It’s the first question when you plan a raised bed, and the answer decides what you can grow and how much soil you’ll buy. It comes down to two things: what you plant, and what’s under the bed.

The Quick Answer

Twelve inches works for most vegetable gardens.

Depth by What You Grow

• 6 to 8 inches: lettuce, radishes, spinach, and most herbs, as long as the ground underneath is loose soil the roots can reach into.
• 12 inches: carrots, onions, beans, and most other vegetables.
• 18 inches or more: tomatoes, squash, potatoes, and other big-rooted crops.

What’s Underneath Matters

On loosened garden soil, roots grow down past the bottom of the bed, so a shallower bed can work. On hard clay, concrete, or a patio, the roots only get what’s in the bed, so go 18 inches or deeper. If your yard stays wet, a deeper bed keeps roots above the soggy ground.

Taller for Your Back

A bed 24 to 30 inches tall lets you garden with little or no bending.

Width Matters Too

Keep it no wider than 4 feet if you can reach in from both sides, and about 2 to 3 feet against a fence or wall. Never step in the bed; it packs the soil down.

Depth Costs Soil

A 4-by-8-foot bed 12 inches deep holds 32 cubic feet of soil, about 1.2 cubic yards. At 18 inches, it’s 48 cubic feet. Price your soil before you pick a depth, and fill with a raised-bed mix or a blend of topsoil and compost, not just dirt dug from the yard.

My guide walks you through building a raised garden bed, step by step.

Get the full guide: How to Build a Raised Garden Bed by Hank Palmer, on Amazon