A “yard” of mulch in landscaping refers not to a linear measurement but to a unit of volume called a cubic yard. This volumetric measurement is standard for ordering bulk landscape materials like soil, gravel, and shredded wood. A cubic unit is necessary because mulch is applied across a two-dimensional area at a specific depth, creating a three-dimensional volume. The goal is to calculate how much square footage one cubic yard of material will cover at the desired thickness.
The Essential Formula for Mulch Coverage
One cubic yard is equivalent to 27 cubic feet, which is the fixed volume used in all coverage calculations. To determine the square footage one cubic yard covers, this volume must be divided by the depth of the mulch layer. Since application depths are usually measured in inches, a conversion factor is required to align the units.
There are 12 inches in one foot; therefore, a depth of three inches equals \(0.25\) feet (\(3 \div 12\)). The coverage area is found by dividing 27 cubic feet by the converted depth in feet. For simplicity, landscapers often use the constant 324, which calculates the square footage covered by one cubic yard at a one-inch depth (27 cubic feet multiplied by 12 inches).
The formula to find the required cubic yards is to multiply the total square footage by the desired depth in inches, and then divide that result by 324. This calculation provides the precise volume of material needed to cover the area uniformly.
Standard Coverage Rates by Depth
The coverage rate of a single cubic yard changes inversely with the depth of the application. For a thin layer, such as refreshing an existing bed, a depth of 2 inches is often used. At this thickness, one cubic yard of mulch covers approximately 162 square feet. This depth is the minimum recommended for effective weed suppression and moisture retention.
The standard application depth for new beds or general landscaping is 3 inches. Applying mulch at this thickness ensures a good balance between moisture conservation, temperature regulation, and material cost. At a 3-inch depth, one cubic yard covers about 108 square feet. This is the most common rate used for quick estimates in residential projects.
For areas requiring heavy weed control or entirely new garden beds, a 4-inch depth is sometimes preferred. This thicker layer provides a greater barrier against sunlight, inhibiting weed germination more effectively. When spread at 4 inches deep, one cubic yard of mulch covers approximately 81 square feet. Applying more than four inches is not recommended, as excessive depth can restrict oxygen to plant roots.
Calculating for Irregular Areas and Real-World Adjustments
Measuring a landscape area is straightforward for simple squares or rectangles, but many garden beds feature curved or irregular boundaries. For non-rectangular shapes, the most accurate method involves segmenting the area into smaller, recognizable geometric shapes. A large curved bed, for example, can be approximated by breaking it down into a series of rectangles and triangles. Calculating the square footage of each section individually and adding them together provides the total area.
Once the total square footage is determined and the volume of mulch is calculated, a final adjustment is necessary. Mulch is a loose, organic material that settles over time and may be applied unevenly during spreading. It is standard practice to increase the final calculated volume by an additional 10 to 15 percent. This buffer covers material waste, spillage, or natural compaction that occurs after delivery and during application.