Gravel calculator tonnes and bulk bags

How the gravel calculator gets its answer

Gravel is sold by weight, but you measure a job by volume. So the calculator works out the volume first, length times width times depth, adds your allowance for waste and uneven ground, then multiplies by the weight of a cubic metre of the material you picked. For 800kg bulk bags it divides the tonnes by the bag weight. For 750 and 500 litre bags it divides the volume by the bag size. Either way it rounds up to whole bags.

The default bag is 800kg, the figure behind Breedon's aggregates calculator, which allows 1.25 bags for every tonne. Plenty of merchants fill to 850kg, Carvers and SCBS among them. The calculator cannot be set to 850kg, so with those bags its count can come out a bag or so high, leaving a little spare.

Here is the working for a typical single drive: 8m long, 3m wide, 20mm shingle laid 50mm deep over a finished sub-base, with the default 10% allowance.

• Area: 8 × 3 = 24m².
• Volume: 24 × 0.050 = 1.20m³. Plus 10% = 1.32m³.
• Weight: 1.32 × 1.6 tonnes per m³ = 2.11 tonnes.
• Bulk bags: 2,112kg ÷ 800kg = 2.64, so 3 bags.
• Loose: if you buy it tipped by the tonne, order 3 tonnes.

What each material weighs per m³

We could not find a loose bulk density for these products on Tarmac's, Hanson's, Breedon's or Brett's public data sheets. So the figures below come from what suppliers actually sell: the weight and volume printed on their bulk bags, and the factors their own calculators use. Where sources disagree, the card gives the range and says which figure the calculator uses.

Natural stone varies from one quarry and one load to the next, and wet material weighs more than dry. Treat the tonnes as a close estimate. If your supplier quotes a different weight per m³, type it in.

How deep for driveways, paths and sub-bases

A gravel drive is two layers: a compacted sub-base that carries the car, and a thin layer of loose stone on top. Skimp on the first and pile on the second, and you get a drive that feels like a beach underfoot and ruts under the tyres.

For the sub-base, Marshalls' driveway installation guide says 150mm should be enough for most domestic driveways, with the design based on the ground and the loads. Its patio guide gives 100mm for most domestic patios. Both say to lay it in layers no thicker than 75mm and compact each one before the next goes on. Soft clay, or a van on it every day, calls for more.

For the stone on top, The Cobble Shop suggests 30 to 40mm of 10mm stone, rising to 50mm on a drive, and 40 to 50mm of 20mm stone, a little deeper on a drive. It also warns that 10mm stone on a drive may tend to rut, while 20mm settles and binds together better.

So the same 8m by 3m drive needs its sub-base worked out as a separate run of the calculator. Pick MOT Type 1 and enter 150mm.

• Volume: 24 × 0.150 = 3.60m³. Plus 10% = 3.96m³.
• Weight: 3.96 × 2.2 = 8.71 tonnes.
• Bulk bags: 8,712kg ÷ 800kg = 10.89, so 11 bags, or 9 tonnes loose.
• Both layers together: about 10.8 tonnes of stone for one drive, before any edging, membrane or sand.

Bulk bags or a loose load

A bulk bag is often called a tonne bag, but it rarely holds a tonne. The bags we checked hold 800 to 850kg, and that weight fills roughly half a cubic metre. Hanson's data sheet puts one bag of its 20mm gravel at 12m² when spread 50mm deep, which is 0.6m³. Measure from the volume, not from the name on the bag.

Over a handful of bags, a loose tipped load usually works out cheaper. Breedon suggests a loose load once you need six bags or more and the site allows it. Gravelbase says its tipper loads start at roughly 10 tonnes and need 3.5m of access and somewhere firm to tip. The 11 bags of Type 1 above sit right on that line, so price it both ways.

Bags have their own access problem. Breedon's standard bulk bag delivery is a tail-lift lorry and a pump truck to the kerb, and the ground has to be flat, level and solid: broken concrete, grass, gravel and slopes are no good. Its crane off-load lorries can lift bags over walls and fences within reach, and on orders of more than six bags Breedon says they may even save you money.

Gravel, drainage and planning permission

In England, a new front driveway in gravel does not need planning permission, whatever its size. The government's guidance on permeable surfacing of front gardens names gravel as a permeable surface, alongside permeable block paving and porous asphalt. Laying more than 5m² of impermeable surface in a front garden, with no way for the rain to run off to a lawn or border, does need permission. Listed buildings have different rules, and a conservation area or national park can need permission for work that would not need it elsewhere, so check with the council there.

The same guidance has a catch worth knowing. It describes Type 1 as containing fine material that makes it hard for water to pass through quickly, and contrasts it with open graded sub-bases, such as 4/20 or Type 3, which let water flow freely through. On heavy clay that will not soak water away, it says a connection to the drains may be needed.

It is also honest about loose gravel: simple and cheap to build, but it scatters, forms waves, and does not suit a steep drive. A strip of block paving or tarmac at the entrance keeps it off the pavement.

What the calculator does not do

It turns a measured area into an order. It does not design a driveway. Ground that is soft, wet or made up, a steep slope, or regular heavy vehicles need someone to look at the site, and Marshalls asks to be contacted about any drive on an incline of 15° or more.

Pricing a drive or path for a customer? List the stone, the sub-base and your labour in the quote template. The concrete calculator works out the concrete for a strip footing under an edging.

• No prices. Stone, haulage and crane charges change from one region and one load to the next, so get a quote from your merchant.
• No excavation. It does not work out the spoil you dig out, or the skip or grab lorry to take it away.
• No extras. Edgings, geotextile membrane, gravel grids and drainage are separate items.
• Densities are averages. A wet load, a different quarry or a decorative stone can weigh more or less. Use your supplier's figure when you have one.

Common questions