The ideal depth for a raised garden bed depends primarily on the types of plants you want to grow, but generally, a depth of 12 to 18 inches (30 to 45 cm) is recommended for most vegetables and herbs.

• For shallow-rooted plants like herbs and lettuce, about 6 inches of soil depth is sufficient.
• Root vegetables such as carrots and radishes require around 12 inches.
• Heavier feeders and larger plants like tomatoes, cucumbers, squash, and kale do best in beds that are 16 to 18 inches deep.
• For optimal productivity and to accommodate almost any annual vegetable, 16 to 18 inches is ideal.
• If you plan to grow deeper-rooted perennials such as berries or asparagus, beds of 20 to 24 inches deep may be necessary

Raised beds less than 8 inches deep are generally not recommended because they may restrict root growth and drainage, but 8 to 12 inches can suffice for many situations, especially if drainage is good

. In summary, a raised garden bed depth of at least 12 inches is a good minimum, with 16 to 18 inches being optimal for most vegetable gardening needs, and deeper beds (up to 24 inches) reserved for plants with extensive root systems or special requirements