Is 6 inches deep enough for a raised bed?

For shallow-rooted crops such as lettuce, spinach, radishes and many herbs, often yes, especially if the bed sits on loosened ground that roots can grow into. Most other vegetables do better with 12 inches or more.

How deep should a raised bed be for tomatoes?

Tomatoes root deeply. Aim for 18 to 24 inches of loose soil, which can be bed depth plus loosened ground underneath if the bed is open at the bottom.

Does a raised bed need a bottom?

Not over soil. Leaving it open lets roots and water move down into the ground below, which effectively makes the bed deeper. Beds on paving, decks or balconies need a base and good drainage.

How much more soil does a deeper bed need?

Soil goes up in step with depth. A 4 × 8 bed needs 16 cubic feet at 6 inches, 32 at 12 inches, 48 at 18 inches and 64 at 24 inches.

How Deep Should a Raised Bed Be? Soil Depth by Crop

For most vegetable gardens, 12 inches of soil is a good all-round depth. Beds over open ground can be shallower; beds on paving, or for deep-rooted crops, do better at 18 to 24 inches.

Linocut print of a cut-away raised bed showing lettuce, pepper, carrot and tomato roots at different depths

Raised bed depth by crop

Plants put most of their feeding roots in the top foot of soil, but some send roots much deeper. These are rough groupings; root depth also depends on variety, soil and watering.

Root depth	Crops (examples)	Suggested soil depth
Shallow	Lettuce and salad greens, spinach, radishes, green onions, most herbs, strawberries	6–12 in
Medium	Beans, peas, peppers, cucumbers, bush squash, cabbage family, beets, chard, short carrots	12–18 in
Deep	Tomatoes, winter squash and pumpkins, long carrots, parsnips, asparagus, artichokes	18–24 in or more

“Soil depth” here means loose soil the roots can grow through. That can be the bed itself plus loosened ground underneath.

Over open ground or on a hard surface?

Bed on soil or lawn. Roots can keep going below the bed. Loosen the ground with a fork before you fill it, and a 6 to 12 inch bed often behaves like a deeper one. This is the cheapest way to get depth, because the bottom of the root zone is free.

Bed on paving, a patio or a deck. The bed is all the soil there is. Plan on 12 inches at the very least, and 18 to 24 inches for tomatoes, squash and root crops. Make sure water can drain out of the bottom.

Bed over poor or compacted ground. Heavy clay, builder’s rubble or a compacted yard behave more like paving at first. Go a little deeper, loosen what you can underneath, and the ground will improve over a few seasons as roots and worms work into it.

How depth changes the soil bill

Every extra inch adds the same amount of soil, so a bed twice as deep needs twice as much. For a 4 × 8 ft bed:

Soil depth	Cubic feet	Cubic yards	1.5 cu ft bags
6 in	16	0.59	11
8 in	21.3	0.79	15
12 in	32	1.19	22
18 in	48	1.78	32
24 in	64	2.37	43
30 in	80	2.96	54

No extra for settling is included. Use the raised bed soil calculator for your own bed sizes, or the chart in how much soil for a raised bed.

Height for comfort

Depth for roots and height for your back are different questions. Beds around 24 to 30 inches tall let many people sit on the edge or garden with less bending. If you build that tall for comfort but your crops only need 12 to 18 inches of soil, you can:

Fill the lower part with coarser, cheaper material and keep at least 12 inches of good soil on top.
Expect more settling, since anything woody or leafy in the bottom will rot down. Top up each spring.
Price it first. A 4 × 8 bed filled 30 inches deep needs 80 cubic feet, close to 3 cubic yards.

Common heights from standard lumber

Wooden beds usually end up at the height of whole boards. Actual widths of US dimensional lumber are smaller than the name says:

Build	Bed height	Typical soil depth
One 2×6	5.5 in	4–5 in
Two 2×6	11 in	9–10 in
One 2×12	11.25 in	9–10 in
Three 2×6	16.5 in	15 in
Two 2×10	18.5 in	17 in
Two 2×12	22.5 in	21 in

Soil depth assumes filling to an inch or two below the top. Many metal beds are sold in heights of about 11, 17 and 32 inches; measure yours.

What to fill a deep bed with

The deeper the bed, the more it matters that the soil holds its structure. A topsoil-based blend settles less than a very light compost or peat mix. The raised bed soil mix calculator splits the volume into a recipe, and the compost calculator works out a yearly top-up.

The crop groupings are general guidance. Your local extension service can give depths suited to your climate and soil.

Questions gardeners ask

Is 6 inches deep enough for a raised bed?: For shallow-rooted crops such as lettuce, spinach, radishes and many herbs, often yes, especially if the bed sits on loosened ground that roots can grow into. Most other vegetables do better with 12 inches or more.
How deep should a raised bed be for tomatoes?: Tomatoes root deeply. Aim for 18 to 24 inches of loose soil, which can be bed depth plus loosened ground underneath if the bed is open at the bottom.
Does a raised bed need a bottom?: Not over soil. Leaving it open lets roots and water move down into the ground below, which effectively makes the bed deeper. Beds on paving, decks or balconies need a base and good drainage.
How much more soil does a deeper bed need?: Soil goes up in step with depth. A 4 × 8 bed needs 16 cubic feet at 6 inches, 32 at 12 inches, 48 at 18 inches and 64 at 24 inches.

How deep should a raised bed be?