How the number is worked out
rectangle area = length × width
circle area = π × (diameter ÷ 2)²
ring area = circle area (outer) − circle area (bare trunk circle)
cubic feet = area × (depth wanted − already there) ÷ 12
cubic yards = cubic feet ÷ 27
Bulk mulch is sold by the cubic yard, bagged mulch by the cubic foot, so the calculator gives both. Bags are the cubic feet divided by the size printed on your bag, rounded up.
Worked examples
A 10 ft × 4 ft bed, 3 in deep, starting bare:
- 10 × 4 = 40 ft², × 3 ÷ 12 = 10 ft³ = 0.37 yd³
- With bags marked 2 cu ft: 10 ÷ 2 = 5 bags
Five trees, each with a 6 ft ring and a 1 ft bare circle at the trunk, 3 in deep:
- Ring: π × 3² − π × 0.5² = 27.49 ft²
- Per tree: 27.49 × 3 ÷ 12 = 6.87 ft³
- Five trees: 34.36 ft³ = 1.27 yd³
Topping up the same bed from 1 in to 3 in only needs 2 in of new mulch — two thirds of the bare-ground amount. That is what the “already there” box is for.
Why the ring has a hole in the middle
The ring shape leaves a bare circle around the trunk, because mulch is usually kept back from the bark rather than piled against it. Set that circle to 0 if you want a solid disc.
What this calculator is not for
It doesn't pick a mulch type or tell you how deep a particular plant wants it. The 2–4 inch range above is a general figure for most products from the extension guide cited below; check the guide or your local extension office for your plants and climate. Fresh mulch also settles, so what you spread may not stay at the depth you measured.
Sources
- University of Missouri Extension, Mulches (G6960). Checked 2026-09-24.
“A 2- to 4-inch layer of most products is needed to insulate the soil effectively and prevent weed growth.”
- NIST, NIST Handbook 44 (2026), Appendix C — General Tables of Units of Measurement. Checked 2026-09-24.
“1 728 cubic inches (in3) = 1 cubic foot (ft3) 27 cubic feet = 1 cubic yard (yd3)”