How do I determine the mantel shelf length for a fireplace?

There are numerous factors and preferences to consider when choosing the length of the shelf. Many mantels are the size of the wall width and the body tapers in from the widest part, the shelf.  Some designs are square and would not require the full width of the wall, but it is wise to assure the box is centered on the wall.  If it is not, then you will likely have more sheetrock left on one side than the other side when installing any surround, accentuating the fact that the box is off centered as this example: 

Off center fireplace surround
There are some fireboxes that are built forward into the room in which case you can wrap the entire surround on the sides to the back wall for a grander look with depth.  If you have room on the side, you can even continue the shelf on both sides.  If you have bookshelves on each side this is not feasible unless the wall is tapered away from the shelf areas.
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boxed fireplace mantel

If you have a tv over the fireplace, it looks best to have a shelf that is wider than the tv. 

Custom Empress
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What is standard fireplace width?

Our most popular styles are 68” wide, but ours will range from 55”-84” wide with some custom designs even larger.  The wall and the firebox width as well as the ceiling height provide insights on choosing a size that is appropriate for your fireplace.  The product should be balanced with the wall and room size.

What is the standard fireplace height?

It is important to consider the height of the ceiling, the height of the firebox, and the location of the firebox off the floor in the wall.  If you plan to place a tv above the fireplace surround, 50”- 65”  is preferred for most floor plans,  but if your room is large and you are seated back substantially from the fireplace wall, you can go higher.  If you are investing in a nice surround, you do not want it to look puny with a huge tv above.   Your eyes can look upwards without your head tilting up to see the tv.  It is best to sit in the location your furniture would be, look at the wall and mark where you think you want to tv to be.

If you are not planning on having a tv above, the height of your ceiling is important, as well as if you have items to place on the mantel or above it such as a mirror or artwork.  Two story ceilings allow for a much taller shelf and the ability to raise the firebox to place a 15” H seat under the box for additional seating in the room.