How large of a hole should be dug when planting a tree?

FAQ
Question: 

How large of a hole should be dug when planting a tree?

Answer: 

When planting balled and burlapped and container-grown trees in well-drained soils, dig a hole that is 2 to 3 times wider than the diameter of the tree’s rootball.  The depth of the hole should be 2 or 3 inches less than the height of the rootball.  Slope the sides of the hole so the top of the hole is several inches wider than the bottom. 

Poorly drained sites are difficult locations for many trees.  When planting in poorly drained soils, the width of the hole should be 2 to 3 times the diameter of the tree’s rootball.  The depth of the planting hole should be approximately two-thirds of the height of the rootball.  When placed in the hole, the top one-third of the rootball should be above the surrounding soil.  When backfilling, place soil to the top of the rootball and gradually slope it down to the surrounding soil line.