What this calculator works out
Ordering gravel, sand, ballast, topsoil or bark comes down to two questions: how much space does it need to fill, and how heavy is that once it's a real material rather than a shape on paper. This tool answers both in one pass. You give it a shape and a size, a depth, and what you're laying, and it turns that into a volume, then a weight, then a practical order: tonnes, bulk bags and 25kg bags.
The shape and depth together give a volume, exactly as they would for any box-shaped space: a flat area, times how deep the layer goes, is the volume that layer fills. The volume, multiplied by the material's typical loose bulk density, gives a weight. That weight is what a supplier actually sells by, whether that's a loose tipped load, a bulk bag, or a stack of small bags from a builders' merchant.
How the maths works
Area first, from whichever shape fits the space. A rectangle is length times width; a circle uses its diameter, halved to a radius and squared, times pi; a triangle is half its base times its perpendicular height.
Depth turns that flat area into a volume, once both are in the same unit of length. A 50 square metre area, 5cm (0.05m) deep, is a 2.5 cubic metre volume.
Weight comes from the material's bulk density, meaning the weight of one cubic metre of that material loose and settled, not compacted or packed down. Gravel and building sand both sit at around 1.6 tonnes per cubic metre for typical loose material, ballast (a sand and gravel mix) is denser at around 1.75 tonnes per cubic metre, topsoil is lighter at around 1.4 tonnes per cubic metre once it has settled a little in transit, and bark or mulch is much lighter again, typically 0.3 to 0.45 tonnes per cubic metre depending on how coarse and how dry it is. This calculator uses 1.6, 1.6, 1.75, 1.4 and 0.35 tonnes per cubic metre respectively as representative figures; always check the bag or delivery ticket against your own supplier's stated density, since grading, moisture and stone type all shift the true number a little.
Working the example through: 10m by 5m is 50 square metres; 50 square metres at 5cm deep is 2.5 cubic metres; 2.5 cubic metres of gravel at 1.6 tonnes per cubic metre is 4.0 tonnes.
Why depth matters more than people expect
Area gets most of the attention because it's the number you can picture, a patch of ground. Depth is easy to treat as an afterthought, a rounding decision made on site. That's backwards, because weight scales with volume, and volume scales directly with depth: double the depth of a layer over the same footprint and you double the volume, and therefore double the weight and the order. Go from 5cm to 10cm over the same 50 square metres and the job jumps from 2.5 cubic metres to 5 cubic metres, from roughly 4 tonnes of gravel to roughly 8. Nothing about the area changed; only the depth did.
That's also why "how deep" is worth deciding properly rather than guessing. A driveway laid at a decorative-bed depth won't carry a car's weight or stop tyres pushing stone into the mud underneath; a border laid at driveway depth wastes a genuinely large amount of material and money for no visual benefit. Getting the depth right for the job matters at least as much as getting the area right.
Choosing a depth
As a starting point, decorative beds, borders and areas around planting typically use about 5cm of gravel, enough to give solid coverage and suppress light weed growth without wasting material. Footpaths generally sit a little deeper, around 5 to 7.5cm, since they take foot traffic and need to resist being kicked or scuffed down to bare ground. Driveways need the most: a top decorative layer is often still only around 5 to 10cm, but it normally sits on a separate compacted sub-base, commonly 10 to 15cm of a denser aggregate like MOT Type 1, that does the structural work of carrying vehicle weight. This calculator works out the volume and weight of the layer you tell it about; it doesn't add a sub-base automatically, since not every job needs one and the two layers are usually bought and calculated separately.
One practical note on preparation: a permeable weed membrane laid under the gravel, before it goes down, stops most weeds pushing up through the stone and keeps the gravel from slowly working its way down into the soil beneath, which keeps the layer looking full and even for longer.
