Understanding your result
The unadjusted volume describes the empty space. The order volume includes your allowance. Each component is rounded to its own whole-bag quantity, so the purchased total may exceed the order volume.
Once the bed dimensions are set, the Plant Spacing Calculator counts rows and planting positions with edge clearance.
The formula
The compost percentage splits the adjusted volume; soil is the remainder. Dividing those two volumes by their respective bag sizes produces separate shopping quantities. The model does not infer a mix from the crop name.
Worked example
One 2 m by 1 m bed with 30 cm left to fill holds 600 L. A 10% order allowance raises this to 660 L. A 30% compost share gives 198 L compost and 462 L soil: five and twelve 40 L bags respectively.
How to use this calculator
- Measure the inside walls and the remaining depth to fill.
- Set the number of same-size beds and your chosen compost percentage.
- Enter an allowance and the actual bag sizes for both ingredients.
Inside dimensions and existing material
Use the open interior rather than the outside dimensions of the frame. Timber thickness can noticeably change a small bed estimate. Existing soil, built-in reservoirs, and other occupied space reduce the depth that needs filling. If your beds differ in size, calculate them separately and combine the component volumes before placing a bulk order.
Choosing a soil mixture
The compost share is adjustable because suitable mixtures depend on the existing soil, crop, drainage, and material quality. University extension guidance can help assess those conditions. This tool treats soil and compost as two volume components; it does not account for sand, aeration amendments, or chemical nutrient content. The allowance is extra material ordered relative to the empty volume, not a physical shrinkage model.
Assumptions & limitations
What this calculation assumes
- Beds are rectangular and identical.
- Ingredient proportions are measured by loose volume.
What to keep in mind
- Settlement and mixing can change final volume; no fixed shrinkage factor is prescribed.
Common questions
Can I use this for one premixed product?
Set compost to zero and use the soil bag field for the premixed product. All volume will be assigned to that component.
Does 30% compost mean 30% by weight?
No. It means 30% of the calculated loose volume. Materials with different densities will have different weight shares.