Topsoil, compost and mulch are all bought by volume — area × spread depth — then converted to bags or tons by the material’s weight. The method is identical for all three; only the sensible depth and the density change. Get the volume right and the rest is a lookup.
Step 1: the volume
Multiply the bed area by the depth, keeping units consistent. A 200 sq ft (19 m²) bed at 3 in (0.25 ft) of mulch is 200 × 0.25 = 50 cu ft, or 50 ÷ 27 = about 1.9 cubic yards (1.4 m³). Topsoil at 4 in over the same bed is about 2.5 cubic yards.
Step 2: bags, or tons
A cubic yard is 27 cu ft, so it takes about 14 bags of 2 cu ft mulch or 27 bags of 1 cu ft topsoil. By weight, screened topsoil runs about 1 US ton per cubic yard (1.2 t/m³) dry, so the 2.5 yd³ of topsoil above is roughly 2.5 US tons.
| Material | US tons / yd³ | t / m³ |
|---|---|---|
| Screened topsoil (dry) | ~1.0 | ~1.2 |
| Compost | 0.5–0.6 | 0.6–0.7 |
| Bark mulch | ~0.25 | ~0.3 |
Wet soil weighs noticeably more — it matters for what a trailer or pickup can legally carry.
Topsoil & Mulch Calculator
Enter the area and spread depth — it returns the cubic yards (m³), bags and US tons (tonnes) for soil, compost or mulch, with depth presets per material.
How deep to spread
Mulch: 2–3 in (50–75 mm) — thick enough to suppress weeds and hold moisture, thin enough to let water and air through. Keep it off stems and trunks; piled mulch rots bark.
Topsoil: about 4 in (100 mm) for a new lawn (6 in on poor ground), 8–12 in (200–300 mm) for planting beds, and 1–2 in as a top dressing over existing soil. Buy screened — the money saved on unscreened loads goes straight back into picking out stones and weeds.
Bags or bulk?
Bags win for small jobs and clean access, but anything over about half a cubic yard is cheaper loose or in bulk bags. A standard pickup bed holds only about 1 cubic yard (0.7 m³) struck level — and a full yard of wet soil overloads most single-axle trailers, so split big deliveries.