How the compost calculator works
Whether you dig compost into a new bed, top up a vegetable plot or rake a thin film over a lawn, the quantity is a slab: surface area times thickness. The calculator converts your layer from centimetres or inches to metres, multiplies it by the ground area and adds 10%, because compost is sold by loose volume and slumps once it is tipped, raked and watered.
Compost = length × width × layer thickness × 1.10The weight uses 600 kg per cubic metre (about 1,000 lb per cubic yard), a fair figure for screened, moist garden compost. Dry peat-free potting mixes are lighter, wet municipal green-waste compost can reach 800 kg/m³, so treat the weight as an estimate for the wheelbarrow, the car boot or the trailer rating.
Worked example
A new vegetable bed of 6 m × 3 m covers 18 m². Spreading 5 cm of compost before digging it in takes 18 × 0.05 = 0.9 m³, or 0.99 m³ with the allowance. That is 990 litres: 25 bags of 40 L, or one bulk bag of 1 m³, which weighs close to 600 kg – too much for most car trailers in one trip. In US units, a 20 ft × 10 ft bed with a 2 inch layer needs 200 × 2 ÷ 12 = 33.3 ft³, 36.7 ft³ with the allowance, which is 1.36 cubic yards or 37 bags of 1 cu ft, roughly 1,370 lb.
How thick to apply compost
| Job | Layer | 1 m³ covers | 1 yd³ covers |
|---|---|---|---|
| Lawn top-dressing after aeration | 0.5–1 cm (¼–⅜ in) | 100–200 m² | 860–1,300 ft² |
| Yearly top-up of vegetable beds | 2.5–5 cm (1–2 in) | 20–40 m² | 160–320 ft² |
| Improving clay or sand before planting | 5–8 cm (2–3 in), dug into 20 cm | 12–20 m² | 110–160 ft² |
| Mulch ring around shrubs and trees | 5 cm (2 in) | 20 m² | 160 ft² |
| New raised bed mix (30–40% compost) | Fill depth × 0.35 | – | – |
Common pack sizes are 40, 50 and 60 L in Europe; 0.75, 1, 1.5 and 2 cu ft in the US and Canada. A 1 m³ bulk bag replaces 25 bags of 40 L; a cubic yard from a landscape yard replaces 27 bags of 1 cu ft.
Buying and using compost well
- Match the type to the job. Screened, fine compost suits lawns and seed beds; coarser soil conditioner is fine for digging into heavy clay.
- Do not overdo it in one go. Burying 15 cm of fresh material starves the soil of oxygen and can lock up nitrogen. Two moderate yearly layers work better.
- Ask for a certificate. Reputable suppliers test green-waste compost for weed seeds, contaminants and pH, and can show the results.
- Keep it off stems. A collar of compost against tree trunks or shrub stems keeps bark wet and invites rot; leave a hand's width clear.
- Home heaps shrink. A bin of garden waste loses roughly half to two thirds of its volume by the time it is ripe, so a 1 m³ heap yields about 0.3–0.5 m³ of finished compost.
Frequently asked questions
How much compost do I need for a 4x8 raised bed?
A 4 ft × 8 ft bed is 32 ft². Adding a 2 inch top-up needs 32 × 2 ÷ 12 = 5.3 ft³, about six 1 cu ft bags. Filling a new 12 inch deep bed with a 35% compost mix takes about 11 ft³ of compost.
How many bags of compost make a cubic metre?
A cubic metre is 1,000 litres, so 25 bags of 40 L, 20 bags of 50 L or about 17 bags of 60 L.
How much does a cubic yard of compost weigh?
Finished, moist compost weighs roughly 800–1,200 lb per cubic yard (about 480–700 kg/m³), depending on moisture and how much mineral soil it contains.
How much compost should I add to my lawn?
Top-dress at 0.5–1 cm (¼–⅜ in), ideally right after core aeration. A 100 m² lawn needs 0.5–1 m³; a 1,000 ft² lawn needs roughly 0.75–1.2 cubic yards.
Can I plant straight into pure compost?
It is better not to. Most vegetables and ornamentals grow best in soil with 25–40% compost mixed in; pure compost dries out fast, shrinks and can be too rich in salts for young plants.
Last reviewed: 2026-09-27 · How we calculate