How Much Gravel for a Soakaway?
Quick Answer
1–3m³ of clean stone for a typical domestic soakaway
Based on: soakaway pit volume filled with 20–40mm clean gravel, ~0.3 void ratio for water storage
How We Calculated This
A traditional rubble-filled soakaway stores water in the voids between stones. The void ratio of clean gravel is approximately 30%, meaning only 30% of the pit volume stores water:
- Example pit: 1.5m × 1.5m × 1.5m = 3.375m³
- Gravel needed to fill pit: 3.375m³
- Weight: 3.375 × 1.7 = ~5.7 tonnes
- Effective water storage: 3.375 × 0.3 = ~1.0m³
Soakaway Crates vs Gravel
Modern soakaway crates have a void ratio of ~95%, meaning a much smaller pit is needed for the same storage volume. A 1m³ crate system stores as much water as ~3.2m³ of gravel fill. Crates are now the preferred method for Building Regulations compliance.
Building Regulations
- Soakaways must be at least 5m from any building
- Minimum depth: 1m below invert of incoming pipe
- A percolation test (BRE 365) determines if your soil is suitable
- Clay soils generally cannot support soakaways
Gravel Specification
- Size: 20–40mm clean (washed) gravel
- Do not use: Crusher run, limestone dust, or mixed aggregates (they block voids)
- Wrap the pit in geotextile membrane to prevent soil migration
Last updated: April 2026