The short answer
Most garden crops need at least 10 inches of soil, according to the University of Georgia Extension. How much more depends on what is under the bed.
Over poor or compacted ground, the University of Maryland Extension says to spread at least 8 inches of mix on top, and Colorado State University Extension calls 8 to 12 inches minimal where the soil below is not suitable for crops. Where the whole root zone sits in the fill, Washington State University Extension says to build beds at least 15 to 18 inches deep. On a patio or other hard surface, Maryland gives at least 8 inches for leafy greens, beans and cucumbers, and 12 to 24 inches for peppers, tomatoes and squash. Every figure here is soil depth, the fill you can plant in, not the height of the walls.
How deep the roots go
The UC Master Gardener program groups vegetables by where most of their roots grow in good soil. In a bed on open ground, roots can carry on into the soil below the bed; on paving they cannot.
| Group | Main roots in the top | Examples |
|---|---|---|
| Shallow-rooted | 6 to 12 in | Cabbage, cauliflower, lettuce, celery, sweet corn, onion, white potato, radish |
| Moderately deep-rooted | 1 to 2 ft | Snap bean, carrot, cucumber, eggplant, peas, pepper, summer squash |
| Deep-rooted | 2 to 4 ft | Asparagus, globe artichoke, cantaloupe, pumpkin, tomato, watermelon |
These are root zones in open ground, where nothing stops the roots. They do not set the depth of a bed with a floor or paving under it: Maryland grows tomatoes, peppers and squash in 12 to 24 inches on a hard surface, and cucumbers in 8, though their roots reach 1 to 4 ft in open soil. For a contained bed, plan on the depths extension services grow crops in; for a planter on legs, the planter guide lists them crop by crop.
Open ground, paving or a planter on legs: what changes
The same depth means different things depending on what is under the bed: open ground, or a hard surface such as paving, a deck or a balcony, or the floor of a planter on legs.
| What changes | Open-bottom bed on soil | Hard surface or floored planter |
|---|---|---|
| Where roots go | Through the fill into the ground below, further if you loosen it | Only into the fill: its depth is all the root room there is |
| Soil depth to plan | At least 10 in for most crops (University of Georgia); 8 in of mix over poor ground (Maryland); 8 to 12 in where the ground is unsuitable (Colorado State) | 8 in for leafy greens, beans and cucumbers, 12 to 24 in for peppers, tomatoes and squash (Maryland); 15 to 18 in where the whole root zone is in the fill (Washington State) |
| Where water goes | Down into the ground | Out through drain holes or a permeable base onto the surface, so plan where it runs |
| What carries the weight | The ground | The surface, or the planter’s legs: each inch of watered potting mix adds about 3.3 to 7 lb on every square foot |
A deeper bed on a deck, balcony or roof is a heavier one. The hard-surface guide gives the weight at each depth; it is an estimate to take to whoever assesses the structure, not an approval.
How tall should a raised bed be?
Tall enough for the soil your crops need plus an inch or two at the top, then as tall as your back and knees ask for. The soil sets the minimum: at least 10 inches for most crops (University of Georgia Extension), 15 to 18 inches where nothing usable lies below (Washington State University Extension), with the walls 1 to 2 inches higher for mulch (Iowa State University Extension).
Comfort sets the rest. The University of Georgia Extension gives 24 inches for gardening from a wheelchair and 36 inches to avoid bending while standing; the University of Minnesota Extension says 27 inches is generally comfortable for wheelchair users. A tall bed does not have to be filled with bought soil: filling a deep bed shows how to fill the bottom with wood, and the tall bed plan builds one 22 inches high.
On open ground, loosen what is underneath
If a bed is shallower than 10 inches, UGA says to till the existing soil below it so roots can go on down. Colorado State adds a warning about layers: a sharp change from one soil to another slows water and air and can stop roots, so mix the first few inches of fill into the ground below.
What each depth costs to fill
For a 4 × 8 ft bed measured inside the walls, every inch of depth adds 2.67 cubic feet of soil. Bags are 1.5 cubic feet, rounded up.
| Soil depth | Cubic feet | Cubic yards | Bags of 1.5 ft³ |
|---|---|---|---|
| 8 in | 21.33 | 0.79 | 15 |
| 10 in | 26.67 | 0.99 | 18 |
| 12 in | 32 | 1.19 | 22 |
| 18 in | 48 | 1.78 | 32 |
| 24 in | 64 | 2.37 | 43 |
The 4 × 8 ft page and the other bed sizes give the same table for your size, and the deep-bed guide covers ways to fill a tall bed with less bought soil.
Wall height is not soil depth
Fill a little short of the top. Iowa State University Extension suggests leaving 1 to 2 inches for mulch, and notes that new soil often settles, leaving a lip. Washington State University Extension adds that bed soil keeps subsiding as it settles and breaks down, so plan to top it up every year or two.
The soil you can plant in is the wall height less that space at the top, less anything in the bottom that is not soil, such as logs or gravel. A planter on legs adds a stand underneath: Vego’s Classic Elevated bed stands 32 inches tall and holds 12 inches of soil. Order soil for the depth you will fill, and choose a planter by the soil depth its maker states, not its height.
Try it with your numbers
Put in your own bed and the raised bed soil calculator works out the answer as you type.
Open the raised bed soil calculatorReferences
- Raised Garden Bed Dimensions (Circular 1027-4), University of Georgia Cooperative Extension.
- Growing Vegetables in Raised Beds, University of Maryland Extension.
- Block Style Layout in Raised Bed Vegetable Gardens (GardenNotes #713), Colorado State University Extension.
- Raised Beds: Will They Benefit Your Vegetable Garden? (FS075E), Washington State University Extension.
- Irrigation, UC ANR Master Gardener Program.
- Creating Raised Bed Planters, Iowa State University Extension and Outreach.
- Raised bed gardens, University of Minnesota Extension.
- Vego Classic Elevated Garden Bed, 2 × 4 ft, Vego Garden.