Gloria Glover Topsoil is the mineral-rich upper layer of soil, and it usually forms only part of the best raised-bed mix. For most raised garden beds, we can use quality topsoil for about 50% to 60% of the fill, then add compost and an aerating material to improve fertility, drainage, and structure.
To calculate how much you need, multiply the bed’s internal length by width by depth. That gives total fill volume; multiply that result by the topsoil percentage to find the topsoil volume.
What Is Topsoil—and How Is It Different From Compost and Potting Mix?
Topsoil is the upper layer of natural soil, made primarily from mineral particles such as sand, silt, and clay, with some decomposed organic matter. Its texture and quality vary widely. Screened topsoil with a loose, crumbly texture is generally more useful for a raised bed than dense soil containing rocks, construction debris, or large clods.
Topsoil is not the same as compost. Compost is decomposed organic material—such as plant matter and manure—that adds nutrients and supports soil organisms. It improves topsoil but does not replace its mineral structure. Because compost continues to break down, filling a bed with compost alone can lead to settling and excessive nutrient levels.
Topsoil is also different from potting mix. Potting mix is usually a manufactured, soilless blend of materials such as coconut coir, peat moss, perlite, or vermiculite. It is designed for containers and seedling roots, while topsoil is natural mineral soil and is typically too dense for use alone in pots.
Topsoil Calculator: How to Calculate Volume for a Raised Garden Bed
Multiply Length × Width × Depth
Measure the bed’s inside length and width, then measure the planned soil depth. Convert all measurements to feet before calculating:
Length (ft) × width (ft) × depth (ft) = total cubic feet
For depth in inches, divide by 12 first. For example, a 10-inch fill depth is 10 ÷ 12 = 0.83 feet. Use the finished soil depth rather than the height of the frame if the bed will not be filled completely.
Convert Cubic Feet to Cubic Yards
Bulk soil is commonly sold by the cubic yard, while bagged products are often labeled in cubic feet. Convert the result with this formula:
Total cubic feet ÷ 27 = total cubic yards
The calculator result is the volume of the complete soil mix, not automatically the amount of topsoil. To calculate topsoil specifically, multiply total volume by the topsoil share. For example, a 60% topsoil mix requires total volume × 0.60. Buying about 5% to 10% extra can account for settling and minor measuring differences.
How Topsoil Fits Into a Raised Garden Bed Soil Mix
Choose a Topsoil Share, Not a Topsoil-Only Fill
A practical starting formula for raised garden bed soil is:
- 50% to 60% topsoil: the mineral base that provides body and water-holding capacity.
- 30% to 40% compost: organic matter and gradual nutrient release.
- 10% to 20% aerating material: such as aged fine bark, leaf mold, or another suitable coarse amendment.
Choose the lower end of the topsoil range when the soil is heavy or drainage is limited. Use more topsoil when the product is loose, well-screened, and the bed already has excellent drainage. Mix the materials thoroughly before planting rather than layering them, which can create abrupt drainage changes.
Worked Example: A 4 × 8-Foot Bed at 12 Inches Deep
A 4 × 8-foot bed filled to 12 inches has a depth of 1 foot:
4 × 8 × 1 = 32 cubic feet
Convert that volume to cubic yards:
32 ÷ 27 = 1.19 cubic yards
Using a 60% topsoil, 30% compost, and 10% aerating-material mix, the quantities are:
- Topsoil: 19.2 cubic feet, or about 0.71 cubic yard
- Compost: 9.6 cubic feet, or about 0.36 cubic yard
- Aerating material: 3.2 cubic feet, or about 0.12 cubic yard
For convenient ordering, we could round slightly upward to about 20 cubic feet of topsoil, 10 cubic feet of compost, and 4 cubic feet of aerating material.