How Much Polished Pebbles Do You Need per Square Metre?

Published 2026-11-03 · By Gardenstone Editorial Team · Educational article from the SWME TRADE team

How Much Polished Pebbles Do You Need per Square Metre?

Plan on 56–64 kg of polished pebbles per square metre for the workhorse 30–40 mm grade laid one stone deep — one tonne covers about 16–18 m², and a 20 ft container of 24–27 tonnes covers roughly 380–430 m². Smaller 20–30 mm stone needs only 40–48 kg/m², while 40–50 mm needs 72–80 kg/m². The rule that fixes the order: set the laying depth equal to the stone band, no deeper.

That single sentence prevents the two most common quantity errors with polished stone. Ordering a blanket 50 mm depth for a 30 mm grade buys a second layer nobody sees, and quoting tonnage without naming the band makes two suppliers' figures impossible to compare. Work through the bands below, convert your area once, and add 8–12% for edges, curves and settlement.

Why the Depth Rule — Not a Weight Chart — Decides the Order

Polished pebbles are display stone: a polished bed shows exactly one layer, and a second layer buried under the first contributes nothing but cost. Unlike crushed gravel, which is compacted into a structural depth, polished pebbles are placed so each stone's finished face is visible. That is why the correct depth for any polished grade is its nominal top size — 30 mm stone at 30–35 mm, 40 mm stone at 40 mm — and why a depth figure copied from a gravel chart almost always over-orders by 25–40%.

There are two legitimate exceptions. First, a bedding trick used on large display beds: fill the lower two-thirds with an unpolished or tumbled grade of the same colour family and reserve the polished stone for the top single layer, which cuts polished tonnage by 50–60% at almost no visual cost. Second, resin-bound or grid-locked surfaces, where the stone is proportioned by mix design rather than loose depth. For everything else, one visible layer is the specification.

How Many Kilograms and Stones per Square Metre at Each Size Band?

Coverage follows bulk density (loose polished pebbles run about 1.5–1.7 tonnes per cubic metre) multiplied by the laying depth. At 20–30 mm laid 25–30 mm deep, expect 40–48 kg/m² — one tonne covers 21–25 m². At 30–40 mm laid 35–40 mm deep, expect 56–64 kg/m² — one tonne covers 16–18 m². At 40–50 mm laid 45–50 mm deep, expect 72–80 kg/m² — one tonne covers 12–14 m². Larger 60–90 mm feature stones, set as a single layer, run 96–120 kg/m², or 8–10 m² per tonne.

Piece counts matter as much as weight for display and retail work. A tight single layer holds roughly 1,100–1,300 stones per square metre at 30 mm, 650–750 at 40 mm and 420–480 at 50 mm; per tonne that is about 25,000–30,000 stones at 30 mm, 10,000–12,000 at 40 mm and 5,000–6,500 at 50 mm. A 20 kg bag therefore holds roughly 500–550 polished pebbles at 30 mm, 210–240 at 40 mm and 110–130 at 50 mm — numbers buyers use to check bagged display grades and shelf-coverage claims.

What Else Moves the Quantity: Grading, Shape, Edges and Waste

Grading changes the arithmetic. A single-size grade packs to roughly 38–42% voids, so the membrane can peek through the gaps; a graded mix (say 20–40 mm with its fines retained) packs tighter, to about 28–32% voids, and needs roughly 10–15% less depth for the same visually opaque cover. Shape matters too: flatter, oval pebbles lie face-up and cover about 10–15% more area per tonne than near-round ones, which is worth knowing before you compare two quotations per square metre.

Then add the site allowances. Edges, curves and cut-outs waste stone, first-season settlement sinks the layer 5–10 mm, and spillage happens on every site: 8–12% on top of the calculated area is the normal allowance. For border strips the per-metre figure is quicker: a 100 mm wide strip at 40 mm deep takes about 6–7 kg per running metre, so one tonne lays roughly 150–160 metres of polished edging.

How Do You Convert Square Metres into Tonnes, Bags and Containers?

Worked example: a 120 m² courtyard in 30–40 mm polished pebbles at 35–40 mm. At 56–64 kg/m² the bed needs 6.7–7.7 tonnes; add 10% and the order is about 8 tonnes — roughly 400 bags of 20 kg. At container scale, a 20 ft box carries 24–27 tonnes, which covers about 380–430 m² at this spec, so the courtyard takes about a third of a container and the balance is normally filled with adjacent bands or other colours from the same lot.

The container is weight-limited, not volume-limited: 25 tonnes of polished pebbles occupies only about 15–17 m³ of the box, which is why mixing two or three size bands or colours in one shipment costs no extra freight and is the standard way first orders are structured. Palletised bag loads give up 10–15% of payload to pallet height and spacing, so quote palletised and loose-in-container as separate lines before you compare prices.

How Do You Choose Polished Pebbles so the Quantity Holds Up?

Choosing well is part of choosing the right quantity, because grade consistency is what keeps your calculated kilograms honest. Specify through-polished stone — mechanically polished so the gloss is the stone itself — rather than wax- or resin-coated stone, where a applied film carries the shine and wears off outdoors within 6–18 months, dulling the bed long before the tonnage is used up. A quick check on samples: scratch with a steel point (a coat lifts and peels) and wipe with white spirit or acetone (a coat transfers to the cloth).

