Mulch is a beneficial addition in many garden settings. Accurately determining how much mulch to purchase to apply at an appropriate depth is important to be more efficient and save time and money.
Follow the steps below to calculate the amount of mulch needed.
Find the Area | Determine Mulch Depth | Calculate the Volume | More Information
Start by Finding the Area of the Planting Bed
To determine the amount of mulch needed for a particular area, it is necessary to know the area or square footage of the planting bed.
Area of Rectangular Bed
A simple rectangular bed is one of the most straightforward bed shapes to determine the area. Simply, multiply its length by its width.
Area = length × width
Area of Triangular Bed
To determine the area of a triangular-shaped planting bed, Measure the length of the base and then the height (the distance from the top point down to the base). Then, use the formula below to determine square footage.
Area = base × height × 0.5
Area of Circular Bed
For a round planting bed, multiply pi (π=3.14) by the radius squared. The radius equals the distance from the center of the circle to the edge.
Area = 3.14 × radius2
Area of a Semicircle Bed
For a semicircle (also called a half-moon or half-circle) bed, start by finding the length of the radius. For a semicircle, the radius equals the length of the flat/straight edge divided by two. Then, use the formula for the area of a circle and multiply it by one-half.
Area = 3.14 × radius2 × 0.5
Area of Oval (Elliptical) Bed
An oval bed (more accurately referred to as an ellipse) can be found after collecting two measurements. Measure from the center of the bed to the furthest edge point. This is called the major radius. Then, going back to the center, at a right angle to the major radius, measure from the center to the bed edge - this is the minor radius. Multiply those two numbers with pi (π=3.14) to get the area.
Area = 3.14 × major radius × minor radius
Area of an Irregular-Shaped Bed
Many garden beds have a more irregular shape. For beds comprising straight lines, simply break the bed apart into triangles, rectangles, and/or semicircles. Find the area of each piece and add them together.
For more complex irregular shapes, such as a bed shaped like a kidney bean, the bed area can be estimated with decent accuracy by first gathering several measurements. Find the length of the longest line within the shape - this is the length line. Along this length line, measure perpendicular width lines (at a 90° angle) to the length line. The distance between the width lines (called the segment width) must be equal. For example, you might measure the width of the bed every 3 feet along that length line. You can measure any number of width lines as long as they are equidistant from each other. The more width lines you measure, the more precise your area calculation will be. To find the area, add all of the width line lengths together and multiply it by the segment width.
Area = (sum of all width lines) × segment width
Determine Your Mulch Depth
Many hardwood chipped or shredded mulches are spread between 2 and 4 inches thick. Learn more about the best depth of mulch to use in this article: Using Mulch in the Garden.
Because you've most likely measured your area in square feet, you need to convert the depth from inches to feet. Use the table below to help determine the desired mulch depth in feet.
| Mulch Depth in Inches | Mulch Depth in Feet |
|---|---|
| 0.5 | 0.041 |
| 1 | 0.083 |
| 1.5 | 0.125 |
| 2 | 0.166 |
| 2.5 | 0.208 |
| 3 | 0.25 |
| 3.5 | 0.291 |
| 4 | 0.333 |
| 4.5 | 0.375 |
| 5 | 0.416 |
| 5.5 | 0.458 |
| 6 | 0.5 |
Determine the Amount of Mulch Needed
To determine the volume of mulch you need to purchase, multiply the area of the garden bed by the depth of mulch in feet.
Volume of Mulch = Area of Garden Bed × Height of Mulch (in feet)
For example, if you wish to freshen up the mulch layer on a 265 ft2 perennial garden bed with 1.5 inches of mulch, your calculation would look like this:
Volume of Mulch = 265 ft2 × 0.125 ft = 33.125 cubic feet
Convert to Cubic Yards
Mulch is commonly sold by the cubic yard - whether that be in bulk or in 1 or 2 cubic-yard bags. Once you've determined the amount of mulch needed in cubic feet, divide by 27 to get the amount in cubic yards. For example, for the garden bed mentioned above, your calculation would look like this:
Volume of Mulch = 33.125 ft3 / 27 ft3 = 1.22 cubic yards.
More Information
- Using Mulch in the Garden
- How to Determine Plant Quantity for Planting Beds
- Planting Trees in the Landscape
- Care of Newly-Planted Trees
- How to Plant Perennials
- How to Plant Bare Root Plants
- Tips for Shopping and Selecting Quality Plants
Photo credit: Ozgur Coskun/AdobeStock