Decorative Stone Calculator

By Gary Hodson · 15+ years UK gardening experience · Figures cross-referenced against named UK aggregate supplier specifications · Last reviewed 4 July 2026

The maths is the same for every decorative stone: area × depth = volume, then volume × density = weight. Decorative stone in the UK is sold by weight — bulk bags, 25kg bags or loose tonnes — so the density of your chosen stone decides how much you order. This calculator uses the bulk density figures that Mainland Aggregates publishes on its own product pages: approximately 1.6 tonnes per cubic metre for 40mm blue slate chippings,¹ 40mm plum slate² and 20mm Cotswold chippings,³ 1.8 tonnes per cubic metre for 20mm golden gravel,⁴ and 1.7 tonnes per cubic metre for 20mm dove grey limestone.⁵

One deliberate boundary: pea gravel and standard 20mm gravel are not in the dropdown. They behave differently, they are priced differently, and they have their own page — our gravel calculator covers pea shingle, 20mm gravel, MOT Type 1 and general-purpose aggregate. This page is for the decorative stuff you choose for its looks: slate, Cotswold, golden and grey limestone chippings.

Supplier-Verified Densities and Coverage

Stone Type	Density (t/m³)	Bulk Bag Coverage	Max Laying Depth
Slate chippings, 40mm (blue/grey)	Approx 1.6	~12m² at 40mm (850kg bag)	40mm
Plum slate chippings, 40mm	Approx 1.6	~12m² at 40mm (850kg bag)	40mm
Cotswold stone chippings, 20mm	Approx 1.6	~10m² at 40–50mm (850kg bag)	40–50mm
Golden gravel / limestone, 20mm	Approx 1.8	~10m² at 50mm (850kg bag)	40–50mm
Dove grey limestone chippings, 20mm	Approx 1.7	~10m² at 50mm (850kg bag)	40–50mm

All five rows are the figures Mainland Aggregates states on the individual product pages (accessed 4 July 2026 — Sources 1–5). "Approx" is the supplier's own word: bulk density moves a little with moisture and how the stone packs, so treat the tonnage as a close estimate, not a lab measurement.

Bulk Bags, Explained

A UK "bulk bag" (dumpy bag, tonne bag) of decorative stone typically holds around 850kg — that is the bag weight Mainland Aggregates states for all five stones above,³ and Cloburn Quarry supplies its granite in "approx. 850kg" bulk bags too.⁸ Some suppliers fill to 800kg — Decorative Aggregates sells 800kg bulk bags.⁷ At the cited densities of 1.6–1.8 tonnes per cubic metre, an 850kg bag works out at roughly 0.47–0.53 cubic metres of stone. This calculator uses 850kg per bulk bag and 25kg per small bag — 34 small bags to one 850kg bulk bag — and always rounds up, because you cannot buy part of a bag.

Match the Stone Size to the Job

Stone size drives both the look and the laying depth. Decorative Aggregates' guidance is that 10mm aggregates are usually laid at 30–40mm — fine for borders and light-use paths — while 20mm aggregate needs at least 40–50mm, rising to 50–60mm where vehicles run over it.⁶ In practice that means smaller 10–14mm stone suits paths and pot-topping where a thin, even layer matters, and 14–20mm stone suits larger open areas and anywhere near a driveway. Larger, more angular chippings are better suited to driveways and walkways because they move less underfoot than rounded stone.⁹ For the driveway surface itself — and the compacted foundation under it — use our gravel calculator and sub-base calculator rather than this page.

Slate vs Rounded Gravel — They Behave Differently

Slate chippings are flat plates rather than chunky nuggets. They lie flat, knit together and shed rain off their surface, which is why an 850kg bag of 40mm slate covers around 12m² at 40mm while the same bag of 20mm Cotswold covers about 10m² at the same depth.¹³ The flip side: flat slate can be slippery on slopes when wet, and it is less comfortable to walk on barefoot than rounded stone. Rounded and semi-rounded chippings — Cotswold, golden gravel — roll more, so they need edging on any gradient, but they give that classic crunch underfoot and a softer cottage-garden look. Colour matters too: slate darkens dramatically when wet, Cotswold buff stays warm and pale, and grey limestone reads almost silver in bright light.

