Mulch Calculator

In short Mulch is sold by the cubic yard or by the bag, and gardens are measured in square feet, so every mulch question is the same conversion: area times depth. The number worth carrying in your head is that ONE CUBIC YARD COVERS 108 SQUARE FEET AT 3 INCHES DEEP. Change the depth and the coverage changes in exact proportion — 162 square feet at 2 inches, 81 at 4 — because coverage multiplied by depth in inches is always 324. That falls straight out of a cubic yard being 27 cubic feet and a foot being 12 inches, so it is a fact about arithmetic rather than about mulch. Bags follow the same rule: a 2 cubic foot bag covers 8 square feet at 3 inches, and 24 divided by the depth in inches for any other depth. Two to three inches is the working depth. It is enough to stop most weed seed germinating and to slow evaporation, and it is thin enough that rain still reaches the soil. Four inches is for coarse bark on paths where nothing is meant to grow. Deeper is not better, and against a tree it is actively harmful — a cone of mulch piled up the trunk holds moisture against bark that is not built to stay wet, and it is the standard way people kill young street trees over about five years. Pull it back a hand span from any stem, which is why this page offers a ring shape with a gap in the middle. Weight is the one figure here that is a guess rather than a calculation. Bark runs around 400 kilograms per cubic metre dry and can weigh half again as much soaked; compost is nearer 650 and screened topsoil about 1,250. Those are typical figures, not constants, and moisture moves them more than material does. Buy by volume, and use the weight only as a sanity check on what turns up on the lorry.