A raised bed is a box, and its soil is the volume of the box. The two things that trip people up are the depth to use and what to do when there is more than one bed.
Volume per bed (cu ft) = length (ft) × width (ft) × fill depth (ft)
Total = volume per bed × number of beds
Cubic yards = cu ft ÷ 27
Bags = cu ft ÷ cu ft per bag, rounded up
Two 8 × 4 ft beds filled 12 inches deep
One bed holds 8 × 4 × 1 = 32 cu ft. Two identical beds hold 64 cu ft · 2.37 yd³, and the list says 2.37 yd³.
Worked example 2 beds, each 8 × 4 ft, filled 12 in deep
| Material | Calculated | Buy |
|---|---|---|
| Raised-bed soil2 beds, 12 in fill depth, no allowance | 2.37 yd³ | 2.37 yd³ |
This example assumes
- Soil fills the bed to the fill depth, with no extra allowance
- 1.5 cu ft bags for the bag count
- Two beds of the same size
For bags, the planner shows the same soil as 43 × 1.5 cu ft bags.
Fill depth is not bed height
Use the depth of soil, not the height of the frame. If a 12 inch frame is filled to 10 inches, enter 10. Dropping the fill depth to 8 inches on the same two beds changes the order from 2.37 yd³ to 1.58 yd³. The depth you choose depends on what you grow and how you fill the bed, which is a gardening decision this guide does not make.
Odd shapes
A bed that is not a rectangle is still made of shapes you can measure. An L-shaped bed is two rectangles. A triangular corner bed is half of base × length. A round bed is πr² × depth. Calculate each piece’s volume, add them, and treat the total as one bed, or split the shapes into separate runs of the planner.
What changes the estimate?
- Fill depth, which scales the volume directly
- The number of beds and whether they are identical
- Bag size or bulk delivery
- Whether you are filling with one soil or a mix, since the planner lets you enter your own percentages and splits the total accordingly
This guide does not recommend a soil mix. If you split the volume into a mix, the planner uses percentages that you enter.