Ordering loose material for a garden or driveway comes down to two steps. First the volume: how much space you're filling, which is simply the area times the depth — and that figure is the same whether you fill it with feather-light topsoil or dense sub-base. Then the weight, because suppliers sell by the tonne, and that's where the material's bulk density comes in: the same hole takes far more tonnes of MOT type 1 than of topsoil. Divide by a bulk bag's capacity and you have the bags to order.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: volume × bulk-density aggregate estimation.
The aggregate equations
Convert the depth to metres (50 mm = 0.05 m) so the volume lands in cubic metres. Volume is purely geometric — it doesn't care what the material is — so the only place the material matters is the density that turns cubic metres into tonnes. A bulk (jumbo) bag holds roughly 0.85 tonnes or about half a cubic metre; loose tipped loads are cheaper per tonne once you're past a few bags.
Worked example — a gravel border
Scenario: Covering 50 m² with 50 mm of decorative gravel (bulk density 1,500 kg/m³), in 0.85 t bulk bags.
The border needs 2.5 m³ of gravel, weighing about 3.75 tonnes, which is five bulk bags. The volume would be identical for topsoil, but at 1,300 kg/m³ that's only 3.25 tonnes; switch to compacted MOT type 1 sub-base at 2,000 kg/m³ and the same 2.5 m³ jumps to 5 tonnes. Because the volume is fixed by area and depth, choosing the right density is what gets the order — and the cost — right.
Frequently Asked Questions
volume = area × depth(m); tonnes = volume × density/1000. 50 m² at 50 mm gravel = 2.5 m³ ≈ 3.75 t.
~1.5 t/m³ for gravel. Topsoil ~1.3, sand ~1.6, ballast ~1.75, MOT type 1 ~2.0. Moisture shifts it.
~0.85 t per bag. tonnes ÷ 0.85, rounded up: 3.75 t ≈ 5 bags. Loose tipped is cheaper for large amounts.
Decorative 40–50 mm; gravel driveway 50 mm over sub-base; MOT sub-base 100–150 mm.
Volume is the same; weight differs by density. 2.5 m³ is 3.25 t topsoil but 5 t MOT type 1.