About the Mulch Calculator
Mulch is sold and applied by volume, but garden beds are measured by area. This calculator bridges the two: you give it the length and width of your bed and the depth of mulch you want, and it returns the volume of mulch you need in cubic yards and cubic feet, plus how many bags to buy. That turns a guessing game at the garden center into a single number.
The formula
Volume is area multiplied by depth. Because bed dimensions are usually in feet and mulch depth is chosen in inches, the depth is converted to feet by dividing by 12, and the result is converted to cubic yards by dividing by 27 (there are 27 cubic feet in a cubic yard). Combining those two conversions gives the shortcut most landscapers use:
- Cubic yards = Length (ft) × Width (ft) × Depth (in) ÷ 324
- The 324 comes from 12 (inches per foot) × 27 (cubic feet per cubic yard).
- Cubic feet = Length (ft) × Width (ft) × Depth (in) ÷ 12, which you divide by your bag size to get the number of bags.
How deep should mulch be?
For most flower beds and around shrubs, a 2 to 3 inch layer is ideal: deep enough to suppress weeds and hold soil moisture, but not so deep that it starves roots of oxygen. Coarse bark can go up to 4 inches. Avoid piling mulch against trunks and stems ("volcano mulching"), which traps moisture and invites rot and pests. When refreshing an existing bed, measure only the additional depth you need to top it back up, not the full target depth.
Common reference points
- One cubic yard of mulch covers about 100 square feet at 3 inches deep, or roughly 160 sq ft at 2 inches and 80 sq ft at 4 inches.
- A standard bagged mulch bag holds 2 cubic feet; it takes 13.5 bags to equal one cubic yard. Bulk mulch is often sold by the 3 cubic foot bag (9 per cubic yard) or by the yard from landscape suppliers.
- One 2 cubic foot bag covers about 8 sq ft at 3 inches deep or 12 sq ft at 2 inches.
Estimating irregular beds
The length × width formula assumes a rectangle. For an L-shape, split the bed into rectangles and add the areas. For a circle, use area = π × radius². For a rough curved bed, estimate an average length and width. Once you have the area in square feet, the rest of the math is identical. It is normal to round up and add 5 to 10 percent, since mulch settles and beds are rarely perfectly flat.