The mulch formula
Mulch is sold by volume, so the job is to turn an area and a depth into a volume — then convert that volume into the units the supplier uses.
volume (cu ft) = length × width × ( depth ÷ 12 )
cubic yards = volume ÷ 27
bags = volume ÷ bag size
Depth is entered in inches, so dividing by 12 converts it to feet. Dividing the cubic-foot volume by 27 gives cubic yards (a yard is 3 ft, and 3 × 3 × 3 = 27).
Worked example
A 20 ft × 10 ft bed mulched 3 inches deep, using 2 cubic foot bags:
Area: 20 × 10 = 200 sq ft.
Volume: 200 × (3 ÷ 12) = 50 cubic feet.
Cubic yards: 50 ÷ 27 = 1.85 cu yd.
Bags: 50 ÷ 2 = 25 bags.
Tip: at roughly two cubic yards or more, bulk delivery is usually cheaper per unit than buying bags, and far easier than hauling 25 bags by hand.
How deep should mulch go?
A layer of 2 to 3 inches suppresses weeds and holds moisture without suffocating roots. Thicker than 4 inches can trap too much water and starve roots of air. Keep mulch pulled back a couple of inches from stems and trunks to avoid the "mulch volcano" that invites rot and pests.