How Deep Should Gravel Be?

The Short Answer

By Gary Hodson · Ex-Army. I do the sums so you buy the right amount. · Depths and sums checked against the sources below on 8 October 2026 · Last reviewed 8 October 2026

Gravel goes 25 to 40mm deep in most projects, and the size of the stone moves that range. Pavingexpert, an independent UK paving guide, says the layer should be at least twice the stone size and no more than about three times it, and that 25 to 40mm is “just about ideal” in the vast majority of projects.¹ Applied to 10mm stone, the rule gives 20 to 30mm, which is our arithmetic. For 12mm gravel it gives 24 to 36mm, which Pavingexpert rounds to 25 to 40mm. For 20mm stone the rule gives 40 to 60mm. Decorative Aggregates, a supplier, gives 40 to 50mm for 20mm stone in a border and 50 to 60mm for 20mm stone on a drive.⁶

That is the top layer only. A path or drive also needs a compacted sub-base underneath, because loose stones do not carry a load on their own.¹ Going deeper with the gravel does not fix a weak base. It just gives the wheel or foot more loose stone to sink into.

Gravel Depth by Job

Depths are the finished surface layer. Sub-base depths are compacted thickness unless marked loose. Two sources are cited because they disagree slightly on drives; the next section explains why that mostly sorts itself out. Our sub-base calculator and decorative stone calculator use the same sub-base and decorative depths.

Stone Size Sets the Depth

The rule is simple. The layer must be thick enough to cover what is under it, and thin enough to stay firm. Pavingexpert puts the floor at twice the stone size, because a thinner layer gets scuffed away and exposes the sub-base. It puts the ceiling at about three times, because deeper than that the surface turns unstable.¹ For a typical 12mm gravel that is 24 to 36mm, which Pavingexpert rounds to 25 to 40mm.

The table is our arithmetic from Pavingexpert's two-times and three-times rule. Pavingexpert itself says 25 to 40mm is just about ideal in the vast majority of projects, and that smaller gravel such as 6mm can go shallower and larger gravel such as 18mm deeper.¹ So for 10mm stone, 25mm is Pavingexpert’s working minimum and 30mm is the top of the two-to-three-times rule. Dig out to the compacted sub-base plus the gravel depth you pick. Use the compacted depth, not the loose depth, for this sum. Pavingexpert’s dig depths of 100mm, 135mm and 180mm allow for about 25mm, 35mm and 30mm of gravel over their bases, so they do not cover the deeper depths quoted above. For more gravel, dig deeper by the difference: 40mm of gravel on a 150mm base needs a 190mm dig, and 50mm needs 200mm.

Drives are where the advice disagrees. Pavingexpert says 10mm is the most popular drive gravel and gives 25 to 40mm for the surface layer.¹ Decorative Aggregates, a supplier, says 10mm stone is usually laid 30 to 40mm deep, but on a driveway at least 50mm, and 50 to 60mm for 20mm stone.⁶ Those two do not match for 10mm stone, and we cannot reconcile them for you. If you are buying a particular product, follow the depth on its own specification. If it gives none, ask the supplier before you order.

The Layers Under the Gravel

A gravel path or drive has three layers: the ground, a sub-base, and the gravel on top. The sub-base does the work. It should be crushed stone with a good mix of lumps and fines, so there are few voids, and builders' merchants stock DTp Type 1 for the purpose.¹ It is compacted with a plate compactor, and a layer shrinks as it goes down: Pavingexpert's rough guide is about 30 per cent, so 130mm of loose Type 1 ends up near 100mm.⁵

If the sub-base needs to be thicker than 150mm, it goes in as layers of 150mm or less, each one compacted before the next.⁵ One tonne of Type 1 covers about 5 square metres at 100mm compacted and about 3.5 at 150mm.¹ Use the sub-base calculator for your own area.

Pavingexpert also suggests running a plate compactor over the first thin spread of gravel, then topping up and compacting again, so the stone beds into the sub-base and the sub-base stays covered if the surface gets scuffed.¹

Membrane: Where It Helps and Where It Does Not

There are two different jobs, and they get mixed up.

