How deep should a raised bed be?

6 to 12 inches. University of Illinois Extension says raised beds are commonly built 6 to 12 inches tall, and can be taller. A bed built for wheelchair access should be 24 inches tall.

Width matters as much as depth: Illinois Extension advises no more than 4 feet wide if you can reach the bed from all sides, and no more than 2 feet otherwise, so you never step on the soil. Fill it with a 1:1 mix of compost and garden soil or topsoil. As the compost breaks down the bed settles, so you may need to add soil and compost each year.