The branching factor b for a tree is the average number of children of the tree's vertices.

For our problems, b will have as upper bound the size of the move list, with the actual value being significantly lower.

(Compare b to the average graph adjacency list size a.)

For a Full Tree of Depth	The Tree's Size Is
0	1
1	b
2	b × b
3	b × b × b
...	...
d	bd