Stone Calculator

Background.

Stone is sold by weight but laid by volume, and most ordering mistakes happen in the conversion. The quarry weighs the lorry; you measure a parking pad or a path base with a tape. Getting from one to the other needs two facts a tape cannot give you: how much a cubic metre of that particular stone weighs as it sits in the heap, and how much extra loose stone it takes to reach a finished thickness once the layer has been compacted.

The first is bulk density, and it is not the density of rock. Solid granite weighs roughly 2,700 kg per cubic metre, but a heap of crushed pieces is partly air, so a cubic metre in the truck weighs far less. How much less depends on grading — a clean single-size stone leaves bigger gaps than a graded mix whose fines fill them — and on moisture, because a wet load carries water in those gaps. The 1,600 kg/m³ default is a starting value, not a property of any named product; the yard can usually give you the figure for what it sells.

The second is the allowance. A base that must finish 100 mm thick needs more than 100 mm of loose stone spread on it, and some always ends up pressed into soft ground, spilled at the edges or left behind on the drive. One percentage covers all of it, added on top of the finished volume.

After that the arithmetic is short: length × width × finished depth, scaled up by the allowance, multiplied by bulk density. Volume and weight are both shown, so a quote priced either way can be checked. What the page cannot know is how your supplier sells — minimum loads, bag sizes, or whether their ton is a metric tonne.

What is stone calculator?

The order mass is the weight of loose stone to ask a supplier for so that, once spread and compacted, it fills a rectangular area to a chosen finished depth. It rests on three quantities: the compacted geometric volume (the space being filled), the order volume (that space enlarged by the waste and compaction allowance), and the loose bulk density — the mass of one cubic metre of stone including the air between the pieces, the property ASTM C29/C29M describes how to measure. The result is in metric tonnes of 1,000 kg.

How to use this calculator.

1. 01Tape the length and width of the bay, path or pad in metres. For an L-shaped or irregular area, split it into rectangles, run each one separately and add the tonnages.
2. 02Enter the compacted depth: the thickness the layer must have after rolling or plate compaction, not the loose depth you spread. 100 mm is 0.1 m; 75 mm is 0.075 m.
3. 03Replace the 1,600 kg/m³ default with the supplier's loose bulk density for the exact product if you can get it, and ask whether it was measured dry or as delivered.
4. 04Set the waste / compaction allowance. A soft subgrade, ragged edges or heavy compaction call for more; 0% gives the bare geometric quantity with no margin at all.
5. 05Compare the tonnes or the order volume with how the yard sells — by the tonne, by the bag or by the cubic metre — and round up to whole loads or bags.

The formula.

Order mass (t) = L × W × D × (1 + a/100) × ρ ÷ 1,000. L, W and D are length, width and finished compacted depth in metres, so their product is the compacted volume in cubic metres. The allowance a, in percent, enlarges that to the loose volume to order; 8% turns 1.8 m³ into 1.944 m³. Multiplying by ρ, the loose bulk density in kg/m³, gives kilograms, and the division by 1,000 gives metric tonnes. Every term enters as a straight multiplier, so a 10% error in depth or density becomes a 10% error in the order. Depth is the one people get wrong most easily: a base specified at 150 mm rather than 100 mm needs half as much stone again. Keep the density at its loose value. The allowance is what accounts for the stone settling tighter under compaction, so entering a higher compacted density as well would count that settlement twice.

A worked example.

Frequently asked questions.

References& sources.

1. [1]ASTM International. ASTM C29/C29M. Bulk density and voids in aggregate (PAYWALL). Retrieved 2026-08-06. access: gated.
2. [2]National Institute of Standards and Technology. NIST SI Guide, 2008 edition. Appendix B unit relationships. Retrieved 2026-08-06.
3. [3]International Code Council. 2021 International Residential Code. Building construction context. Retrieved 2026-08-06.