The area of every bed you add, minus the areas you leave out.
Volume = net area × the depth you’re adding, in cubic yards and cubic feet (or m³ and litres).
Order volume after the extra you choose, and bag count from your bag’s label in Store Mode.
Choosing a depth
Advice varies by mulch type and soil. Iowa State University Extension suggests 3–4 in of wood chips or shredded bark on well-drained sites and 2–3 in on heavy soils (source). University of Kentucky Extension says no deeper than 2–3 in, and about 2 in for finely ground mulch (source). All of them warn against piling mulch against tree trunks. The quick picks here are choices, not a recommendation.
How far does mulch go?
Area covered by one cubic yard and by one 2 cubic foot bag at common depths
Depth
1 cu yd covers
One 2 cu ft bag covers
1 in
324 sq ft
24 sq ft
2 in
162 sq ft
12 sq ft
3 in
108 sq ft
8 sq ft
4 in
81 sq ft
6 sq ft
Arithmetic: 1 cu yd = 27 cu ft = 324 sq ft at 1 in deep. Check the volume on your bag; sizes vary.
Worked example: a bed with a tree in it
Bed: 10 × 12 ft = 120 sq ft. Tree ring left out: a 2 ft circle = 3.14 sq ft. Net: 116.86 sq ft.
At 3 in (0.25 ft): 116.86 × 0.25 = 29.21 cu ft = 1.08 cu yd.
In 2 cu ft bags with no extra: 29.21 ÷ 2 = 14.6 → 15 bags.