Filling raised beds takes more soil than most people expect. A single 8 × 4 ft bed, 12 inches deep, holds 32 ft³, which is more than a cubic yard for two beds and over 20 bags at 1.5 ft³ each. This raised bed soil calculator works out the volume for any number of rectangular or round beds, adds an allowance for settling, and splits the total into the mix recipes university extension services recommend. Iowa State Extension suggests equal parts topsoil, organic matter such as compost, and coarse sand, or roughly 70% topsoil and 30% compost. University of Minnesota Extension suggests ⅔ to ½ topsoil with ½ to ⅓ compost.
For in-ground beds or lawn top-dressing, use the topsoil calculator. To mulch the paths, try the mulch calculator.
How to use the raised bed soil calculator
- Measure the inside length and width of the bed, or the diameter of a round bed.
- Enter the soil depth you plan to fill. A bed with 12-inch sides is usually filled an inch or two below the rim.
- Enter the number of identical beds.
- Choose a mix recipe. The calculator splits the volume into topsoil, compost and sand.
- Enter a bag size if you’re buying bags. UMN Extension notes that buying in bulk by the cubic yard is usually cheaper for large beds.
Formula
Each component of the mix is its fraction of the total volume. Iowa State Extension's raised bed guidance calculates the soil needed the same way: length × width × height.
Worked example
Two 8 × 4 ft beds filled 12 inches deep, with 10% extra for settling, in a 70/30 mix.
- One bed: 8 × 4 × 1 = 32 ft³. Two beds: 64 ft³.
- With 10%: 70.4 ft³ = 2.61 yd³ (1,993 L).
- Mix: 1.83 yd³ topsoil + 0.78 yd³ compost.
- In 1.5 ft³ bags: 70.4 ÷ 1.5 = 46.9, so 47 bags, which shows why bulk delivery is worth pricing.
Soil per bed (no allowance)
| Bed | 6 in deep | 12 in deep | 18 in deep |
|---|---|---|---|
| 4 × 4 ft | 8 ft³ | 16 ft³ | 24 ft³ |
| 8 × 4 ft | 16 ft³ | 32 ft³ | 48 ft³ |
| 10 × 4 ft | 20 ft³ | 40 ft³ | 60 ft³ |
| 12 × 4 ft | 24 ft³ | 48 ft³ (1.78 yd³) | 72 ft³ (2.67 yd³) |
| 6 ft round | 14.1 ft³ | 28.3 ft³ | 42.4 ft³ |
How deep should a raised bed be?
University of Missouri Extension (guide G6985) notes that most plants need at least a 6- to 12-inch rooting zone, though deeper is better. It adds that beds built higher than 18 to 24 inches require retaining walls with foundations and supports. If the bed sits on loosened native soil, roots can grow down into it, so a shallower bed may work. Beds on hard ground or a patio need the full depth in the bed. MU suggests beds about 4 feet wide when you can reach from both sides, and 3 feet wide from one side, so you never step on the soil.
UMN Extension advises against filling raised beds with potting soil, peat or coir alone, which dry out quickly. Choose a loam or sandy loam topsoil and plant-based compost. Have the mix tested and adjust nutrients and pH; Iowa State suggests a pH of about 6.0–6.5 for most vegetables. The lawn fertilizer calculator converts soil-test recommendations into product amounts.
Caveats and assumptions
- The settling allowance is a planning margin, not a measured rate; mixes high in compost settle more.
- Bag sizes and bulk material quality vary; check the label and ask suppliers what is in their blends.
- Coarse sand is only part of one recipe; fine sand mixed with clay can make soil denser, not looser.
- Use untreated or appropriately rated lumber for food beds and follow local extension advice.
Frequently asked questions
How much soil do I need for a 4 × 8 raised bed?
At 12 inches deep, 32 ft³, which is about 1.19 yd³ or 22 bags of 1.5 ft³. At 6 inches deep, half that.
How many bags of soil for a raised bed?
Divide the bed volume in cubic feet by the bag size. A 4 × 8 × 1 ft bed takes 32 ÷ 1.5 = 22 bags of 1.5 ft³, or 16 bags of 2 ft³.
What is the best soil mix for raised beds?
Iowa State Extension suggests equal parts topsoil, compost and coarse sand, or about 70% topsoil and 30% compost. UMN Extension suggests ⅔–½ topsoil with ½–⅓ compost.
How deep should a raised garden bed be?
MU Extension says most plants need at least a 6–12 inch rooting zone, and deeper is better. Beds over 18–24 inches need retaining walls.
Is it cheaper to buy soil in bags or in bulk?
For more than a cubic yard, bulk delivery is usually cheaper, as UMN Extension notes. Bags make sense for one or two small beds.
Sources
- Iowa State University Extension — Creating Raised Bed Planters — equal-parts and 70/30 mixes, length × width × height, pH 6.0–6.5
- University of Minnesota Extension — Raised bed gardens — topsoil/compost proportions, avoid potting soil-only fills, bulk vs bags
- University of Missouri Extension — Raised-Bed Gardening (G6985) — rooting depth, bed width, retaining walls
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.