What Is the Compost Calculator?
This calculator tells you how much compost you need to cover a garden bed at a chosen depth. Enter the area in square feet and the desired application depth in inches, and it returns the volume needed in both cubic yards (how compost is usually sold in bulk) and cubic feet.
How to Use It
Measure the length and width of your bed and multiply them to get the area in square feet. Decide on a depth — most gardeners apply 1–3 inches of compost as a topdressing, or up to 6 inches when amending new beds. Enter both values and read off your result.
The Formula Explained
Because depth is measured in inches but area in feet, the depth is first divided by 12 to convert it to feet. Multiplying area (ft²) by depth (ft) gives the volume in cubic feet. Since bulk compost is sold by the cubic yard, the cubic feet are then divided by 27 (the number of cubic feet in a cubic yard).
$$\text{Cubic Yards} = \dfrac{\text{Area (ft}^2) \times \text{Depth (in)} / 12}{27}$$
Worked Example
For a 500 ft² bed at 2 inches deep:
$$500 \times (2 \div 12) = 83.33 \text{ cubic feet}$$
Dividing by 27 gives about 3.09 cubic yards of compost.
FAQ
How deep should compost be? A 1–2 inch topdressing is typical for established beds; 3–6 inches works for building or heavily amending soil.
How much does a cubic yard weigh? Compost weighs roughly 800–1,100 lb per cubic yard depending on moisture content.
Can I use this for mulch or soil too? Yes — the volume math is identical for any bulk material spread at a uniform depth.
