How much mulch, topsoil or compost do you need?
Bagged mulch is sold by volume — 2 cubic feet is the standard bag in the US, 50 litres in metric countries — while bulk mulch and topsoil arrive by the cubic yard or cubic metre and are billed by volume, not by weight. The conversion everyone gets wrong is bags per cubic yard: at 2 ft³ per bag there are 13.5 bags in a cubic yard, so a 10 ft × 10 ft bed at 3 inches deep needs 0.93 cubic yards, which is 13 full bags.
The arithmetic
- Volume = area × depth. 3 inches is 0.25 ft, so 100 ft² at 3 in = 25 ft³ = 0.93 yd³ = 0.71 m³.
- Bags per yard: 2 ft³ bags → 13.5 per yd³; 1.5 ft³ bags → 18; 3 ft³ bags → 9; 50 L bags → 15.3 per yd³.
- Weight depends on the material: shredded bark mulch is light (about 700 lb/yd³), compost around 1,000 lb/yd³ and screened topsoil about 2,000 lb/yd³ — ten times heavier for the same volume of mulch, which is why bulk soil needs a delivery truck rather than a car boot.
Depths worth using
Mulch does its job at 2–4 inches: thinner and weeds push through, thicker and the layer sheds water and starves roots of oxygen. Keep it a few inches clear of trunks and stems to avoid rot. Topsoil for a new lawn or a reseeded patch wants 4–6 inches worked into the surface; for amending an existing bed, 2–3 inches tilled in is plenty. A raised bed filled from scratch needs 8–12 inches of soil mix — that is a different order of magnitude, so use the same calculator with the real depth.
Bags or a bulk delivery?
Under about one cubic yard, bags win: you can carry them to the bed, store leftovers and buy exactly what you need. Past two cubic yards, bulk is typically half the price per yard, but it arrives in a heap where the truck can reach — so factor in a wheelbarrow and an afternoon. Compost and topsoil also compact and settle by 10–20% after the first watering, so ordering 10% extra is normal practice.
Frequently asked questions
How many bags of mulch are in a cubic yard?
Thirteen and a half 2 ft³ bags, since a cubic yard is 27 ft³ and 27 ÷ 2 = 13.5 — buy 14. With 1.5 ft³ bags it is 18 per yard, with 3 ft³ bags 9, and with 50 litre bags about 15 per cubic yard (or 20 bags per cubic metre).
How deep should mulch be?
Two to four inches. Two inches is enough to block light and hold moisture on established beds; four inches is the practical maximum because deeper layers shed water, keep oxygen out of the root zone and can encourage rot around trunks. Top up in spring rather than adding a thick layer all at once.
How much soil do I need for a raised bed?
A 4 ft × 8 ft bed filled 10 inches deep needs about 26.7 ft³, or one cubic yard of soil mix — roughly 13 bags of 2 ft³, or about half a bulk delivery. For a 12-inch fill the same bed takes 1.19 cubic yards, so check the depth you actually want before ordering.
Is bulk mulch cheaper than bags?
Almost always, once you pass one or two cubic yards. Bulk mulch is usually quoted per cubic yard delivered and can be half the cost per yard of bagged material, but it arrives in a pile where the truck can park, so you need a wheelbarrow and a free afternoon. Bags stay the better deal for small beds and touch-ups.
Should I convert my measurements to metres or feet?
Either — the calculator converts internally. Enter feet and inches for an imperial bed, or metres and centimetres for a metric one. The volume is shown in cubic yards, cubic metres, cubic feet and litres at the same time, so you can compare a bag price with a bulk quote without doing arithmetic.
How accurate is the weight estimate?
It is a planning figure, not a scale ticket. Bulk density varies with moisture, how fine the material is shredded and how much it has settled; dry shredded mulch can be 550 lb/yd³, wet compost over 1,200 lb/yd³. Use the custom density field if your supplier quotes a specific number.