Build a hearth for a fireplace that is 16 inches deep, according to "This Old House" writer Harry Sawyers. This is the standard requirement for municipalities and homeowner associations. The hearth should also extend a minimum of eight inches on either side of the fireplace.
Add a hearth that is at least 18 inches wide around a wood stove. Not all wood stoves have hearths, so this also includes the floor protection around the wood stove.
Design an outdoor fireplace or fire pit so the hearth is at least 20 inches wide. However, the wider the fire pit, the less likely it is stray sparks will land on dry grass. Consider making the hearth wider if you live in a dry, windy area.