Mulch is sold by the cubic yard or the bag, and a bed is measured in square feet, so the sum in between is the one everybody gets wrong. It is not hard: one cubic yard covers 324 square feet an inch deep, so at three inches it covers 108.
What matters more than the arithmetic is the depth, and that is where most calculators stop. Two extension services put it plainly, and this page uses their numbers rather than a round one someone liked:
- Fine wood and bark mulches should not be more than 2 to 3 inches deep, because fine particles pack down and shut out air and water.
- Coarse material such as pine bark nuggets can be as deep as 4 inches, because air moves through it.
- Grass clippings or shredded leaves should never be deeper than 2 inches — they mat.
- Around a tree, 2 to 4 inches and no more, kept off the trunk: 3 to 5 inches clear of a young tree, 8 to 10 of a mature one. Mulch heaped against a trunk starves the roots of oxygen and kills the inner bark, which is the “volcano” you see in car parks.
Measured an odd-shaped bed on the calculator? Follow its link here and the area comes with it.
Gravel and topsoil run on the same arithmetic, with what each of them weighs. For a lawn, the sod calculator counts rolls and pallets; for anything else sold by volume there is the plain cubic yard calculator.