How to figure out how much soil you need

The whole calculation comes down to volume: length × width × depth. The only trick is keeping your units straight. Measure the inside of the bed in feet, convert the depth from inches to feet (divide by 12), and multiply the three together. That gives you cubic feet. Divide by 27 and you have cubic yards, which is how bulk soil is sold.

A worked example: a 4 × 8 ft bed filled 10 inches deep is 4 × 8 × (10 ÷ 12) = about 26.7 cubic feet, or just under one cubic yard. The calculator above does this for any size, shape, and depth, and converts it into how many bags to buy — but it’s worth understanding so the numbers make sense.

How deep should the soil be?

Depth is the number people get wrong most often, and it’s the biggest lever on cost. You don’t fill the full height of a tall bed — you fill the depth your plants’ roots actually need. Leafy greens and herbs only need 6 inches. Most vegetables like tomatoes, peppers, and beans need 8–12 inches. Root crops like carrots, beets, and potatoes need 12–18 inches.

If your bed sits directly on soil or grass rather than a hard surface, roots can grow down past the frame, so you rarely need to fill a tall bed all the way. Filling only the depth you need — not the full height of the box — is the single biggest way to cut your soil bill.

The right fill mix (and why it matters)

Don’t fill a raised bed with bagged topsoil alone. On its own it compacts, drains poorly, and starves plants. A balanced bed mixes structure, nutrition, and drainage. The calculator splits your total into 50% topsoil for body and structure, 30% compost to feed soil life and release nutrients slowly, and 20% aeration — perlite, coarse sand, or coconut coir — to keep the blend light so roots get oxygen and water drains properly.

Bagged or bulk — which is cheaper?

Bagged soil is sold by the cubic foot and is convenient for small jobs. Bulk soil is sold by the cubic yard and delivered loose. The rough break-even: once you need more than about 10–15 bags, bulk usually wins on price per cubic foot — but factor in delivery fees and the fact that a loose pile lands on your driveway and has to be wheelbarrowed in. For one or two beds, bags are simpler. For a whole garden’s worth, price out a bulk delivery.

Common questions

How many bags of soil for a 4×8 raised bed?

At 10 inches deep, a 4 × 8 ft bed needs about 26.7 cubic feet — roughly 18 bags of 1.5 cu ft soil, or about 21 once you add 15% for settling. At 6 inches deep it’s closer to 11 bags. Enter your exact depth in the calculator above for the precise number.

How much is a cubic yard of soil?

One cubic yard equals 27 cubic feet — enough to fill a 4 × 8 ft bed about 10 inches deep, or cover roughly 100 square feet at 3 inches. Bulk soil is priced by the yard; bagged soil by the cubic foot.

Can I reuse last year’s soil?

Yes. Most beds just need topping up, not replacing. Add an inch or two of fresh compost each spring to replace settled volume and renew nutrients. Full replacement is rarely necessary unless you’ve had a disease problem.

A note on weight before you load the truck

Soil is heavier than people expect. A single cubic yard of garden soil runs roughly 1,800–2,200 pounds — about a ton. A standard half-ton pickup can’t safely haul a full yard of wet soil in one trip. If you’re picking up bulk, ask the supplier for the weight and split it across loads, or pay for delivery.

Got a different bed size or shape? The calculator at the top of this page handles any dimensions — just plug in your numbers and it’ll do the rest. And if you’re planning more than one bed, use the “Number of beds” field to get your total order in one shot. Good luck with the build.