Sand Sub-Base Calculator

Volume = area × compacted depth, in cubic metres. That volume is multiplied by the material density (MOT Type 1 ~1,950 kg/m³, sharp sand ~1,600 kg/m³) to get weight, then a wastage allowance is added for over-excavation and compaction loss. Tonnes are converted to ~850kg bulk bags for ordering.

At 100mm compacted depth, 1 m² needs about 0.1 m³ ≈ 0.2 tonnes of MOT Type 1, so roughly 4 m² per 850kg bulk bag. A typical 12 m² patio at 100mm takes about 2.4 tonnes (3 bulk bags) of sub-base plus a sharp sand blinding.

MOT Type 1 is a graded crushed stone (63mm to dust) that locks together when compacted, leaving few voids and a stable, free-draining base. Unscreened scalpings or mixed hardcore have an inconsistent grading that compacts unevenly and can settle later, so they are not a like-for-like substitute under paving.

For a patio or garden path: 100mm of compacted MOT Type 1 (75mm is the absolute minimum for a path). For a driveway taking vehicle loads: 150-200mm. A shed or outbuilding base: 100-150mm. Always compact in ~50mm layers with a plate compactor (whacker plate) — a single deep layer will not compact properly.

On soft or clay ground, lay a permeable geotextile membrane across the whole excavated area BENEATH the sub-base. It stops the aggregate punching down into the subsoil (pumping) while letting water drain through. Overlap sheets by at least 300mm. A separate weed membrane can also sit just under the paving to suppress weeds growing up through joints.

Sharp sand has angular, coarse grains, drains freely and is used for the bedding/blinding layer under paving. Building (soft) sand is finer and smoother and is for mortar and bricklaying. Using building sand under slabs holds water and leads to frost heave and settlement — do not substitute one for the other.