How Much Drainage Gravel Do I Need?

How We Calculated This

Drainage gravel fills the trench around the perforated pipe. The volume is the trench cross-section multiplied by the run length:

• Trench length: 10m
• Trench width: 300mm (0.3m)
• Gravel depth: 300mm (0.3m) — 100mm below pipe, pipe diameter, 100mm above
• Volume: 10 × 0.3 × 0.3 = 0.9m³
• Less pipe volume: ~0.095m³ (negligible, but accounted for)
• Net gravel volume: ~0.8m³
• Weight: 0.8 × 1.7 = ~1.5 tonnes (pea gravel at 1.7 t/m³)

Recommended Gravel Types for Drainage

• 20mm pea gravel: most common, smooth and round, excellent drainage
• 20mm clean gravel: angular, good for filter drains
• 10mm pea shingle: finer, used for smaller diameter pipes
• 40mm clean stone: for larger soakaways and French drains

Do not use: limestone (can clog over time), crusher run/Type 1 (too many fines), or recycled aggregates with mixed content.

Standard Drainage Trench Build-Up

1. Geotextile membrane: line the trench to prevent silt ingress
2. Gravel bed: 100mm of gravel at the base
3. Perforated pipe: 110mm or 160mm, holes facing down
4. Gravel surround: fill to 100mm above the pipe
5. Geotextile wrap: fold the membrane over the top
6. Topsoil backfill: to ground level

Minimum Fall for Drainage

Building Regulations require a minimum fall of 1:80 for 110mm pipes (12.5mm per metre). For land drains and French drains, a minimum of 1:100 is acceptable. Always check the fall with a spirit level or laser level before backfilling.

Typical UK Costs

• 20mm pea gravel (bulk bag, ~850kg): £50–£80
• 20mm pea gravel (loose, per tonne): £30–£50 + delivery
• 110mm perforated pipe (25m coil): £25–£45
• Geotextile membrane (per m²): £0.50–£1.50