Soil Calculator
Calculate soil for raised beds and garden top-ups. Subtract existing soil, convert bags and cubic yards, and compare bagged and bulk prices.
How to calculate soil volume
Measure the inside length and width of the bed, then enter the soil depth you want. For a top-up, enter the average depth already present. The calculator subtracts that existing layer and multiplies the remaining depth by the bed area and number of identical beds.
Cubic yards = cubic feet ÷ 27
An 8 ft × 4 ft raised bed filled to 12 inches needs 32 cubic feet, or about 1.1852 cubic yards. If it already holds 8 inches of soil, the 4-inch top-up needs only 10.6667 cubic feet. With a 10% allowance, that becomes 11.7333 cubic feet, which requires 6 whole 2-cubic-foot bags.
Compare soil bags and bulk delivery
For that top-up, an example price of $5 per bag gives 6 × $5 = $30. At an example bulk price of $45 per cubic yard, 11.7333 ÷ 27 × $45 = $19.56. Delivery charges and a supplier’s minimum order can change which option costs less.
The bag calculation rounds up once after combining identical beds. Bulk cost uses the full volume before display rounding. Enter the volume printed on the bag, in cubic feet or litres. A bag’s weight alone cannot tell you its volume.
Example: a soil top-up in litres
For a bed 2 m long and 1 m wide, raising the soil from 20 cm to 30 cm adds 10 cm, or 0.10 m. The top-up is 2 × 1 × 0.10 = 0.20 m³, or 200 litres. A 10% allowance makes it 220 litres. Five 50-litre bags supply 250 litres, leaving 30 litres beyond that allowance-adjusted requirement.
Depth, soil type and settling
The tool estimates quantity, not a soil recipe or a suitable rooting depth. Choose those for your plants and bed construction. The RHS explains that bed depth depends on what you plan to grow and that the fill should be allowed to settle before final topping up.
Extra material is an allowance you choose. It does not automatically calculate compaction or predict how much a particular soil will settle. Measure uneven existing soil at several points and use a representative depth. For beds with different dimensions, calculate them separately.
Estimate a surface mulch layer separately with the mulch calculator. For a general volume calculation without bag pricing, use the cubic yard calculator.
Gardening reference: RHS: how to make a raised bed. Unit definitions: NIST volume conversion factors.
Common questions
Can I use this for a circular bed?
Yes. Choose Circle and enter its internal diameter. The area is π × (diameter ÷ 2)². The depth and bag calculations then work the same way.
Can I convert soil volume directly into tons?
Only if you know the bulk density of the supplied soil. Moisture and composition change its weight. Ask the supplier for that value and use the cubic-yards-to-tons guide.
What if the bed already has enough soil?
When existing depth equals desired depth, the additional volume is zero. An existing depth greater than the target is flagged because this tool does not calculate excavation.
Questions about a result? See how our calculators are built or report a calculation issue.