How is the quantity calculated?

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.

How many bulk bags of MOT Type 1 per square metre?

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.

Why MOT Type 1 rather than scalpings or hardcore?

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.

How deep should a paving sub-base be?

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.

Where does the weed membrane go?

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.

What is the difference between sharp sand and building sand?

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.

GardenCalcs

Sand Sub-Base Calculator

Work out sharp sand and sub-base aggregate for paving foundations.

Area

Total area to be excavated and prepared for paving

Project TypeSets a recommended sub-base depth: patio/path 100mm, driveway 150mm, shed base 125mm

Sub-Base Depth

Compacted MOT Type 1 — 10cm patios/paths, 15-20cm driveways. Excavate ~20% deeper to allow for compaction.

Sand Depth

Sharp sand blinding layer — typically 3-5cm screeded level

How We Calculate This

This calculator estimates MOT Type 1 sub-base aggregate and sharp sand for paving foundations.

Sub-base

Tonnes = Area × Depth × Density. Compacted MOT Type 1 density is ~1,950 kg/m³ (loose/delivered material is nearer 1,500, so allow for compaction). Lay and compact in ~50mm layers.

Sharp sand

The sand blinding layer sits on top of the sub-base. Typically 30-50mm deep, screeded level as the final bed for slabs. Sharp sand density is ~1,600 kg/m³ air-dry (heavier when damp).

Membrane

On soft or clay ground, lay a permeable geotextile membrane across the whole excavated area beneath the sub-base to stop the aggregate sinking into the subsoil. A weed membrane can also sit just under the paving to suppress weeds through the joints. Allow 10% extra for overlaps (overlap sheets by at least 300mm).

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.