Always Lay a Membrane on Soil

Whatever stone you pick, put a woven weed membrane between soil and stone. It does two jobs: it blocks weeds coming up through the display layer, and it stops the chippings slowly disappearing into the ground over winter — the main reason decorative borders go thin and patchy after a year or two. Overlap sheets by at least 150mm and pin them down. Planting through it is easy: cut a cross, fold the flaps back, plant, fold the flaps back around the stem.

Depth is where most people overspend. Deeper than the supplier's recommended depth does not look better — it feels soft underfoot and buries money. The presets in the calculator map to the three depths that cover almost every decorative job:

Depth	When to Use It	Approx Weight per m²
30mm	Topping up an existing stone area, pot dressing, thin decorative cover over membrane in low-traffic borders — and the usual lower bound for small 10mm aggregates6	48–54kg
40mm	The standard for 40mm slate chippings — Mainland Aggregates says lay them at no more than 40mm1 — and the lower bound of the 40–50mm range for 20mm chippings3	64–72kg
50mm	Full fresh coverage with 20mm stone — Decorative Aggregates' recommended standard installation depth for stone up to 20mm, giving even coverage with no gaps6	80–90kg

Weight per m² is derived from the cited densities (1.6–1.8 t/m³): for example 1m² × 0.04m × 1.6 t/m³ = 64kg. If you are covering bare soil for the first time, do not go under 40mm with 20mm stone — thinner layers show membrane through the gaps within weeks.

Decorative stone is a forgiving landscaping job, but the difference between a border that looks sharp for five years and one that is weedy and patchy by next spring is all in the preparation. Work through these five steps in order.

1. Measure Properly

Break awkward shapes into rectangles and circles, measure each, and use the "+ Add this area" button in the calculator to stack them into one total. Subtract anything inside the area that won't be stoned — planting pockets, stepping stones, a bird bath base. If in doubt, round the area up slightly: a part-used bulk bag in the shed is cheaper than a second delivery charge.

2. Prepare the Base

Strip vegetation and rake the ground roughly level. For borders and beds you do not need a compacted foundation — firm, level soil is enough. Kill or remove perennial weeds first; membrane slows them, it does not stop an established bramble. If the area takes foot traffic, tread or tamp the soil firm so the finished surface does not dish.

3. Lay the Membrane

Use a woven geotextile membrane, not the thin felt-like sheet — cheap membrane tears and weeds push through within a season. Overlap joins by 150mm minimum, run it up under any edging, and pin every metre or so. Cut crosses (not holes) where plants go through.

4. Spread to the Right Depth

Tip, rake and check depth as you go with an offcut of timber marked at your target depth — 40mm for 40mm slate, 40–50mm for 20mm chippings (see the depth guide above). Spread from boards if the ground is soft, and keep a little stone back for settling and top-ups.

5. Edge It

Loose stone migrates — into the lawn, over the path, everywhere. Any decorative area that meets grass or paving needs a physical edge: metal strip, timber board, brick soldier course or a raised sett. It is an inexpensive way to keep the area looking deliberate rather than scattered.

Decorative stone is priced per bag, and the spread between stones is wide. Rather than quote vague ranges, here are real, verifiable bulk-bag prices from named UK suppliers, all checked on 4 July 2026:

Product	Bag Size	Price (inc. VAT)	Supplier
20mm red granite aggregate	850kg bulk bag	£86.40	Cloburn Quarry8
20mm plum slate chippings	800kg bulk bag	£125.58	Decorative Aggregates6
20mm Cotswold buff chippings	800kg bulk bag	£132.53	Decorative Aggregates7
20mm polar white marble gravel	800kg bulk bag	£178.76	Decorative Aggregates6

Three practical notes. First, compare the delivered price — delivery terms differ by supplier and postcode, and a cheap bag with an expensive pallet delivery is no bargain. Second, watch the bag weight when comparing: an 850kg bag at £130 is cheaper per kilogram than an 800kg bag at £126. Third, quarry-direct stone (like Cloburn's granite) is often markedly cheaper than the same class of stone through a decorative reseller — worth a search if a quarry sits in your region.

The calculator deliberately does not put a single cost figure on your result: with prices ranging from £86 to £179 per bag for different stones at the same size, any single "estimated cost" would be misleading. Multiply your bulk-bag count by the price of the specific product you are buying.