Raised Bed Soil Calculator
A raised bed is filled to its height like a container, not dug into the ground. Enter the bed's size for the total fill — and how it splits into topsoil and compost.
Total fill
0.99cu yd
- Total fill
- 26.7 cu ft
- Topsoil / garden soil
- 18.7 cu ft
- Compost
- 8 cu ft
- Bags (if bagged)
- 18 bags
How it’s calculated
Total fill = length × width × (height ÷ 12) cubic feet, ÷ 27 for cubic yards. The mix splits that by your compost percentage: compost = total × %, the rest topsoil or garden soil. Bags = total ÷ the bag's cubic feet, rounded up. Bulk (by the cubic yard) is far cheaper once you're past a few bags.
Worked example
A 4 × 8 ft bed filled 10 in deep: 8 × 4 × (10 ÷ 12) = 26.7 cu ft = 0.99 cu yd. At 30% compost, that's about 8 cu ft of compost and 18.7 cu ft of topsoil — or ⌈26.7 ÷ 1.5⌉ = 18 bags if buying bagged.
FAQs
- How deep should a raised bed be?
- 8–12 inches of fill suits most vegetables and flowers; go deeper (or open the bottom to the ground below) for carrots, parsnips, and other root crops. Deeper beds need more soil — and more than you'd guess, since a bed's volume is its full height, not a thin layer.
- What should I fill a raised bed with?
- A blend — commonly topsoil or garden soil with around a third compost for fertility and structure; many gardeners also add a handful of an aeration material like perlite. Don't fill with pure compost: it's too rich, holds too much water, and shrinks as it breaks down. Use the compost percentage to set your blend.
- Why does the bed need topping up later?
- The organic matter in the mix decomposes and settles, so the level drops over the first season or two. Plan to top up with compost each year — and order a little extra at the start, since the settled volume ends up below the fresh-fill figure.
Sources
- Total fill = L × W × (height ÷ 12) ÷ 27 (geometry); mix split = total × compost %, remainder soil; bags = total ÷ bag size, rounded up. The compost % is an editable planning input (gardener preference), not a fixed constant.