Bulk materials are used by volume and sold by weight, and converting between the two needs a bulk density that accounts for the air in a loose heap. One tonne of gravel occupies about 0.6 cubic metres and covers roughly 12 square metres at 50 mm deep, because loose gravel is around 40 per cent air at a bulk density near 1.68 tonnes per cubic metre.
Get the density wrong and the delivery is a third out. Get the depth wrong and it is out by whatever fraction you misjudged, because volume is linear in depth and every one of these calculations is area times depth.
What does each material weigh?
Working figures for loose material as delivered. They vary with moisture and grading, which is why an order should always round up.
| Material | Bulk density | One tonne covers |
|---|---|---|
| Gravel | ~1.68 t/m³ | ~12 m² at 50 mm |
| Sand | ~1.6 t/m³ | ~10 m² at 60 mm |
| Concrete (cured) | ~2.4 t/m³ | n/a: ordered by volume |
| Bark mulch | ~0.4 t/m³ | ~20 m² at 75 mm |
Note that concrete is the odd one out. Ready-mix is ordered by the cubic metre or cubic yard directly, because the supplier is delivering a volume of wet material rather than a weight of dry aggregate.
How much waste should you allow?
Between 5 and 10 per cent for most jobs, and more for anything poured onto ground you have not prepared. A slab onto compacted hardcore with clean formwork might lose 3 per cent; the same slab onto a dug-out garden with soft spots can swallow 15 per cent, because concrete finds every dip.
Trench footings are the worst case. The sides slump inward as you pour and the actual volume can exceed the calculated one substantially, which is why footings are usually ordered with a deliberate over-allowance rather than a percentage.
The other direction matters too. Ready-mix is sold in quarter-cubic-yard or quarter-cubic-metre steps, so the practical order is the calculated figure plus waste, rounded up to the next step, and a 10 × 10 ft slab at 4 inches, which calculates to 1.23 cubic yards, becomes a 1.5 cubic yard order.
Is cement the same as concrete?
No, and the distinction decides what you buy. Cement is the binder; concrete is cement plus sand plus aggregate plus water. The mix ratios you will be quoted are volumes of the three ingredients in order, so the familiar 1 : 2 : 4 is a single measure of cement carrying two of sand and four of stone.
The yield is the part that surprises people: seven parts of dry material produce roughly five parts of finished concrete, because the fine material fills the voids between the coarse. Ordering seven cubic metres of ingredients for seven cubic metres of concrete leaves you two short.
The mix ratio follows the job. A 1 : 2 : 4 is general-purpose for slabs and footings, a 1 : 3 : 6 is a weaker fill for backfilling, and a 1 : 1.5 : 3 is a stronger structural mix. Mortar drops the aggregate entirely and runs around 1 : 4 cement to sharp sand.
Which sand does the job need?
Three sands do three different jobs and swapping them causes real failures. Grain shape is what separates them, and it is why substituting one for another causes real failures rather than cosmetic ones. Sharp sand has coarse angular grains that lock together, which is what you want under slabs, in a screed and in concrete. Building sand is fine and rounded, which makes a mortar that spreads well and is deliberately weaker. Kiln-dried is sharp sand with the moisture taken out and the grading tightened, so it will run down a broom into a paving joint.
Using building sand in concrete is the common error. The rounded grains give a weaker mix, and the result looks fine until it starts to spall a winter or two later.
When is loose cheaper than bags?
Sooner than most people expect, particularly for mulch. The arithmetic is unkind to bags: a cubic yard is 27 cubic feet, so it takes thirteen and a half of the standard two-cubic-foot sacks to fill one. At the prices those sacks are usually sold at, the crossover comes somewhere around eight or ten of them — past that point a loose delivery wins even once the haulage is counted, and it keeps getting further ahead.
The counterweight is access and handling. A bulk bag needs somewhere to be dropped and a barrow route to where it is going, and a tonne of gravel is a genuinely long afternoon. Bags are more expensive per unit and they arrive in quantities a person can carry.
Questions people ask
How deep should gravel be? Around 50 mm for a path and 50 to 75 mm for a driveway over a proper sub-base. Deeper than that on a drive is worse rather than better: the gravel ruts instead of packing.
How deep should mulch be? About 75 mm, which suppresses weeds without stopping water reaching roots. Deeper starves the bed; shallower lets light through and the weeds come back.
Is gravel heavier when wet? Yes, appreciably, which matters if you are ordering by weight after rain. Suppliers quote dry-ish figures and the lorry weighs whatever arrives.
How many bags of cement per cubic metre? For a 1 : 2 : 4 mix, about thirteen 25 kg bags — or six to seven 50 kg bags — per cubic metre of finished concrete, plus the sand and aggregate. That is roughly 320 kg of cement, which is what the concrete calculator works from. Check the mix you are actually using rather than assuming.
Work out the volume first, convert to weight second, and round up to what the supplier sells in. The gravel, sand and mulch calculator does the density conversion for all three loose materials, which differ by a number rather than by a method, and converts between loose yards and bags. The concrete and cement calculator works in ready-mix units and splits a mix into its parts.