15x15 ft bed at 2in of mulch

Mulch is sold by volume, not by weight or bag count, so the math starts with a simple volume formula: bed length times bed width times desired depth, all converted to the same unit. This calculator keeps length and width in feet and depth in inches, so it divides the depth by 12 before multiplying — a 10 ft by 10 ft bed at a 3-inch depth works out to 10 × 10 × (3 ÷ 12) = 25 cubic feet.

Cubic feet is useful for buying bagged mulch, but bulk mulch from a landscape supplier is almost always priced and delivered by the cubic yard, which is 27 cubic feet. This calculator divides the cubic footage by 27 to give you that number too, so you can compare a bulk delivery quote against a stack of bags at the garden center and see which is actually cheaper for your project size.

For bag counts, this tool assumes the most common bag size sold in the US: 2 cubic feet per bag. It rounds the bag count up to the next whole bag, since you can't buy a fraction of a bag and running short mid-project means an extra trip to the store. If your local supplier sells 1 or 3 cubic foot bags instead, just divide your cubic footage by that bag size directly.

Depth matters more than people expect: 2 inches is typically enough to suppress weeds around established perennials, 3 inches is the standard recommendation for most garden beds and around trees and shrubs, and going past 4 inches can actually suffocate roots and hold too much moisture against stems, encouraging rot. Piling mulch directly against a trunk or stem — a practice sometimes called a "mulch volcano" — is also worth avoiding regardless of total depth.

This calculator gives you the volume for a single freshly-applied layer. Organic mulches like shredded bark or wood chips settle and decompose over a season, typically losing 20-30% of their depth by the following year, so many gardeners plan on a lighter annual top-up rather than recalculating a full fresh-bed depth every time.