Area, depth, then allow for compaction
Bulk materials are simple until the compactor arrives. Length times width times depth gives the finished volume. If the material will be compacted, which is the case for crushed rock under a slab, road base under paving and decomposed granite on a path, the loose volume off the truck has to be larger than the finished volume, because the compactor squeezes the air out. Crushed rock loses about a fifth to a quarter of its loose volume, so you order about 1.25 times the finished figure. Mulch and drainage gravel are not compacted and are ordered as measured, plus a little for waste.
order (loose m³) = length × width × depth × compaction factor × (1 + waste)
Cubic metres or tonnes
Landscape yards sell some materials by the cubic metre and some by the tonne, and delivery trucks are rated by weight. The conversion is the loose density, and it varies enormously: a cubic metre of crushed rock weighs about 1.55 tonnes, sand about 1.5, garden soil about 1.2, and mulch only about 0.4. That is why the calculator shows both. It also means a 6 × 4 trailer rated at 750 kg carries half a cubic metre of crushed rock, but a full cage of mulch.
| Material | Loose density | Compaction | Typical depth |
|---|---|---|---|
| Crushed rock / road base 20 mm | 1.55 t/m³ | ×1.25 | 75 to 150 mm under paving and slabs |
| Drainage gravel / screenings | 1.5 t/m³ | ×1.0 | 300 mm behind retaining walls |
| Decomposed granite / toppings | 1.5 t/m³ | ×1.2 | 75 to 100 mm on paths |
| Washed / bedding sand | 1.5 t/m³ | ×1.1 | 30 mm under pavers |
| Garden soil mix | 1.2 t/m³ | ×1.15 | 200 to 300 mm in garden beds |
| Topsoil under turf | 1.3 t/m³ | ×1.15 | 100 to 150 mm |
| Mulch | 0.4 t/m³ | ×1.0 | 75 mm, settles to 50 |
| Scoria | 0.8 t/m³ | ×1.0 | Drainage and lightweight fill |
Trailer loads
A standard 6 × 4 box trailer holds about 0.5 cubic metres level and about 0.7 with a cage. The rated load is the real limit for heavy materials: a 750 kg trailer is full at half a cubic metre of crushed rock or sand, while the same trailer will take a heaped cage of mulch. Over about two cubic metres of anything, a delivery is cheaper than the trips.
Worked example
A 10 × 4 m paved area needs 100 mm of compacted crushed rock. The finished volume is 4 m³. With the 1.25 compaction factor and 5 percent waste, order 5.25 m³, which is about 8.1 tonnes. That is a small truck delivery, or eleven trailer loads at half a cubic metre, which nobody should do. The same 40 m² mulched at 75 mm is 3 m³ loose and 3.2 with waste, about 1.3 tonnes.
Common questions
How much does a cubic metre of crushed rock weigh?
About 1.55 tonnes loose. Sand 1.5, soil 1.2, mulch 0.4.
How much mulch per square metre?
0.075 m³ at 75 mm, so a cubic metre covers about 13 m².
How much soil does a 6 × 4 trailer hold?
About 0.5 m³ level or 0.7 with a cage, but check the rated load.
How much extra for compaction?
About 25 percent for crushed rock and road base.
Quantities only. Densities vary with moisture and grading; the supplier's figures for their product take precedence.