How Deep Should a Raised Bed Be? Soil Depth by Crop (and Bags to Fill It)

It depends on what is under the bed and what you grow. On pavement or other hard ground, University of Maryland Extension suggests at least 8 inches of soil for leafy greens, beans and cucumbers, and 12 to 24 inches for tomatoes, peppers and squash. On garden soil, University of Minnesota Extension says many gardeners may not need a bed raised more than a few inches.

Free calculator: once you pick a depth, our free raised bed calculator works out the soil for rectangle, circle and oval beds, splits it into topsoil and compost, and prices bags against bulk. No sign-up.

Soil depth by crop

Leafy greens, beans, cucumbers — at least 8 in of soil on a hard surface (UMD)

Tomatoes, peppers, squash — 12 to 24 in of soil on a hard surface (UMD)

Bed on garden soil — a bed raised just a few inches may be enough (UMN)

Depth here means inches of soil, not the height of the frame. Leave a little room below the rim so water does not wash soil over the edge.

How many bags to fill each depth

Counts are 1.5 cu ft bags of raised bed mix, rounded up, with 10% added for settling.

4 × 4 ft — 8 in: 8 bags; 12 in: 12 bags; 18 in: 18 bags; 24 in: 24 bags

3 × 6 ft — 8 in: 9 bags; 12 in: 14 bags; 18 in: 20 bags; 24 in: 27 bags

4 × 8 ft — 8 in: 16 bags; 12 in: 24 bags (1.3 cu yd); 18 in: 36 bags (2 cu yd); 24 in: 47 bags (2.6 cu yd)

4 × 12 ft — 8 in: 24 bags; 12 in: 36 bags; 18 in: 53 bags; 24 in: 71 bags

Deep beds use a lot of soil, so price a bulk delivery too. Enter your own bed in the free raised bed calculator to compare bags with cubic yards.

What to fill it with

Topsoil and compost. University of Minnesota Extension suggests about one-half to two-thirds topsoil, with plant-based compost making up the rest. Too much compost holds too much water, and potting mix dries out too quickly.

On a hard surface. University of Maryland Extension suggests compost and a soilless growing mix in a 1:1 ratio. It also notes that bagged "organic soil" usually contains no topsoil.

Width counts too. For beds at ground level, UMN suggests up to 5 ft wide if you can reach from both sides, or 2.5 ft from one side. For taller beds, use your arm's reach so you can get to the center.

Raised beds that fit these depths (#ad)

Land Guard Galvanized Raised Garden Beds, 2-Pack — two oval steel beds, 8 × 4 × 1 ft each, with open bases so roots can reach the ground below. The maker lists 57.12 cu ft for the pair, about 28.6 cu ft per bed, or 20 bags of 1.5 cu ft to fill one to the rim. See it on Amazon

Vego Garden 17 in Modular Raised Bed — coated steel panels that the maker says assemble into six shapes. At 17 in tall it fits the 12 to 24 in depth suggested for tomatoes and peppers. See it on Amazon

As an Amazon Associate, Curb Calc earns from qualifying purchases.

Raised bed depth FAQ

Is 12 inches deep enough for a raised bed? For greens, beans and cucumbers, yes, even on a hard surface. For tomatoes, peppers and squash on pavement, 12 inches is the low end of the 12 to 24 inch range from University of Maryland Extension.

How deep should a raised bed be for tomatoes? 12 to 24 inches of soil on a hard surface, per University of Maryland Extension.

How many bags of soil for a 4 × 8 raised bed? With 12 inches of soil, 24 bags of 1.5 cu ft, including 10% for settling. Filled 10 inches deep, about 20.

Can I fill a raised bed with potting mix? University of Minnesota Extension says potting mix dries out too quickly in raised beds. Use a blend of topsoil and compost.

Sources: University of Maryland Extension, Soil to Fill Raised Beds; University of Minnesota Extension, Raised bed gardens.

Related: How to get raised beds ready for winter.