Raised bed depth is one of the first decisions you will make when planning a build, and it is one of the most common sources of overthinking. The internet is full of conflicting advice — some sources say six inches is fine, others insist on two feet or more. The truth is that the right depth depends on a few specific factors: what you plan to grow, what the ground beneath the bed looks like, and your budget for soil. Here is a practical breakdown.
The Six-Inch Minimum
Six inches of quality soil is enough for shallow-rooted crops like lettuce, herbs, radishes, strawberries, and most annual flowers. If your raised bed sits on top of decent native soil — meaning the roots can continue down past the bottom of the frame — six inches works surprisingly well for a wide range of plants. The bed frame in this case is really just defining the growing area and improving the top layer of soil.
The limitation of a six-inch bed is that it depends entirely on what is below it. If the bed sits on compacted clay, gravel, concrete, or landscape fabric that blocks roots, six inches is not enough for most vegetables. In that scenario, you need the bed itself to contain the full root zone.
The Twelve-Inch Sweet Spot
Twelve inches is the depth most gardeners should aim for. It provides enough root space for the vast majority of vegetables, herbs, and annual flowers without requiring an enormous volume of soil. Tomatoes, peppers, squash, beans, and root vegetables like carrots and beets all perform well in 12 inches of good soil. Even if the bed is placed on a hard surface, 12 inches gives roots room to develop and provides enough soil mass to retain moisture through hot afternoons.
Twelve-inch beds also hit a practical sweet spot for lumber. Standard dimensional lumber comes in widths that make 12-inch walls easy to build — two stacked 2x6 boards, or a single 2x12. This keeps construction simple and cost-effective.
When to Go Deeper
Beds deeper than 12 inches make sense in a few specific situations. If you are gardening on concrete, asphalt, or rooftops, an 18- to 24-inch bed gives you a more forgiving root zone and better moisture buffering. Deep beds are also beneficial for accessibility — a 24-inch bed reduces bending and can be gardened from a seated position, which matters for gardeners with mobility limitations.
Large root vegetables like parsnips and daikon radishes prefer 18 or more inches of loose soil. Fruit bushes like blueberries, which have deeper root systems than most annuals, also perform better in deeper beds.
The Cost of Depth
Soil is the biggest ongoing expense in raised bed gardening, and volume increases fast with depth. A standard 4-by-8-foot bed that is 6 inches deep holds about 16 cubic feet of soil. At 12 inches, that doubles to 32 cubic feet. At 24 inches, you are looking at 64 cubic feet — roughly two and a half cubic yards — which can cost anywhere from $80 to $250 depending on your soil source. Factor this into your planning, especially if you are building multiple beds.
Practical Recommendations
For most vegetable gardens built on open ground: 12 inches. For beds on hard surfaces or for accessibility: 18 to 24 inches. For herb gardens and shallow-rooted crops on decent soil: 6 to 8 inches is workable. When in doubt, go with 12 inches. It handles the widest range of crops, uses a reasonable amount of soil, and keeps build costs manageable. You can always add depth later by stacking an additional frame on top of an existing bed.