When you need this calculation
Filling new raised beds, topping up beds that have settled, or pricing bagged soil against a bulk delivery. Bed kits are usually sold by outside dimensions (4 × 8 ft, 4 × 4 ft, 1.2 × 2.4 m) and heights in inches or centimeters, while soil is sold in cubic feet, liters or cubic yards.
How the soil is worked out
Multiply length × width × the depth of soil you’ll add, then multiply by the number of beds. The calculator converts inches to feet (÷ 12) or centimeters to meters (÷ 100) first, then gives the total in cubic feet, cubic yards, liters and bags, rounding bags up.
Use the inside dimensions of the bed if the walls are thick. A 4 × 8 ft frame built from 2-inch lumber is a little smaller inside.
How deep to fill
Kansas State University recommends 10 to 12 inches of rich soil for vegetables. Shallow-rooted crops such as lettuce and herbs manage with less; deep-rooted crops and large plants do better with more. Leave an inch or two below the top edge so water and mulch don’t spill over.
Common mistakes
Ordering for the frame height, not the soil depth. A 12-inch-tall bed filled to within an inch of the top needs 11 inches of soil.
Forgetting settling. New soil settles in its first season, so plan to top up.
For a level area rather than beds, use the topsoil calculator. To plan what goes in, use the plant spacing calculator.