Tonnage Calculator

Background.

Quarries and builders' merchants sell gravel, sand, hardcore and crushed stone by the tonne, but a driveway, a shed base or a trench is measured in metres. Getting from one to the other takes two numbers the site itself cannot give you: how heavy a cubic metre of that particular material is, and how much extra to order so the finished layer does not come up short.

The volume is plain geometry. Length times width times the finished, compacted depth gives the space the material has to fill; a 6 m by 3 m pad at 100 mm deep holds 1.8 m³ once it is rolled flat.

The weight is where estimates go wrong. Bulk density is the mass of a cubic metre of material as it arrives, air gaps included, and it depends on the rock, on how the particles are graded and on how wet the load is. A single-size stone leaves large voids and weighs less per cubic metre than a well-graded mix whose fines fill the gaps between the bigger pieces. That is why density is a field you set here rather than something looked up from a material name. The 1,600 kg/m³ default is a placeholder; replace it with the figure on your supplier's data sheet.

Then there is the shortfall. Loose stone settles and tightens when it is spread and rolled, some is lost into soft ground or left on the drive where the lorry tipped it, and sub-bases are rarely dead level. The allowance enlarges the volume before it is turned into mass, so 10% means ordering a tenth more than the bare geometry.

What comes out is a quantity for an order form, not a weighbridge ticket. Check the yard's density, its minimum load and the unit it sells in before you commit.

What is tonnage calculator?

Order tonnage is the mass of loose bulk material needed to fill a rectangular layer to a given compacted depth once an allowance for waste and compaction has been added. It is worked out in three stages, all shown: the compacted geometric volume (length × width × depth), the order volume (that volume enlarged by the allowance) and the mass in metric tonnes (order volume × bulk density ÷ 1,000). Bulk density is the mass per cubic metre of the material including the voids between particles, the property ASTM C29 measures for aggregates.

How to use this calculator.

1. 01Measure the area in metres and enter its length and width. For an L-shaped or irregular area, split it into rectangles, run each one separately and add the tonnes together.
2. 02Enter the finished depth after compaction, also in metres: 100 mm is 0.1 and 75 mm is 0.075. Typing 100 into this field would make the order a thousand times too big.
3. 03Enter the bulk density in kg/m³ for the exact product from the supplier's data sheet. If they quote tonnes per cubic metre, multiply by 1,000, so 1.6 t/m³ becomes 1,600.
4. 04Set the allowance to cover spillage and uneven ground, plus compaction if your density figure is for loose material. Set it to 0 only when you want the bare geometry as a check.
5. 05Put the tonnes on the order. If the yard sells by the cubic metre instead, use the order volume.

The formula.

Three multiplications, with no material table or hidden constant behind them. Compacted volume V = L × W × D, in cubic metres. Order volume = V × (1 + a/100), where a is the allowance percentage. Mass = order volume × ρ ÷ 1,000, where ρ is the bulk density in kg/m³ and dividing by 1,000 turns kilograms into metric tonnes. Because every step is a straight multiplication, a 10% error in any single input becomes a 10% error in the tonnage. In practice depth is the measurement most often misjudged on site: laying 120 mm where you planned 100 mm adds a fifth to the material. Density is the input with the widest spread between products, so it is the one most worth getting from the supplier rather than guessing.

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.