Sub-base and aggregates
Type 1 sub-base tonnes
- where
- compacted volume (m³)
- area (m²)
- compacted thickness (mm)
- Type 1 to order (t)
- compacted density (t/m³)
- compaction and level allowance (%)
Explain this equation
This works out how many tonnes of Type 1 sub-base to order for a given area and finished, compacted thickness.
A is the area and t the compacted thickness in millimetres, so A × t / 1000 is the compacted volume Vc in m³. ρ is the compacted density and c is an allowance for compaction, uneven formation and tolerance.
Tonnes equal volume times density. The allowance (1 + c/100) covers the extra you always need because the formation is never perfectly level and loose stone compacts down.
Sense check: 10 m² at 100 mm is 1 m³, about 2.1 t, and 2.3 t with a 10% allowance. A 50 m² drive at 150 mm needs around 16–17 t.
Watch out: t is the finished compacted thickness, not the loose depth you spread before rolling.
Tonnes of Type 1 (MOT) sub-base for a drive, path, patio or slab base, from area and finished compacted thickness.
Enter your values to see the result.
Good to know
Type 1 is the granular unbound sub-base of Specification for Highway Works Series 800, clause 803. Quarries quote 2.0–2.2 t/m³ in place, 2.1 t/m³ as a working figure, and suggest adding 5–10% for compaction, uneven formation and measuring tolerance. Lay and compact in layers of about 75–100 mm with a plate or roller at about optimum moisture; thicker single layers stay loose at the bottom. Typical thicknesses: about 100 mm for paths and patios, 150 mm or more for drives, more on soft ground.
Common mistake
Using loose (delivered) volume for a compacted depth. Order by weight at compacted density, then add the allowance.
Limits
Density varies by rock type (limestone, granite, recycled). Thickness design for roads and heavy loads needs a pavement design.
Sources
- Specification for Highway Works, Series 800 Road pavements: unbound, cement and other hydraulically bound mixtures (clause 803 Type 1)
- Holcim UK, Type 1 sub-base technical data sheet (to SHW Series 800 clause 803; lay and compact at optimum moisture)
- Armstrongs Aggregates, MOT Type 1 sub-base: how much you need (2.0–2.2 t/m³, 2.1 working figure; add 5–10%; 75–100 mm layers)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS 8500, BS EN 206, BS EN 752, Building Regulations Part H, manufacturers’ data and your engineer’s design. Structural concrete and foundations need a competent designer.