Mulch, Gravel and Topsoil: How Deep Should Each Layer Be?

Recommended depths

How much does one unit cover?

One cubic yard covers 324 sq ft at 1 in deep, so at 2 in it covers 162 sq ft, at 3 in 108 sq ft and at 4 in 81 sq ft. One cubic metre covers 20 m² at 50 mm, 13.3 m² at 75 mm and 10 m² at 100 mm. These figures let you sanity-check any order.

Volume, weight and bags

Mulch and topsoil are usually sold by volume: bags of 2 or 3 cu ft (mulch), 0.75–1.5 cu ft (soil) or 40–80 L in metric markets, and bulk by the cubic yard or cubic metre. Gravel and crushed stone are more often sold by weight, so volume must be converted with a density.

Settling and compaction

• Mulch settles after rain; allow about 5–10% extra.
• Topsoil settles 10–20% as it is watered and walked on.
• Crushed stone sub-base compacts 15–20% under a plate compactor.
• Decorative gravel and pea gravel hardly compact; 5% is plenty.

Worked example: a garden refresh

1. Beds: 40 × 4 ft = 160 sq ft of mulch at 3 in → 40 cu ft → 1.48 cu yd → about 20 bags of 2 cu ft, or 1½ yd bulk.
2. Gravel path: 25 × 3 ft = 75 sq ft at 2 in → 12.5 cu ft → 0.46 cu yd → about 0.6 short tons.
3. New lawn patch: 15 × 10 ft = 150 sq ft at 4 in topsoil → 50 cu ft → 1.85 cu yd, plus 15% settling → about 2.1 cu yd.

Ordering from a landscape yard

Bulk yards usually sell by the cubic yard (US) or cubic metre or tonne (UK and Australia) and have a minimum delivery. Ask how they measure — some load by bucket volume, which is approximate. Give them your calculated volume including the settling allowance and round up to their increment. Ask whether the price includes delivery and whether the truck can tip where you need it; a large tipper needs firm ground and overhead clearance.

Prepare a tipping area on a tarp or board near the work area, and avoid tipping on a lawn you want to keep. For gravel and soil, a single wheelbarrow holds about 3–6 cu ft (80–150 L) depending on size, so a 3 cubic yard load is roughly 15–25 barrow trips.

Layering materials correctly

Several jobs combine layers. A gravel path often uses compacted crushed stone at the bottom, a membrane and a decorative top layer. A raised bed may use coarse material at the base, then topsoil, then compost. A patio uses sub-base, bedding sand and pavers. Calculate each layer separately with its own depth and its own allowance, and order them in the sequence you will use them so they do not block each other on the driveway.

Sustainability tips

Local arborists often have wood chip that is cheaper than bagged mulch, and some councils give away compost. Reclaimed crushed concrete can make a good sub-base where allowed. Buying bulk instead of bags avoids plastic, and ordering the right quantity avoids leftover material being dumped. Keep any surplus covered for top-ups next season.

A quick coverage check

Before you order, divide the area by the coverage figure for your depth from the table above. If a 300 sq ft bed at 3 in comes out at about 2.8 cubic yards, you are in the right range; if your number is ten times larger or smaller, a unit was mixed up. This single check catches most ordering mistakes.

Bags or bulk?

Bags are clean and easy to carry but expensive per unit volume. Bulk deliveries are cheaper above roughly 1–2 cubic yards or cubic metres, but you need a place to tip the load, a tarp to protect the driveway and a wheelbarrow. Bulk bags (around 0.7–1 m³) are a good middle ground in the UK and Australia.