Enter your bed size and how deep you want the mulch, and get the cubic yards, cubic feet and number of bags to buy — plus the cost.
Mulch is measured by volume, so you need three numbers: the length and width of the bed in feet, and how deep you want the mulch in inches. Multiply length by width to get the square footage, multiply that by the depth in feet (inches ÷ 12), and you have cubic feet. There are 27 cubic feet in a cubic yard, which is how bulk mulch is sold.
Bagged mulch usually comes in 2 cubic foot bags, so it takes about 13.5 bags to make one cubic yard. Bags are easy for small beds; once you get past roughly 6–8 bags, bulk mulch by the cubic yard is almost always cheaper and less to haul. A single cubic yard covers about 100 square feet at 3 inches deep, or about 160 square feet at 2 inches.
For most beds, 2 to 3 inches holds moisture and blocks weeds. Around trees and shrubs, 3 to 4 inches is fine — but keep mulch pulled a few inches back from trunks and stems so they don't rot. Piling it deeper than about 4 inches can smother roots.
This is a planning aid. Coverage figures are rounded rules of thumb — actual coverage varies with how you spread and pack the mulch. Always round up and buy a little extra.