Here is an easy way to understand how to calculate your mulch needs.

First remember there are 27 cubic feet in one cubic yard.
One cubic yard covers 324 Sq. ft. with depth of one inch.

Now - take your square footage, divide by 324, and multiply by the depth in inches.  This is the number of cubic yards that you need.

Example - Distance of 45 ft. x 80 ft. = 3,600sq. ft.
3,600 divided by 324 = 11.11 x 2 inches deep =22.25 yds