Fireplaces come in all shapes and sizes, and in all types of materials. Many contemporary fireplaces have no mantel at all, while others feature elaborate mantels in carved wood trim or fancy stonework. There is no national standard for the height of a mantel, although there is a minimum distance to keep wood away from the firebox opening. Most mantels are sized by standard firebox sizes and the aesthetic concerns of the room where the fireplace is featured.
National Fire Protection Association
The National Fire Protection Association provides standards for how close wood trims should be to the firebox opening. A 10-inch thick mantel should be at least 19-inches above the firebox. Checking the national code for the type and quantity of material being used for the mantel will help determine the minimum height of the mantel for your application. Local codes should also be consulted as their instructions may vary from the national code.
Facing Material and Firebox
The fireplace firebox is the inner box where the fire actually burns. The facing material is the fire-resistant masonry, stone or tile work that surrounds the firebox to provide an additional safety zone. Most fireplaces have standard-size fireboxes. The facing of the fireplace will vary depending on the builder and what materials they are using. Many companies have mantel surrounds designed to fit the standard-size of firebox. Identify the standard mantel for the size of your firebox, and tape the size around your firebox so you can see how it fits the room.
The Room and Ceiling Height
Some rooms are large. When a fireplace is set in a large room, the scale of the fireplace must rise to fit the volume of the space. If the room is both wide and tall, then the fireplace will often be a custom size. In this case the mantel is proportionally enlarged to match the aesthetic scale of the room, and the fireplace. In some cases the firebox may be so large that a person can stand inside the box.
The Right Look
Once you have determined the minimum height, and you have taped standard fireplace surrounds and mantels on the wall around your firebox, now it is time to determine what you like, personally. If the mantel looks too low, too wide, too narrow, too small, too tall or otherwise wrong, change it. You are allowed to move the mantel as high as you want and to shape it to fit the style and decor of your room. You should try to select a height that feels good to a majority of the people around you since you may want to sell your home at some point and you don't want to have to change the mantel later.