How to use this calculator
Measure between the inside faces of the bed, then choose the soil depth you actually need to add. If the bed already contains soil or a base layer, enter only the remaining fill depth.
The formula
A worked example
An 8 × 4 ft bed filled 12 inches deep takes 32 cubic feet, or 1.19 cubic yards. With no extra allowance and 1.5 cubic-foot bags, buy 22 bags.
Assumptions & limitations
Assumes a rectangular bed with uniform depth. Settling and compaction are represented only by your extra allowance. This estimates volume, not a soil blend or a plant-specific growing depth.
Common questions
Should I use outside dimensions?
No. Wall thickness occupies space, so use the interior length and width.
Can I calculate only a top-up?
Yes. Enter the thickness of new soil you want to add instead of the full bed height.
Units reference
The calculation follows the method above. This reference supports the units and conversions.
NIST: units and conversion factors ↗Formula and worked example checked against automated test cases. About our methodology.