How to estimate mulch manually
Mulch is sold by volume. If you are ordering a delivery from a local landscape supplier, you will buy it in cubic yards. If you are buying it from a hardware store, it is sold in bags measured in cubic feet.
Cubic yards = Length (ft) × Width (ft) × [Depth (in) ÷ 12] ÷ 27
A standard depth for new garden beds is 3 inches to properly suppress weeds and retain moisture. If you are just topping off an existing mulch bed, 1 to 2 inches is usually sufficient.