Width
Width is determined by the size and scale of the fireplace. A 50-inch-wide fireplace might have a mantel shelf that ranges from 70 to 80 inches wide.
Each mantel is made up of different parts. The legs, or surround, are vertical supports on either side of the fireplace. The shelf, or cap, is the horizontal piece that tops the fireplace and legs. The shelf may be used alone or with a "skirt," a horizontal piece used under the shelf.
Fireplaces have a standard minimum height of 38 inches and a width of 50 inches.