The proper height of a fireplace mantel depends on the height of the hearth, the size of the fireplace's fire box, and the requirements of the local building codes. In addition, the National Fire Protection Association proposes codes relating the width of the mantel to its distance from the fireplace.
The NFPA calls for any mantel that is 2 inches deep to be a minimum of 11 inches from the top of the fireplace's fire box. A 4-inch deep mantel should be 13 inches from the fire box. A 6-inch mantel should be 15 inches away. An 8-inch deep mantel should be 17 inches above the fire box, and a 10-inch mantel needs to be 19 inches above the fireplace.