Compost Calculator
Works out compost for improving soil. Compost is a soil conditioner rather than a growing medium, so the layers are thin — the quantities come out far smaller than for topsoil.
Includes the waste allowance — this is the figure to order.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
volume = length × width × depth × (1 + waste) weight = volume × bulk density
The geometry is trivial; the bulk density is where the answer actually comes from. Compost runs about 600 kg per cubic metre loose, so a cubic metre weighs roughly 0.60 tonnes. Suppliers quote one or the other, which is why both are given.
What to watch out for
Compost is spread thin and worked in:
- Top dressing a bed: 25–50 mm, forked into the top of the soil or left for worms.
- Top dressing a lawn: 10 mm at most, brushed in so the grass is not smothered.
- Improving heavy clay: up to 100 mm dug into the top spit, repeated annually rather than all at once.
It is not a substitute for soil. Planting directly into pure compost gives fast early growth and then a plant that dries out quickly and has nothing to hold it — compost holds little structure and few minerals once the initial nutrients are used up.
Frequently Asked Questions
How much compost do I need for a vegetable bed?
25–50 mm as an annual top dressing. A 6 × 3 m bed at 50 mm needs about 0.9 m³, or roughly 20 bags of 50 litres.
Can I plant directly into compost?
Not for anything long-lived. Compost has little structure and drains or dries unpredictably. Mix it with topsoil — about one part compost to three or four of soil for a raised bed.
How much does a cubic metre of compost weigh?
Around 600 kg, though it varies a lot with moisture. Wet compost after rain can be half as heavy again.