A top-up is an added layer, not the full bed depth.
Geometry = area × depth · Order volume = geometry × loose-volume multiplier × (1 + waste ÷ 100)
Use the average added thickness
For a level top-up, multiply the footprint by the thickness you will add. On uneven ground, take several depth measurements and use a considered average, or calculate separate sections. The calculator assumes an even layer within each section.
Separate the quantity from soil suitability
The result describes space to fill. It does not determine fertility, drainage, soil contamination or whether a particular topsoil suits your plants. Check the product specification and intended use separately. For enclosed beds, use the raised-bed tool and measure inside the boards.
A worked topsoil example
A 4 m × 3 m area topped up by 8 cm needs 0.96 m³. With no additional allowances, 40-litre bags require 24 whole bags. If a supplier quotes per cubic metre, price the order volume; if it quotes per bag, price the rounded bag count.
Separate a top-up from a new fill
A garden with some existing soil needs only the remaining fill. Measure how far the present surface is from the intended level at several points. Where depths differ substantially, save separate sections rather than turning an uneven site into a falsely exact single average.
Keep product choice separate from volume
The arithmetic cannot tell you whether a product is suitable for growing, how it drains, or what is mixed into it. Check the supplier’s product description and specification. A precise cubic-metre result is not a certification of the soil being delivered.
Ask what a bulk bag actually contains
If a listing says only “bulk bag”, ask for its stated volume or mass. Use a volume label directly. A mass label needs a compatible loose bulk density before it can be compared with the space in your garden. Do not infer capacity from a photograph of the bag.
Before you place an order
Check dimensions, bag yield and the price basis. Whole bags round up; bulk orders may have minimum quantities or delivery charges that this calculator does not include. All adjustments are your inputs. Read our method, unit definitions and limits.
Useful questions
Why do bulk bags differ in coverage?
A bulk bag may be sold by mass or by stated volume. Its name does not define a universal capacity. Enter the supplier’s actual quantity and units.
Can I use the tool for soil settling?
Only if you have a relevant project-specific loose-volume multiplier. There is no built-in settling percentage and the tool does not predict soil behaviour.