How Much Compost Do I Need?

Quick Answer

Length × width × depth in metres gives cubic metres; multiply by 1,000 for litres

A 1.8 × 1.2 m raised bed filled 30 cm deep is 1.8 × 1.2 × 0.3 = 0.648 m³, which is 648 litres: 17 bags of 40 L, or one 850 L bulk bag with room to spare.

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Worked Examples

Job	The maths	What to order
Raised bed, 1.8 × 1.2 m, 30 cm deep	1.8 × 1.2 × 0.3 = 0.648 m³ = 648 L (713 L with 10% settling)	18 × 40 L bags or 1 bulk bag
Border mulch, 4 × 1 m at 5 cm	4 m² × 0.05 m = 0.2 m³ = 200 L	5 × 40 L bags
5 patio pots at 15 L each	5 × 15 = 75 L	2 × 40 L bags

Soil and compost settle by 10-20% in the first year, so add roughly 10% to bed and border quantities; the bag counts above already include it for the raised bed. Bag counts are rounded up to whole bags.

Raised Beds and New Beds

Fill depth drives the quantity: about 20 cm suits salad and herbs, 30 cm suits most vegetables, and carrots, parsnips and other deep roots want 45 cm or more. Few beds are filled with pure compost. A good raised bed mix is roughly 60% topsoil, 30% compost and 10% perlite or sharp sand for drainage, and some gardeners simply use 50/50 topsoil and compost. For the 713-litre bed above, the 30% compost share is about 214 litres, so buy six 40 L bags of compost and make up the rest with topsoil. The raised bed soil calculator does the whole sum, including the settlement allowance and bulk bag count.

Borders, Mulching and Soil Improvement

For mulching, the RHS suggests a 5-7.5 cm layer of organic mulch, so every square metre of border needs 50-75 litres. When improving soil rather than mulching, dig the compost into the top 15-30 cm; on established borders it is easier to top-dress and let the worms work it in. Annual additions in autumn or spring are ideal. The mulch calculator and soil amendment calculator cover both jobs.

Pots and Containers

Work from litres per pot: a 30 cm round pot takes about 15-20 L of compost, a hanging basket about 12 L, a growbag about 33 L and a window box about 10 L. Do not fill to the rim; leave a 2-3 cm watering gap. If you add crocks or gravel to the base of deep pots, reduce the compost figure by about 15%. The container compost calculator totals a whole patio’s worth of pots and converts it to 40 L and 70 L bags.

Litres, Bags and Tonnes

One cubic metre is 1,000 litres. The standard UK bag is 40 L (15-20 kg), a bulk or dumpy bag holds around 850 L (0.85 m³) and weighs about a tonne when filled with topsoil, and loose loads are sold per cubic metre. The full bag size table is in our soil and compost volume guide.

If a supplier quotes tonnes, convert via density. Compost is light, about 0.5 tonnes per m³, so 1 tonne of compost is roughly 2 m³ = 2,000 litres. Screened topsoil is much heavier at about 1.4 tonnes per m³, so 1 tonne of topsoil is only about 0.71 m³ = 710 litres. Ordering by volume avoids that confusion entirely; the topsoil calculator shows both.