Gardening
Compost Calculator
Enter bed area and compost depth to get volume and bag count.
How much compost for a bed?
Compost refreshes borders as a thin top-dress or builds fertility when dug into new beds. Volume is again area × depth: roughly 1–3 cm suits many established borders, while new beds may want a deeper layer mixed through the topsoil rather than left as a smothering blanket on delicate crowns. Divide cubic volume by bag size and round up; homemade heaps can be estimated the same way in cubic metres. Soil type changes how much organic matter you want — heavy clay and hungry sandy soils often benefit from different depths. For surface bark see the mulch calculator; for filling raised boxes use raised bed soil.
Worked example
15 m² at 2.5 cm (0.025 m) ≈ 0.375 m³. At 40 L (0.04 m³) bags that is ceil(0.375 ÷ 0.04) = 10 bags; at £4.50/bag ≈ £45.
Limits and assumptions
Planning volume only. Nutrient content, weed seeds in low-grade compost and soil pH are outside this model.
Frequently asked questions
What depth for a light top-dress?
Often about 1–3 cm on established beds, then lightly worked in around plants.
Should I mix compost in or leave it on top?
Mixing into the topsoil suits most beds; a thick unmixed layer can smother crowns — mulch sits on top instead.
Can I use this for homemade compost?
Yes — estimate the cubic volume you will spread the same way, even if it never came in a bag.
Raised beds vs open borders?
Measure the surface area you will treat; deep fills for empty raised beds belong on the raised-bed soil tool.