Garden Bed Soil Mix Calculator
Find exactly how many cubic yards of soil you need to fill a raised garden bed. Use this before buying soil to avoid costly over- or under-ordering.
About this calculator
The volume of soil needed for a raised bed is found by multiplying its three dimensions — length, width, and depth — and then converting the result into cubic yards, the standard unit sold by suppliers. Because length and width are in feet but depth is often measured in inches, the depth must first be converted to feet by dividing by 12. The resulting cubic feet are then divided by 27 (since 1 cubic yard = 27 cubic feet). The formula is: cubic yards = (bed_length × bed_width × (bed_depth / 12)) / 27. Garden type and drainage requirements then guide the recommended soil blend — for example, a classic raised-bed mix is typically 60% topsoil, 30% compost, and 10% perlite or coarse sand for drainage. Knowing the total volume upfront lets you scale each component accurately.
How to use
Suppose your raised bed is 8 ft long, 4 ft wide, and 12 inches deep, and you want a standard vegetable-garden mix. Step 1 — Convert depth: 12 in ÷ 12 = 1 ft. Step 2 — Cubic feet: 8 × 4 × 1 = 32 cu ft. Step 3 — Cubic yards: 32 ÷ 27 ≈ 1.19 cu yd. Step 4 — Apply mix ratios: 60% topsoil = 0.71 cu yd, 30% compost = 0.36 cu yd, 10% perlite = 0.12 cu yd. Round up when ordering to allow for settling.
Frequently asked questions
How many cubic yards of soil do I need for a 4x8 raised bed?
For a 4 ft × 8 ft bed filled to 12 inches deep, you need (4 × 8 × 1) / 27 ≈ 1.19 cubic yards of soil. At a shallower 6-inch depth that drops to about 0.59 cubic yards. Always round up slightly when ordering, because soil settles 10–15% over the first growing season. Most suppliers sell in half-cubic-yard increments, so plan accordingly.
What is the best soil mix ratio for a raised garden bed?
A widely recommended raised-bed mix is 60% topsoil, 30% compost, and 10% coarse perlite or vermiculite. The topsoil provides mineral structure and weight, the compost delivers nutrients and beneficial microbes, and the perlite improves aeration and drainage. For vegetables, some gardeners use a lighter 'Mel's Mix' of equal thirds compost, peat moss, and coarse vermiculite. The best ratio depends on your local native soil, rainfall, and the crops you're growing.
Why does soil depth matter so much for raised garden beds?
Root depth determines how deep your soil needs to be. Shallow-rooted crops like lettuce and herbs thrive in 6 inches of soil, while tomatoes, squash, and most root vegetables need 12–18 inches for healthy development. Insufficient depth restricts root expansion, reduces water retention, and leads to nutrient deficiencies even in a rich mix. A deeper bed also insulates roots from temperature extremes and requires less frequent watering.