Then pin the three variables that move coverage: the size band (a tight, declared band keeps your kg/m² figure valid, while an undeclared wide mix can swing coverage either way), the colour lot (polished stone is sorted by hand, so insist every delivery matches an approved wet sample within an agreed tolerance), and the shape mix (round reads formal and packs deeper; flatter stones cover more per tonne). Approve the grade from a wet sample photographed in daylight — dry polished stone always photographs brighter than it lands.

What Should the Purchase Order Say About Quantity?

Write the quantity so it can be checked at loading, not disputed at destination. Seven lines do it: the size band with explicit oversize and undersize tolerances; the quantity stated three ways — area, depth and kilograms per square metre — so any two confirm the third; through-polished with no wax, resin, dye or coating unless separately declared; the rock type; washed with fines at or below 1%; moisture on a dried-weight basis; and bag net weight within ±2% with a piece-count reference per kilogram for display grades.

Close the order with two protections. Require a wet sealed sample taken at loading and retained by both sides, so every future top-up matches the bed you built; and order the full 8–12% allowance from the same production lot rather than topping up later, because a replacement lot from a different sorting run will read as a patch even when the specification on paper is identical.

Frequently Asked Questions

How much do polished pebbles cover per square metre?

At one stone deep, expect about 40–48 kg per square metre for 20–30 mm pebbles, 56–64 kg for 30–40 mm and 72–80 kg for 40–50 mm. One tonne therefore covers roughly 21–25 m², 16–18 m² or 12–14 m² respectively, and a 20 ft container of 24–27 tonnes covers about 380–430 m² in the 30–40 mm grade.

How many polished pebbles are in a 20 kg bag?

Roughly 500–550 pebbles at 30 mm, 210–240 at 40 mm and 110–130 at 50 mm, based on typical stone weights for polished river pebbles. Counts vary with shape and density, so treat them as order-planning figures and confirm a piece count per kilogram with the supplier for retail display grades.

What depth should polished pebbles be laid at?

One stone deep: set the depth equal to the nominal band, so 30 mm stone goes down at 30–35 mm and 40 mm stone at about 40 mm, over a membrane with a rigid edge. Anything deeper hides polished faces under polished faces and wastes 25–40% of the order; a cheaper tumbled or unpolished bedding layer below the visible layer is the normal way to save.

How do you choose between polished pebble grades?

Choose through-polished stone rather than wax- or resin-coated, since a coating wears off outdoors within 6–18 months; verify with a steel-point scratch and an acetone wipe. Then fix the size band with tolerances, the colour lot against an approved wet sample, and the shape mix — round for formal looks, flatter stones when you want more square metres per tonne.

References & standards

“ISO 9001 is a globally recognized standard for quality management. It helps organizations of all sizes and sectors to improve their performance, meet customer expectations and demonstrate their commitment to quality.”

ISO, ISO 9001:2026 “Quality management systems — Requirements” — official page

• ASTM International — published standards for testing construction and landscape aggregates (grading, density, abrasion).
• ISO — international management and product standards, including ISO 9001 quality management systems.
• BSI Group — the UK national standards body; publishes BS specifications for aggregates and paving.

Products Mentioned

• Decorative Pebbles — Decorative pebbles is the umbrella trade term for washed and graded pebbles selected for visual appeal — covering the color, size
• Garden Pebbles — Garden pebbles are smooth natural stones used for ground cover, garden borders, pathways and decorative landscaping in residential
• White Pebbles — White pebbles are bright, smooth landscaping stones used in Japanese and Zen gardens, modern garden designs, flower beds and premi

Related Applications

Related Reading

• How Do You Lay Polished Pebbles? — You lay polished pebbles by clearing and levelling the base, laying a permeable weed membrane, fixing a rigid edge, then dry-setting a 30–40 mm layer and lightly compacting. One tonne covers about 12–18 m² at a 40 mm depth, and a 20 ft container holds about 24–27 tonnes — enough for 1,400–2,200 m². Keep them in near-view beds and displays rather than driveways, where the gloss coat wears and the round stones roll.
• How Much Do Decorative Pebbles Cost per Tonne? — Decorative pebbles have no single per-tonne price — it depends on rock, colour, size, finish and freight (60–80% of landed cost). See how to convert bag and jumbo prices, coverage per tonne, and how to compare supplier quotes fairly.
• What Are the Best Polished Pebbles for a Water Feature? — The best polished pebbles for a water feature are through-polished — never waxed or resin-coated — 20–40 mm inert stone such as black basalt or grey granite, laid as a waterline band and spillway accent rather than across the whole submerged bed. One tonne covers about 12–14 m² at 50 mm, and a 20 ft container holds 24–27 tonnes.
• How Many Garden Pebbles Are in a Bag? — A 20 kg bag of garden pebbles holds about 0.013 m³ — enough to cover roughly 0.3 m² at 40 mm. Here is the bag-to-tonne-to-square-metre maths, why 20 kg, 25 kg and 50 lb bags are the same product in different wrappers, and how to turn a bag price into a price per ton you can actually compare.