Material Calculator

How to use this calculator

1. Measure the finished depthFor a sub-base, that is the compacted depth you want when the job is done. The compaction allowance then works out how much loose material to order.
2. Set compaction only where it appliesType 1, hardcore and anything rolled or whacked settles by 20% to 25%. Decorative gravel, bark and topsoil do not — leave it at zero.
3. Check the density against your supplierThese are typical figures. Suppliers can give the actual density for the material they are selling, which matters on a large order.
4. Read the coverage rowsKnowing that one tonne covers a certain area at your depth is the quickest way to sanity-check an order.

Density is the number nobody knows

There is a permanent mismatch in the building trade. Jobs are designed and measured in cubic metres — a base 10 metres by 3 at 150 millimetres deep. Materials are sold by the tonne.

The conversion between the two is bulk density, and it varies far more than people expect.

MOT Type 1 sub-base is around 2.1 tonnes per cubic metre. Building sand is 1.6. Topsoil is 1.45. Bark mulch is 0.35.

That is a sixfold range. The same excavation that takes 12 tonnes of Type 1 takes 2 tonnes of bark, because one is crushed rock and the other is shredded wood.

Get the density wrong and the order is wrong by that proportion. Ordering by cubic metres when the supplier sells by tonne, without converting, is one of the most common ways a delivery arrives short or a driveway ends up buried under surplus.

Bulk density is not the same as the density of the material itself. It measures the material as delivered, including the air gaps between the particles. Solid limestone is about 2.7 tonnes per cubic metre; crushed limestone as an aggregate is nearer 2.0, because a third of the volume is space.

That is why the same rock has different densities depending on how finely it is crushed and how well it settles.

Moisture is the other variable, and it moves the figure meaningfully. Wet sand can be 15% heavier than dry sand for the same volume. Suppliers quote dry or damp figures, so after a week of rain a tonne fills less space than the calculation suggests — and after a dry spell it fills more.

Compaction is a separate effect and applies only to materials that are meant to be consolidated. Type 1 laid loose at 200 millimetres compacts to about 150. So to finish at 150 you need to order and lay 200, which is 33% more material than the finished dimension suggests.

Missing that step is why so many sub-base orders come up short, and it is the single most useful thing this calculator does.

Worked example: a 10 × 3 m driveway base

A driveway base 10 metres by 3, finished at 150 millimetres of MOT Type 1.

Finished volume: 10 × 3 × 0.15 = 4.5 cubic metres.

Type 1 compacts by about 20%, so you need 4.5 × 1.2 = 5.4 cubic metres loose to end up with 4.5 compacted.

Add 10% waste for over-dig, spillage and uneven ground: 5.94 cubic metres.

At a bulk density of 2.1 tonnes per cubic metre, that is 12.47 tonnes.

So a base that sounds like four and a half cubic metres needs twelve and a half tonnes ordered. The gap between those two numbers is where most ordering errors live.

That is 14.7 bulk bags at 850 kg each, or 499 small bags at 25 kg — which immediately tells you this is a loose tipped delivery rather than a bagged job.

At £40 a tonne it costs £498.80, or £16.63 per square metre of driveway.

Now change the material. The same 5.94 cubic metres of building sand weighs 9.5 tonnes; of topsoil, 8.6 tonnes; of bark mulch, just 2.08 tonnes.

And one tonne of Type 1 at 150 millimetres compacted covers 1/(2.1 × 0.15) = 3.17 square metres. That single figure is the quickest sanity check there is — twelve and a half tonnes covering thirty square metres works out at about 2.4 square metres per tonne once compaction and waste are included, which is in the right region.

Now a smaller job. A border 8 metres by 1, mulched to 75 millimetres with bark. Volume 0.6 cubic metres, no compaction, 10% waste gives 0.66 cubic metres, at 0.35 t/m³ that is 0.23 tonnes — about 231 kg, or ten 25 kg bags.

That is a bagged job, and calling out a tipper for it would be absurd. The density is what tells you which kind of job you have.

Bulk densities of common materials

The coverage columns are the practical ones. A tonne of bark covers nineteen square metres at a useful mulching depth; a tonne of Type 1 covers three at a useful sub-base depth. That sixfold difference is entirely density, and it is why bark is sold in bags and sub-base by the lorry load.

Ordering, delivery and the things that go wrong

Order generously. Running short mid-job means a second delivery charge, and on a sub-base it can mean a cold joint or an uneven finish where the two loads meet. Ten per cent is a minimum on regular ground and twenty is sensible on anything irregular.

Bulk bags are convenient and cost considerably more per tonne than loose tipped. They are also frequently described as a tonne while holding 800 to 900 kilograms — check the stated weight rather than the name, because a job costed on tonne bags that turn out to be 850 kg comes up short by nearly a fifth.

Loose delivery needs somewhere to tip and something to move it with. A full articulated load is 20 tonnes or more, and a tipper cannot place material where you want it — it lands in a heap. Factor in the barrowing.

Check access before ordering. Weight limits on bridges and private roads, overhead cables, low branches, soft verges and narrow gateways all stop deliveries, and a wasted delivery is usually chargeable.

The driveway itself is worth thinking about. Twenty tonnes on a domestic drive over a shallow soakaway or a drain run can crack it, and delivery drivers will normally decline to enter if they are unsure.

On depths, some rules of thumb worth knowing. A sub-base for a footpath is typically 100 millimetres compacted; for a domestic driveway 150; for anything taking vehicle loads regularly, more. Decorative gravel wants 40 to 50 millimetres over a membrane — deeper than that and it becomes hard to walk on. Bark mulch is 50 to 75 millimetres to suppress weeds effectively.

Membranes matter more than the depth in some cases. Gravel laid straight onto soil sinks into it and needs topping up every year; the same gravel over a permeable membrane stays where it is put.

On waste and disposal, remember that excavating for a sub-base produces spoil that has to go somewhere, and there is usually more of it than the material going back in — soil bulks up by 20% or more when dug. The muck-away calculation is a separate and frequently forgotten cost.

Finally, densities here are for planning. If you are ordering twenty tonnes, a phone call to the supplier asking for their actual figure for that material is worth the two minutes.

Assumptions and limitations

• Densities are typical planning figures and vary with moisture and grading
• Compaction allowance must be set to zero for materials that are not consolidated
• Does not calculate spoil arising from excavation, which bulks up when dug
• Assumes a uniform depth across the whole area
• Bulk bags are assumed to hold 850 kg — check the actual stated weight
• Does not price delivery, which varies by distance and load size
• Not a substitute for a supplier quote on a large order

Questions people ask about this in United Kingdom