Underordering gravel means an extra delivery cost (and awkward thin patches); overordering leaves you with tonnes of excess material. A simple volume calculation, with one adjustment for material density, gives you the exact quantity to order. Here's how.
The Core Formula
Volume (m³) = Length (m) × Width (m) × Depth (m)
Weight (tonnes) = Volume (m³) × Bulk density (t/m³)
Add 10–15% for compaction and waste
Choosing the Right Depth
| Application | Recommended Depth | Notes |
|---|---|---|
| Decorative garden bed | 40–50 mm | Ornamental use only |
| Garden path (pedestrian) | 50–75 mm | On compacted sub-base |
| Driveway (domestic cars) | 75–100 mm | On 100 mm MOT sub-base |
| Heavy vehicle driveway | 100–150 mm | Additional sub-base layers |
| Drainage layer | 150–300 mm | Pipe backfill or soak away |
Bulk Density by Material
| Material | Bulk Density (t/m³) | Notes |
|---|---|---|
| Pea gravel | 1.5 | Decorative, drains well |
| Crushed stone (20mm) | 1.7 | General purpose |
| MOT Type 1 (sub-base) | 2.1 | Compacted hardcore |
| Shingle (10mm) | 1.6 | Drainage, paths |
| Slate chippings | 1.4 | Decorative garden |
| Bark chippings | 0.3 | Not gravel, but often calculated similarly |
Worked Example: Gravel Driveway 5m × 8m
- Area: 5 × 8 = 40 m²
- Depth: 75 mm = 0.075 m
- Volume: 40 × 0.075 = 3.0 m³
- Add 10% compaction: 3.0 × 1.10 = 3.3 m³
- Weight (20mm crushed at 1.7 t/m³): 3.3 × 1.7 = 5.6 tonnes
- Order: 6 tonnes (round up to nearest half-tonne)
Also add 100 mm of MOT Type 1 sub-base: 40 × 0.10 × 1.10 = 4.4 m³ × 2.1 t/m³ = 9.2 tonnes → order 10 tonnes.
UK Gravel Costs (2025)
| Material | Cost per tonne (delivered) | 5.6 tonne driveway cost |
|---|---|---|
| MOT Type 1 | £35–55 | £330–520 |
| 20mm crushed stone | £55–75 | £308–420 |
| Pea gravel | £55–80 | £308–448 |
| Decorative slate | £70–110 | £392–616 |
Do I Need a Membrane?
For driveways and paths, always lay a weed-suppressing geotextile membrane between the sub-base and surface gravel. This prevents weeds, stabilises the gravel, and significantly extends the maintenance-free life of the surface. Do NOT use plastic sheeting — it prevents drainage and causes waterlogging. Permeable geotextile only.