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How the gravel calculator works
Gravel is a volume purchase that you pay for by weight, so the calculation is in three steps: area, then volume, then tonnes. Multiply length by width to get the area in square metres, multiply that by the depth in metres to get cubic metres, then multiply by the bulk density of the material. A 5 m × 3 m border at 50 mm is 15 m² × 0.05 m = 0.75 m³, and at 1.5 tonnes per cubic metre that is 1.125 tonnes before any allowance — 1.24 tonnes once the default 10% is added.
The allowance is not padding. Ground is rarely dead flat, a layer raked out by hand rarely ends up at an even depth, and gravel settles into the ground beneath it. Ten per cent is the usual figure and it is the difference between a finished surface and a trip back to the merchant for four more bags.
Decorative gravel or MOT Type 1?
They are different jobs, and they weigh differently, which is why the calculator has a switch for it. Decorative gravel is the loose finished surface — 10–20 mm rounded shingle, pea gravel or angular chippings — laid 40–75 mm deep over a weed membrane. Because the stones are rounded and loosely packed, it comes out at roughly 1.5 tonnes per cubic metre.
MOT Type 1 is a crushed, graded sub-base that is compacted before anything goes on top of it. Its compacted bulk density is about 1.8 tonnes per cubic metre — the same figure the paving calculator uses, so a patio sub-base and a gravel sub-base agree. A 4 m × 3 m area at 100 mm is 1.32 m³ with the allowance, which is 2.38 tonnes, or three 850 kg bulk bags.
Depth, and what a tonne actually covers
Depth is the number that decides the size of the order, so be honest about it. Decorative gravel wants a minimum of 40 mm to hide the membrane; 50–60 mm looks better and resists being kicked away from edges. MOT Type 1 needs 100 mm compacted for foot traffic and garden furniture, and 150 mm for anything a car will park on, laid in two layers and compacted between them.
As a rough guide, one tonne of decorative gravel covers about 13 m² at 50 mm, while one tonne of MOT Type 1 covers only about 5.6 m² at 100 mm. That is why sub-base orders look so much heavier than people expect. Bulk bags of 850 kg are the practical unit for most domestic jobs; loose loads are cheaper per tonne but need somewhere to tip.
Gravel FAQs
How many tonnes of gravel do I need for 15 m² at 50 mm?
About 1.24 tonnes. That is 15 m² × 0.05 m = 0.75 m³, plus the default 10% allowance, multiplied by 1.5 tonnes per cubic metre for decorative gravel. In practice that is two 850 kg bulk bags, or 50 bags of 25 kg if you are carrying it through the house.
How much does a bulk bag of gravel weigh?
A standard bulk bag holds 850 kg — roughly 0.85 tonnes — which is a bit over half a cubic metre of decorative gravel. They are delivered on a pallet, so you need a hard, flat surface for the lorry to leave them on, and a spade and wheelbarrow to move the contents.
Why is MOT Type 1 heavier than decorative gravel?
Because it is crushed and compacted rather than loose and rounded. Angular, graded stone packs together with fewer voids, so the same volume holds more mass — about 1.8 t/m³ against roughly 1.5 t/m³. Ordering a sub-base by the same tonne figure as your decorative gravel will leave you a layer short.
How deep should gravel be on a driveway?
A gravel driveway needs a compacted MOT Type 1 sub-base of at least 150 mm, and 200 mm over soft clay, before the decorative surface. The surface layer itself wants 50 mm minimum, and on a sloped drive run a perforated drainage channel across the threshold. Loose gravel alone, spread on soil, ruts within a season.
Do I need a membrane under gravel?
Yes, unless the gravel sits on a rigid base. A woven or non-woven weed membrane stops weeds germinating through the stone and stops the chippings being gradually swallowed by the soil. Order the area plus about 10% for overlaps between rolls, and pin the edges down before you tip a single barrow of stone.
Can I order gravel in half tonnes?
Many merchants sell loose aggregate in half-tonne increments, and bulk bags are usually priced individually. If you are within a fraction of a whole number of bags, take the extra one — a part bag is not a thing, and a spare bag of gravel will always find a use on a border or a pot.