How to Calculate Bulk Pebble Requirements

Multiply area in square metres by depth in metres to get volume in cubic metres, then multiply by the bulk density of the stone. Most decorative pebble sits around 1.5 to 1.8 tonnes per cubic metre depending on size and stone type.

For a 40 m² bed at 50 mm depth: 40 × 0.05 = 2 m³. At 1.6 t/m³ that is approximately 3.2 tonnes. Add a small allowance for settlement and uneven ground.

At 50 mm depth, expect roughly 75–90 kg per square metre. At 30 mm, roughly 45–55 kg. Use these only for estimating; confirm against the actual density of the grade you order.

Add around 5–10% for spillage, settlement and future top-up, particularly on sloped or irregular sites. On phased projects it is worth ordering the allowance with the first shipment so it comes from the same batch.

On sloped ground, measure the surface area along the slope rather than the flat plan area, since the actual covered surface is larger than it appears on a site plan.

Linear edging and pond surrounds are usually estimated by running length and typical depth rather than area, so keep this calculation separate from your main bed quantity to avoid under-ordering.

For a 25 m² sloped bed measured at 6 m along the slope by an average 4.2 m width, at 50 mm depth: 25.2 × 0.05 = 1.26 m³, giving around 2.0–2.3 tonnes at 1.6–1.8 t/m³. A separate 15 m run of edging at an average 150 mm depth and 100 mm width adds roughly 0.2 m³, calculated and ordered as its own line rather than folded into the bed quantity.

Check these points before finalising the tonnage on an order.

• — Has area been measured along the slope where the ground is not flat?
• — Has a waste and top-up allowance of around 5–10% been added?
• — Have edging or water feature quantities been calculated separately?
• — Has the density figure been confirmed against the actual grade ordered?
• — Is the allowance included in the same batch as the main order?