Advanced options
Based on typical gravel at 1.6 tonnes per cubic metre. Decorative gravel needs about 5cm depth (4-5cm over a membrane); driveways need 5cm of gravel over a proper sub-base. A 850kg bulk bag covers roughly 10 square metres at 5cm.
What The Gravel Calculator Works Out
- Volume: length x width x depth with the depth in metres. A 5 x 4m area at 5cm deep is 1 m3. Decorative gravel over a weed membrane wants 4 to 5cm: too shallow shows the membrane, too deep is hard to walk on.
- Weight: volume x density, with typical 20mm gravel at 1.6 tonnes per m3. Merchants sell by weight, so this is the number you order.
- Bulk bags: the weight divided by the bag size, rounded up. Bulk bags vary between 800 and 900kg depending on the supplier and how wet the stone is, so check before assuming 850kg. A bulk bag will not fit through a side gate on a pallet truck, so agree the crane drop location.
- Choosing gravel: angular stone around 20mm locks together and stays put; anything under 10mm migrates and sticks in tyre treads, and rounded pea gravel rolls underfoot.
- Material (advanced): changes the density: slate chippings are lighter at about 1.4 t/m3, Cotswold gravel heavier at 1.7, compacted MOT Type 1 about 2.1, and bark mulch only 0.4, so the same volume needs very different tonnages.
- Bulk bag weight and allowance (advanced): enter your supplier's actual bag weight, and add 5% to 10% for settlement and uneven ground so you are not a wheelbarrow short at the end.
- Cost in bulk bags vs loose (advanced): bags x your bag price against tonnes x the loose price. Bulk bags typically cost GBP 60 to GBP 90; loose loads by the tonne are cheaper once you need three or more bags, but most merchants only tip 5 tonnes or more and you need somewhere to tip it.
- Small 25kg bags (advanced): how many pre-packed bags the job would take, for tiny areas or where only carry-in access is possible. Beyond about 20 bags a bulk bag is almost always cheaper.
- Coverage per bulk bag (advanced): how many square metres one bag covers at your depth with your material, the shorthand merchants quote (about 10 m2 at 5cm for standard gravel).
- Sub-base (advanced): for a driveway, enter a Type 1 depth (10 to 15cm compacted) to get the tonnes of MOT Type 1 needed underneath. It is a separate product and a separate order from the surface gravel.