How far a tonne actually goes
Because weight and depth are so closely linked, it helps to see coverage the other way round: how much area a single tonne covers at a given depth, for a given material. At gravel's typical 1.6 tonnes per cubic metre, one tonne is 0.625 cubic metres (1 ÷ 1.6). Spread 0.625 cubic metres 5cm deep and it covers 12.5 square metres (0.625 ÷ 0.05); spread the same tonne twice as deep, at 10cm, and it covers only 6.25 square metres, half as far, for exactly the reason above.
| Depth | Coverage per tonne of gravel (1.6 t/m³) |
|---|---|
| 2.5cm | 25.0 m² |
| 5cm | 12.5 m² |
| 7.5cm | 8.3 m² |
| 10cm | 6.25 m² |
The results panel on this page shows the same working for whichever material you've actually selected, since a tonne of much lighter bark covers far more ground than a tonne of gravel at the same depth.
Bulk bags, small bags and loose loads
Once a weight is known, it can be bought a few different ways. A "bulk bag" (also called a jumbo bag, tonne bag or builder's bag) typically holds somewhere around 800 to 900kg of aggregate, commonly quoted as about 850kg, and this calculator uses that 850kg figure to estimate how many bulk bags an order needs, rounded up since suppliers don't split one. Small bags, usually 25kg, are the option seen stacked at builders' merchants and DIY stores, useful for small jobs or top-ups but a slow and comparatively expensive way to buy a large quantity.
For genuinely large orders, loose tipped delivery (a full lorry load poured directly where you need it, or as close as access allows) is normally the most efficient option by weight, provided there's room to tip it and either the whole load gets used quickly or there's somewhere dry to store the rest. Bulk bags cost more per tonne than a loose load but need no more than driveway access and disappear as you use them, which suits smaller gardens, tighter access, or jobs done in stages. Neither option is universally cheaper; it depends on quantity, access and how quickly the material will actually get used, so it's worth asking a local supplier for both prices before deciding.
Whichever way you order it, the 10% compaction and waste allowance on this page is a common, sensible buffer, not a fixed rule: it covers material that settles into an uneven base, gets lost to spillage, or gets used slightly more generously than the theoretical calculation assumed. It matters most for driveways and paths that get walked or driven on and will naturally bed in over time; for a shallow decorative bed that won't be compacted, it's a smaller concern.
Questions people ask
How deep should gravel be for a driveway?
A decorative top layer is typically around 5 to 10cm, but a driveway that carries vehicles normally needs a separate compacted sub-base underneath as well, often 10 to 15cm of a denser aggregate. This calculator works out the layer you enter; add the sub-base as a second, separate calculation if your project needs one.
Why does the weight change so much when I just tweak the depth?
Because weight is driven by volume, and volume scales directly with depth over the same area. Doubling the depth doubles the volume and therefore doubles the weight, even though the footprint on the ground hasn't changed at all.
How big is a bulk bag, and how many do I need?
A bulk bag (also called a jumbo bag or tonne bag) typically holds around 800 to 900kg, commonly quoted as about 850kg, which is the figure this calculator uses. It divides your total weight by that figure and rounds up, since a supplier can't sell part of a bag.
Should I order extra for compaction and waste?
A 10% allowance is a common, widely used buffer for settling, an uneven sub-base and general spillage, and this page offers it as an optional tick rather than folding it in automatically. It's worth adding for driveways and paths that will be walked or driven on; a shallow decorative bed needs it less.
Why is bark or mulch so much lighter than gravel for the same volume?
Bark and mulch are made of loose, air-filled organic material rather than solid mineral stone, so the same cubic metre weighs roughly a quarter as much. Typical loose bark sits around 0.3 to 0.45 tonnes per cubic metre against gravel's roughly 1.6, which is why a bark delivery needs far fewer bags or far less weight than the same volume of stone.
Does a weed membrane actually make a difference?
A permeable membrane laid under gravel before it's spread lets water drain through while stopping most weeds pushing up and stopping the stone from slowly sinking into the soil beneath, which keeps a gravelled area looking full for longer with less topping up later.
Is loose tipped delivery cheaper than bulk bags?
Usually yes per tonne for a large enough order, since it needs no bagging, but it needs space to tip a full load and a way to use or store it promptly. Bulk bags cost more per tonne but suit tighter access, smaller gardens or work done in stages, so the better option depends on quantity and access rather than one being universally cheaper.
This calculator uses representative typical loose bulk densities for each material and standard conventions for bag sizes and over-ordering. Real density varies by supplier, stone type, grading and moisture content, so always confirm the figures on your delivery ticket or with your supplier before placing a large order.