How to use this calculator
- Add each bed. Choose rectangle or round, enter the size in feet and the soil depth in inches. If you have several identical beds, set the quantity instead of adding them one by one.
- Add filler, if any. For deep beds you plan to fill partly with logs or branches, enter the filler depth and the calculator subtracts it.
- Choose an allowance for settling. Fresh soil sinks as it settles and gets watered, so plan for some extra.
- Pick your soil recipe and the bag size you plan to buy.
- Optional: enter prices for a bag and for a cubic yard delivered in bulk, to see which is cheaper.
The formulas
- Rectangular bed: cubic feet = length (ft) × width (ft) × depth (in) ÷ 12
- Round bed: cubic feet = 3.1416 × (diameter ÷ 2)² × depth (in) ÷ 12
- Cubic yards = cubic feet ÷ 27
- Bags = cubic feet of an ingredient ÷ bag size, rounded up
Worked example
Two 4 × 8 foot beds, each 12 inches deep, filled with a 50/50 mix of topsoil and compost, with a 10% allowance for settling:
- Each bed: 4 × 8 × 12 ÷ 12 = 32 cubic feet, so two beds hold 64 cubic feet
- With 10% extra: 64 × 1.1 = 70.4 cubic feet, or about 2.6 cubic yards
- Half topsoil, half compost: 35.2 cubic feet of each, which is 18 bags of 2 cubic feet of each, or about 1.3 cubic yards of each in bulk
Common raised bed soil recipes
| Recipe | What goes in | Good for |
|---|---|---|
| Simple 50/50 | Screened topsoil and compost in equal parts | Large beds on a budget; easy to buy in bulk |
| Square foot mix | ⅓ compost, ⅓ peat moss or coir, ⅓ coarse vermiculite | Light, fluffy beds that hold moisture well |
| Bagged raised bed mix | A ready-made product | Small beds and container-style beds |
Whatever you choose, avoid filling beds with plain native clay or with compost alone. Pure compost shrinks a lot as it breaks down, and heavy clay drains poorly in a bed.
Bags versus bulk
One cubic yard is 27 cubic feet, the same as 13.5 bags of 2 cubic feet or 18 bags of 1.5 cubic feet. Garden centers and landscape suppliers sell soil and compost by the yard, and for bigger projects the price per cubic foot is usually lower than bags. Small loads may carry a delivery fee, so compare the totals, not just the unit price. The calculator shows both side by side once you enter your local prices.
Tips for filling raised beds
- Fill in layers and water as you go. Wetting each few inches helps the soil settle before you plant.
- Top up every year. Beds usually sink a little each season. An inch or two of compost each spring keeps them full and feeds the soil.
- Use untreated wood for filler. Avoid pressure-treated or painted lumber in a vegetable bed.
Frequently asked questions
How much soil do I need for a 4×8 raised bed?
A 4 by 8 foot bed needs 16 cubic feet of soil at 6 inches deep, 24 cubic feet at 9 inches, and 32 cubic feet (about 1.2 cubic yards) at 12 inches. Plan on roughly 10% extra because new soil settles.
How many bags of soil fill a raised bed?
Divide the bed's volume in cubic feet by the bag size. A 4 by 8 by 1 foot bed holds 32 cubic feet, which is 16 bags of 2 cubic feet or about 22 bags of 1.5 cubic feet, before any allowance for settling.
Is it cheaper to buy raised bed soil in bags or in bulk?
For more than about one cubic yard (27 cubic feet), bulk soil delivered by the yard is usually cheaper per cubic foot than bags, though delivery fees can change the math for small orders. Enter both prices in the calculator to compare them for your project.
What can I use to fill the bottom of a deep raised bed?
Many gardeners fill the bottom of beds deeper than about 12 inches with untreated logs, branches or coarse wood, then top them with soil. Most vegetables root mainly in the top 12 inches, so this saves soil. Enter the filler depth in the calculator and it subtracts that volume.
What is the best soil mix for raised beds?
A simple, reliable mix is equal parts screened topsoil and compost. Square foot gardeners often use one third compost, one third peat moss or coir, and one third vermiculite. Bagged raised-bed mixes are convenient for small beds.