Separation, under the sub-base. A proper geotextile laid on the dug-out ground stops the sub-base mixing into soft soil and sinking. Pavingexpert says that is its main purpose, and that it belongs between the ground and the sub-base.² It warns against polythene, building film and damp-proof sheet, because a sheet that water cannot pass through causes drainage trouble.²

Weed control, in a planted area. Here the experts split. The RHS says to lay landscape fabric over the soil before the gravel to prevent weeds.⁹ Beth Chatto Gardens do not use it. Their reasons: it stops them pulling out perennial weed roots, it does nothing against annual weeds that seed into the gravel and root into the fabric, and on paths it wears thin and shows.¹⁰ The RHS adds that where fabric is left out, weeds may become a problem, and so may self-seeded plants.⁹ Neither route removes the weeding. It changes what kind of weeding you do.

Gravel Grids: Shallower Fill, Firmer Ground

A grid is a plastic honeycomb that holds the stones in place. Pavingexpert says these systems make a gravel area far friendlier to feet and wheels and cut scatter and drift.³ It also says the sub-base under cellular systems is generally 100mm to 200mm as a minimum, depending on use and ground.³

Makers' sheets differ, so follow the one for your product. Stone Warehouse says its 40mm-high panels can go straight onto soil, though it recommends at least 100mm of compacted Type 1. It fills the cells with 8 to 20mm stone and covers them with 20 to 35mm, which settles to a 10 to 15mm layer over four to six weeks.⁷ IBRAN says any grid job with vehicles needs a sub-base: 150mm of compacted MOT sub-base, a 10mm sand layer, and 50mm of 20mm angular gravel, which means digging out at least 210mm.⁸ Same idea, different numbers. Do not mix instructions from two brands.

Edging: Gravel Wanders

Pavingexpert is blunt: gravel without an edge restraint will migrate, even inside cell systems.¹ An edge that sits flush with the gravel slows it but does not stop it, because stones get kicked or dragged over the top. An edge with a small upstand does better. At a drive entrance, where a raised edge is not practical, it suggests a dished channel or a low ridge of setts across the opening.¹ Edging goes in before the sub-base, so plan it into your dig depth.¹

Worked Example: A 3m by 6m Drive

The area is 3m × 6m = 18m². We use 1,500kg per cubic metre for 20mm gravel, the same figure as our gravel calculator. Your supplier's density may differ, and denser stone weighs more for the same depth.

Cross-check: Pavingexpert says gravel typically covers 15 to 20m² a tonne at 35mm.¹ Over 18m² that is 0.9 to 1.2 tonnes, and our 40mm figure of 1.08 tonnes sits inside it. The RHS says one cubic metre of gravel covers around 20m²,⁹ which works out at 50mm deep.

Now the sub-base. At 100mm compacted, Pavingexpert's 5m² a tonne means 18 ÷ 5 = 3.6 tonnes of Type 1. Our sub-base calculator uses a denser 2.1 tonnes per cubic metre and gives 1.8m³ × 2.1 = 3.78 tonnes. Order against your supplier's figure. The 100mm option is for cars. For vans, the compacted sub-base goes up to 150mm.

The 100mm of gravel in the last row is there to make a point. It costs 1.6 tonnes more than the 40mm layer, and the surface gets worse.

Mistakes That Cost a Second Delivery

• Gravel over old concrete or tarmac. Pavingexpert calls it the “ball bearing effect”: on a hard base the stones have nothing to bed into, so they roll, and the base shows through in bald patches. Adding depth does not help, and neither does sprinkling sand on top.⁴
• Choosing the depth before the stone. Order 20mm chippings and spread them 25mm deep, and you are below the two-times floor.
• Skipping the edging. The gravel ends up in the lawn, and Pavingexpert notes it does the mower no good.¹
• Trusting the bag label for coverage. The RHS says a 25kg bag covers 0.6m² at 5cm.⁹ At 1,500kg/m³ our calculator gives about 0.33m² for the same bag and depth. The RHS bag figure implies a stone of about 830kg/m³, which is much lighter than typical gravel at around 1,500kg/m³. Trust the weight sum for the product you buy, and check the stated weight and density on its label.
• Using polythene as a weed barrier. It blocks drainage.² Use a permeable fabric or none.

Gravel in a Front Garden

In England, laying more than five square metres of traditional impermeable surface on a front garden needs planning permission. You do not need it if the surface is permeable, or if the rain runs off onto a lawn or border and soaks away. Loose gravel on its own is permeable, but a Type 1 sub-base under it may not be, so the whole build-up has to drain, or the runoff has to reach a suitable permeable area, for a drive over 5 square metres.¹²¹¹ The rules do not cover flats, converted houses or areas with an Article 4 direction, and Wales may differ, so check with your local planning authority.¹¹ The government guidance on permeable surfacing is linked in the sources below.¹²

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