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Gravel calculator
Loose aggregate is sold by weight or in bulk bags, almost never by the square metre, so a gravel order starts by turning the area you are covering and the depth you want into a volume, and the volume into a weight.
Enter the length and width of the area in metres and the depth of gravel in centimetres.
This calculator runs that chain. Enter the length and width of the area in metres and the depth in centimetres; the tool finds the volume in cubic metres, multiplies by the bulk density of the material to give the tonnes to order, and counts how many 850 kg bulk bags that is. Density is the part most calculators hide and most orders get wrong, because different materials weigh very different amounts for the same volume. Dry decorative gravel and chippings come in around 1.5 tonnes per cubic metre, sharp sand nearer 1.6, and compacted MOT type 1 sub-base about 2.0, so the same hole filled with sub-base weighs a third more than the same hole filled with gravel. Set the density to the material you are actually buying and the weight comes out right; leave it on the gravel default for a sub-base and you under-order. Depth depends on the job. A decorative gravel topping over a membrane wants about 50 mm so the ground does not show through and the stones do not scatter; a path or a drive wants a compacted sub-base of 100 to 150 mm under whatever finishes on top. Because loose material settles, the depth you spread is a little more than the depth that ends up compacted, which is one more reason orders tend to fall short rather than long. The 850 kg bulk bag is the common builders-merchant unit and the figure the tool counts against, though loose tipped deliveries are sold by the tonne and small jobs by the 25 kg handy bag. Rounding the bag count up matters here for a practical reason as much as a mathematical one: aggregate deliveries often carry a minimum-load or delivery charge, so a second trip for the last few bags can cost as much as the material, and a small surplus of gravel is easy to use up on a border or a low spot. People reach for this before a gravel driveway, a path, a French drain, a shed base sub-base or a decorative border, all of which are an area, a depth and a material away from a weight to order. It estimates the aggregate only, not the membrane, edging or the bedding sand that some finishes need.
How it works
- Measure the length and width of the area in metres and enter them.
- Enter the depth in centimetres: about 50 mm for decorative gravel, 100 to 150 mm for a sub-base.
- Set the bulk density for the material: about 1.5 for gravel, 1.6 for sharp sand, 2.0 for MOT type 1.
- The tool finds the volume, multiplies by the density for the weight, and divides by 850 kg for bulk bags.
- It rounds the bags up, since aggregate is sold by the bag or the tonne, never part-filled.
tonnes = length x width x (depth / 100) x bulk density
Length times width gives the area in square metres. Multiplying by the depth in metres (the centimetre figure divided by 100) gives the volume in cubic metres. Multiplying the volume by the bulk density in tonnes per cubic metre gives the weight to order, because loose aggregate is priced and delivered by weight. Bulk bags are then the total weight in kilograms divided by 850 and rounded up, since a bag is never part-filled.
- length, width
- area dimensions in metres
- depth
- layer depth in centimetres, divided by 100 for metres
- bulk density
- tonnes per cubic metre, about 1.5 for gravel, 2.0 for sub-base
- 850 kg
- weight of one builders-merchant bulk bag
Density and depth by material
| Decorative gravel, chippings | about 1.5 t/m3 | spread around 50 mm deep |
| Sharp sand | about 1.6 t/m3 | bedding and mixes |
| MOT type 1 sub-base | about 2.0 t/m3 | compacted, 100 to 150 mm |
| Bulk bag | about 850 kg | the merchant tonne bag |
Worked example
A 5 by 4 metre area covered 50 mm deep in decorative gravel at a bulk density of 1.5 tonnes per cubic metre: the area is 20 m2, which at 0.05 m deep is 1.0 m3 of gravel. At 1.5 tonnes a cubic metre that is 1.5 tonnes, or 1,500 kg, which is 2 bulk bags of 850 kg (1.76 rounded up). Switch the material to MOT type 1 sub-base at 2.0 tonnes a cubic metre and the same hole needs 2.0 tonnes, showing how much the density alone changes the order even when the area and depth do not move.
Key facts
- Aggregate is sold by weight, so an area needs a depth before it means anything.
- Density varies a lot: sub-base weighs about a third more than gravel per cubic metre.
- Decorative gravel spreads around 50 mm deep, a sub-base 100 to 150 mm.
- A builders-merchant bulk bag holds roughly 850 kg.
Tips
- Set the density to the real material, not the default, before reading the weight.
- Measure the deepest point on sloping or uneven ground and allow for it.
- Round the order up to dodge a second delivery and its minimum-load charge.
- Lay a weed membrane under decorative gravel so stones do not work into the soil.
Frequently asked questions
Why is gravel sold by weight, not area?+
Loose aggregate has no fixed thickness, so an area tells you nothing about quantity until you set a depth. Once area and depth give a volume, suppliers price it by weight because that is how it is weighed onto the lorry and into bulk bags. The tool turns your area and depth into a volume, then into the tonnes the merchant will quote.
What density should I use?+
Use the figure for the actual material: roughly 1.5 tonnes per cubic metre for dry decorative gravel and chippings, about 1.6 for sharp sand, and around 2.0 for compacted MOT type 1 sub-base. Heavier materials need more tonnes for the same hole, so leaving the density on the gravel default when you are buying sub-base will under-order by about a third.
How deep should the gravel be?+
A decorative gravel layer over a membrane wants about 50 mm so the ground does not show and the stones stay put underfoot. A path or driveway needs a compacted sub-base of 100 to 150 mm beneath the surface finish. Going much deeper than 50 mm on a decorative top just makes it loose and hard to walk on.
How big is a bulk bag?+
A builders-merchant bulk bag, sometimes called a tonne bag, holds around 850 kg of aggregate, the figure this tool counts against, though the exact weight varies a little by material and moisture. Loose tipped loads are sold by the tonne, and small jobs by the 25 kg handy bag. Check what your supplier means by a bag before ordering.
Should I order a bit extra?+
Usually yes. Loose material settles and compacts, so the spread depth is a touch more than the finished depth, and aggregate deliveries often carry a minimum-load or delivery charge that makes a second trip for a few bags expensive. A small surplus spreads easily onto a border, a low spot or a path, so rounding the order up rarely goes to waste.
What about an irregular or sloping area?+
Split an irregular area into rectangles, work out the volume of each, and add them. For a circle use the radius squared times 3.14 times the depth. On a slope, the depth is measured square to the ground, not the horizontal, and a slope that needs levelling can take far more material in the low corner, so measure the deepest point and allow for it.
Things to watch
- Leaving the density on gravel when buying sub-base under-orders by about a third.
- Loose material settles, so the spread depth exceeds the finished compacted depth.
- This counts the aggregate only, not the membrane, edging or bedding sand.
Last updated: 2026
This is an estimate for general guidance, not financial, tax, legal or medical advice. Figures can change and individual circumstances vary. Always confirm with an official source before making decisions.
Built and maintained by Vikas Dulgunde. Editorial standards.