Calculate blocks, backfill gravel, and drainage materials for a retaining wall.
This retaining wall calculator works out how many blocks you need for a wall of a given length and height, plus the backfill, drainage gravel and geotextile fabric behind it. It uses metric or imperial and standard segmental (interlocking) retaining-wall block sizes.
| Block face | Blocks per m² |
|---|---|
| 200 × 100 mm | ~50 |
| 300 × 150 mm | ~22 |
| 450 × 150 mm | ~15 |
A wall 10 m long × 0.9 m high = 9 m² of face. With 300×150 mm blocks (~22 per m²) that's about 198 blocks; add 5–10% waste → ~210 blocks.
Most jurisdictions allow gravity retaining walls up to 4 feet (1.2 m) without engineered design. Always check local building codes.
Geotextile fabric separates the backfill gravel from the surrounding soil, preventing fine particles from clogging the drainage system.
Most segmental retaining walls sit on a compacted gravel base (6–12 inches / 150–300 mm deep). A concrete footing is typically needed for walls over 4–6 feet.
Yes — backfill with free-draining gravel and add a perforated drainage pipe so water does not build up behind the wall and push it over.
Backfill in layers with free-draining gravel directly behind the wall, compacting as you go, and save soil and turf for the very top. Avoid clay, which holds water and swells.
