Raised Bed Soil Calculator: Volume, Bags and Soil Mix
Find how much soil you need for one or more raised beds in cubic feet, cubic yards, litres, and bags. Split the order into soil-mix components, add a settling allowance, and compare bagged and bulk costs.
Choosing the right fill depth and soil mix
Full fill versus top-up
For a new bed, use the inside footprint and the portion of the frame that will contain growing mix. If a false bottom or filler occupies the lower section, calculate only the layer above it. For an established bed, measure from the settled surface to the intended finished level; that gap is the top-up depth.
Soil, compost, and aeration
A full bed generally needs a balanced mineral-and-organic mix rather than pure compost or dense, unamended topsoil. The 60/30/10 preset is a purchasing starting point, while equal thirds reflects a commonly sold three-way style—not a rule for every crop or climate. Use a soil test and supplier specification. Compost-only top-ups may be appropriate in a thin layer, but repeated heavy applications can create excess nutrients.
Settlement and purchasing
Fresh mixes can settle after watering and organic material continues to decompose. Use the adjustable allowance when appropriate, then keep the base measurement visible so you can follow a supplier’s own recommendation. Bagged products suit easy handling and staged work; bulk mix can reduce packaging and cost, but delivery, minimum orders, driveway access, and storage can change the decision. The calculator compares only the prices you enter and does not assume a universal cutoff.
Common raised bed soil amounts
These are base volumes for inside dimensions, before any extra allowance. Representative bag counts use 1.5 ft³ bags and round up.
| Bed size | Depth | Cubic feet | Cubic yards | Litres | 1.5 ft³ bags |
|---|---|---|---|---|---|
| 4 × 4 ft | 6 in | 8 | 0.30 | 227 | 6 |
| 4 × 4 ft | 12 in | 16 | 0.59 | 453 | 11 |
| 4 × 4 ft | 18 in | 24 | 0.89 | 680 | 16 |
| 4 × 8 ft | 6 in | 16 | 0.59 | 453 | 11 |
| 4 × 8 ft | 12 in | 32 | 1.19 | 906 | 22 |
| 4 × 8 ft | 18 in | 48 | 1.78 | 1,359 | 32 |
Raised bed soil formulas
Use one consistent length unit before multiplying. Here, V is volume, L is length, W is width, D is soil-filled depth, r is radius, Lc and Wc are the rectangular cut-out dimensions, Q is identical-bed quantity, P is a component percentage, and E is extra allowance.
Rectangle: V = L × W × D
Circle: V = π × (diameter ÷ 2)² × D
L-shape: V = [(L × W) − (Lc × Wc)] × D
Multiple beds: Vtotal = (V1 × Q1) + (V2 × Q2) + …
Extra volume: Vorder = Vtotal × (1 + E ÷ 100)
Component volume = Vorder × (P ÷ 100)
Bag count = ceiling(component or total volume ÷ bag volume)
Exact constants used: 12 in = 1 ft; 30.48 cm = 1 ft; 1 m = 3.280839895 ft; 1 yd³ = 27 ft³; 1 ft³ = 28.316846592 L; and 1 m³ = 1,000 L. “Ceiling” means every partial bag is rounded up to a whole bag.
Worked soil and bag examples
1. Standard 4 × 8 ft raised bed
At 12 inches deep, first convert depth: 12 ÷ 12 = 1 ft. Base volume = 4 × 8 × 1 = 32 ft³. With 15% extra: 32 × 1.15 = 36.8 ft³ (1.36 yd³ or about 1,042 L). A 60/30/10 mix needs 22.08 ft³ topsoil, 11.04 ft³ compost, and 3.68 ft³ aeration. Using 1.5 ft³ bags, those components round up separately to 15, 8, and 3 bags; a pre-blended mix would require 25 bags.
2. Metric 2 × 1 m bed
At 30 cm deep, convert depth: 30 cm = 0.30 m. Base volume = 2 × 1 × 0.30 = 0.60 m³ = 600 L. With 10% extra, order 660 L. An equal-thirds mix needs 220 L of each component. At 40 L per bag, each component rounds up to 6 bags (18 bags total when bought separately); a pre-blended mix rounds up to 17 bags.
3. Established 4 × 8 ft bed compost top-up
For a 2-inch top-up, convert depth: 2 ÷ 12 = 0.1667 ft. Base volume = 4 × 8 × 0.1667 = 5.33 ft³. With 15% extra: 5.33 × 1.15 = 6.13 ft³ (about 174 L). A custom 0/100/0 mix assigns all 6.13 ft³ to compost. At 1 ft³ per bag, round up to 7 compost bags.
Methodology and sources
Geometry is calculated in cubic feet with the exact conversion constants shown above, then converted for display. Component amounts use the settlement-adjusted order volume; bags always round up. The default 15% allowance is an adjustable planning margin, not a horticultural requirement.
Horticultural guidance was checked against University of Maryland Extension guidance on filling raised beds, which discusses compost ranges, 2–4 inch additions for established beds, and depth for beds on hard surfaces; Oregon State University Extension guidance on using compost, which advises against compost-only full fills, describes up to 25% compost by volume in one fill approach, and notes that new fills settle; and University of Minnesota Extension soil guidance, which describes incorporating 1–2 inches of compost for flowers and vegetables. Recommendations differ with native soil, crops, drainage, and product composition, so local testing and supplier instructions take priority.
Raised bed soil calculator FAQs
How much soil does a 4 × 8 raised bed need?
A 4 × 8 ft bed filled 12 inches deep needs 32 ft³ (1.19 yd³ or about 906 L) before extra allowance. With 15% extra, order 36.8 ft³.
How many bags of soil should I buy?
Divide the settlement-adjusted volume by the volume printed on one bag and round up. For 36.8 ft³ of soil in 1.5 ft³ bags, buy 25 bags.
Should I measure inside or outside dimensions?
Measure the usable inside length, width, or diameter. Outside measurements include the frame and overstate the soil-filled space.
How do I calculate a top-up for an established bed?
Choose Top-up and enter only the missing depth, not the full frame height. For a false bottom, existing soil, or filler, subtract its occupied depth before calculating.
How much compost should I add?
Use local soil-test and extension guidance. A practical preset here is 30% compost for a full fill; for an established bed, extension guidance commonly recommends a 1–2 inch incorporated layer or a 2–4 inch top-up depending on soil and method.
Should I buy bagged soil or bulk soil?
Enter your local bag, cubic-yard or cubic-metre price and delivery charge to compare totals. Without prices, choose based on supplier minimums, access, storage, and handling rather than a universal volume cutoff.
How much extra soil should I allow for settling?
Use the calculator’s configurable 10–20% allowance when the mix is likely to settle. The default is 15%; supplier advice and material composition should guide the final choice.
How do I calculate soil for multiple raised beds?
Set the quantity for identical beds, then use Add another bed for different dimensions. The calculator multiplies each per-bed volume by its quantity and adds every bed group.
Calculator limits
Volumes, bag counts, and costs are planning estimates. Confirm bag labels, bulk supplier minimums and increments, delivery fees, taxes, compaction, and an appropriate soil recipe before ordering. Soil volume is not soil weight; moisture and bulk density vary.