Mulch vs Compost vs Topsoil – What to Use & How Much

Soil & CompostNovember 5, 20262 min readBy The Plant Calculator team

Hands holding a handful of rich garden soil

Garden centres stack them side by side in similar bags, but mulch, compost and topsoil do very different jobs. Using the right one, at the right depth, saves money and gives better results.

Topsoil: building or levelling ground

Topsoil is the mineral base of a garden. Use it to raise levels, fill new beds, level a lawn before seeding or replace poor ground. It is heavy and sold by the bag, by the tonne or by the cubic metre or yard.

Typical depths: 10–15 cm (4–6 in) under a new lawn; 20 cm (8 in) or more for a new bed; a thin 1 cm (½ in) top-dressing to smooth an uneven lawn.

The Topsoil Calculator converts your area and depth into volume, weight and bags, with an allowance for settling.

Compost: feeding the soil

Compost is decomposed organic matter. It feeds soil life, improves structure and helps sandy soils hold water and clay soils drain. Use it to top-dress beds every year, build no-dig beds or mix into new borders.

Typical depths: 2.5–5 cm (1–2 in) as an annual top-dressing; 10 cm (4 in) to start a no-dig bed; a thin 6 mm (¼ in) layer brushed into a lawn.

See the Compost Calculator. To mix compost into existing soil to a target percentage, use the Soil Amendment Calculator.

Mulch: protecting the surface

Mulch is a covering on top of the soil. Bark, wood chip, straw and leaf mould suppress weeds, hold moisture and keep soil temperature even. As organic mulches break down they also add organic matter.

Typical depth: 5–8 cm (2–3 in). Deeper than about 10 cm can starve roots of air, and mulch piled against stems and trunks invites rot.

The Mulch Calculator works out bags or bulk for your beds.

Quick comparison

	Topsoil	Compost	Mulch
Main job	Build or level ground	Feed and improve soil	Cover and protect
Where it goes	Below or as the bed	On top or mixed in	On top only
Typical depth	10–20 cm (4–8 in)	2.5–5 cm (1–2 in)	5–8 cm (2–3 in)
Sold by	Bag, tonne, m³ / yd³	Bag, m³ / yd³	Bag, m³ / yd³

How much to buy: the one formula

All three come down to area × depth. Measure the area in square metres and the depth in metres to get cubic metres, or square feet × depth in feet to get cubic feet (divide by 27 for cubic yards). Add 10–15% for topsoil to cover settling. Above about one cubic metre, bulk is usually much cheaper than bags.