How Deep Should a Raised Bed Be?

For most vegetable gardens, 10 to 12 inches of soil is the sweet spot. It gives roots plenty of room, drains well and doesn’t cost a fortune to fill. Six inches can be enough over good, loosened ground, while beds over concrete, rock or heavy clay, or beds built at sitting height, need to be deeper.

Depth by situation

Situation	Recommended soil depth
Open-bottom bed over good garden soil that you loosen first	6–8 inches
Most vegetable beds	10–12 inches
Root crops (carrots, parsnips, potatoes) or poor, compacted soil underneath	12–18 inches
Bed on a patio, concrete or rock	12–18 inches minimum
Bed at sitting height for easier access	18–24 inches or more
Standing or wheelchair-height bed	30–36 inches

Why depth matters

Roots go deeper than you think

Lettuce, spinach and onions do most of their feeding in the top 6 to 12 inches of soil. Tomatoes, squash and peppers send roots much deeper when they can. In an open-bottom bed, those roots simply carry on into the ground below, which is why a shallow bed over loosened soil works. Over concrete or hardpan they can’t, so the bed has to hold everything.

Drainage

A raised bed drains faster than the ground around it. That helps in wet regions and heavy clay, where in-ground roots can sit in waterlogged soil. In hot, dry climates, very tall beds can dry out quickly, so plan on mulch and regular watering.

Warmth

Soil in raised beds warms up earlier in spring than the ground around it, which lets you plant a little sooner. Deeper beds hold that warmth longer into the evening.

How much soil each depth takes

More depth means more soil to buy. For one 4×8 ft bed, without the extra 10% many gardeners add for settling:

Depth	Cubic feet	Cubic yards	Bags of 1.5 cu ft
6 in	16	0.59	11
8 in	21.3	0.79	15
10 in	26.7	0.99	18
12 in	32	1.19	22
18 in	48	1.78	32
24 in	64	2.37	43

For your own bed sizes, number of beds and soil mix, use the Raised Bed Soil Calculator.

Filling a deep bed for less

Beds deeper than about 18 inches take a lot of soil. Two common ways to cut the cost:

• Fill the bottom with organic material. Logs, branches, leaves and untreated wood scraps in the bottom third rot slowly and hold moisture. Put at least 12 inches of good soil on top for the plants. Expect the surface to sink over a few seasons as the wood breaks down, and top it up with compost.
• Buy in bulk. For more than about a cubic yard, a landscape supplier’s topsoil and compost usually costs much less than bags.

Avoid gravel in the bottom. It doesn’t improve drainage and can make water collect in the soil above it.

Before you fill an open-bottom bed

1. Remove or smother grass. Lay cardboard over the area, wet it, then fill the bed on top. It blocks grass and weeds and breaks down within a season.
2. Loosen the soil underneath with a digging fork if it is compacted, so roots can get through.
3. Add hardware cloth across the bottom if gophers or moles are a problem in your area.

Bed width and length

Depth gets the attention, but width matters as much. Keep beds no wider than 4 feet if you can reach them from both sides, or about 3 feet against a wall or fence, so you never step on the soil. Length is up to you; 8 feet is common because lumber comes in 8-foot boards.

The short answer

Build beds 10 to 12 inches deep unless you have a reason not to. Go shallower only over good, loosened ground, and deeper for root crops, hard surfaces underneath, or easier access. To see how many beds your household needs, try the Garden Planner.