How it works
The calculator works out the volume of one bed, multiplies it by the number of beds and adds any settling allowance you enter.
Soil per bed = Length × Width × Depth
Soil = Soil per bed × Beds × (1 + Settling ÷ 100)
Bags = ⌈ Soil ÷ Bag size ⌉
⌈ ⌉ means rounding up to the next whole bag. A bed 8 ft by 4 ft filled 12 in deep holds 8 × 4 × 1 = 32 cu ft, which is 32 ÷ 27 = 1.19 cu yd. Units are converted internally, so lengths in feet or meters, the depth in inches or centimeters and the bag size in cubic feet or liters all work together.
Worked example
Inputs
- Bed length (inside)
- 8 ft
- Bed width (inside)
- 4 ft
- Soil depth
- 12 in
- Number of beds
- 1
- Bag size
- 1.5 cu ft
- Settling allowance
- 0 %
Result
- Soil to fill
- 32 cu ft (1.19 cu yd)
- Soil per bed
- 32 cu ft
- Bags
- 22
Common mistakes
- Measuring outside the boards. A 2 in thick board takes 4 in off each dimension, which matters on small beds.
- Filling to the full height of the sides. Leave a little room so water and mulch stay in the bed.
- Mixing up cubic feet and cubic yards. One cubic yard is 27 cubic feet; bulk soil is sold by the yard, bags by the cubic foot.
- Forgetting that soil settles. Plan to top up after the first few waterings, or include an allowance.
- Using topsoil alone. Many raised beds are filled with a blend; follow your supplier or extension service for the mix.
Assumptions and limits
- Each bed is a rectangular box filled to the same depth everywhere.
- Volume = length × width × depth × number of beds, plus any settling allowance you enter. Nothing is assumed for settling.
- Bags are rounded up to whole bags. The bulk volume is not rounded.
- The 1.5 cu ft example bag size is from the Miracle-Gro Organic Raised Bed Soil product page; other products differ, and no metric bag size is assumed.
Frequently asked questions
How much soil do I need for a 4 × 8 raised bed?
For a bed 4 ft by 8 ft inside, filled 12 in deep, 4 × 8 × 1 = 32 cubic feet, or about 1.19 cubic yards. At 1.5 cu ft per bag that is 22 bags. Filled 6 in deep, it is half that – 16 cu ft.
How many bags of soil does a raised bed take?
Divide the bed volume by the volume on the bag and round up. Check the bag as well – Miracle-Gro says two of its 1.5 cu ft bags fill a 2 × 2 ft bed 6 in deep, which is 2 cu ft of space, so bagged soil may go less far than its printed volume.
How do I convert cubic feet to cubic yards?
Divide by 27. A cubic yard is 3 × 3 × 3 = 27 cubic feet, so 54 cu ft is 2 cu yd. In metric, 1 m³ is 1,000 L.
How deep should a raised bed be?
It depends on what you grow and on the soil underneath. Ask your local extension service; enter the depth you choose and the calculator works out the soil.
How much extra soil should I buy for settling?
It depends on the mix and how it is placed, so the calculator assumes none. Ask your supplier, and enter their figure as the settling allowance.
Sources
- Miracle-Gro Organic Raised Bed Soil – product page — ScottsMiracle-Gro. Product size 1.5 cu. ft.; "Two 1.5 cu. ft. bags … will fill a 2 ft. x 2 ft. x 6 in. raised bed".
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 4, 2026