Retaining wall block calculator

Blocks, cap blocks and the crushed stone under and behind a garden retaining wall.

A circular paved patio edged with a low stone wall

How it works

We add a buried course below the ground, work out how many courses of blocks reach the top, and multiply by the blocks in each course. Then we add the crushed stone for the levelling pad underneath and the drainage stone behind the wall. Nicolock's installation instructions bury at least 6 in, or 10% of the wall's height if that's more. They set the blocks on a 6 in pad of compacted stone in a trench 6 in wider than the block at the front and back, with 12 in of drainage stone behind.

Worked examples

A 20 ft wall standing 2 ft high, with blocks 12 in long, 4 in high and 7 in deep: 6 in is buried, so the wall is 30 in tall. That's 8 courses of 20 blocks, or 168 blocks with 5% spare, plus 20 caps and about 2.56 cu yd of stone.

A 6 m wall 0.6 m high with 300 × 150 mm blocks: 5 courses of 20, so 105 blocks and about 1.85 m³ of stone.

Height limits

Gravity walls built from small blocks are generally for walls up to about 3 ft (0.9 m) above ground in good soil. Taller walls, walls on slopes or holding up a driveway, and terraced walls need geogrid and an engineer's design. Check the block maker's guide and your local rules.

Common questions

How many retaining wall blocks do I need?

Divide the wall length by the block length for the blocks in one row, then multiply by the number of rows, including one buried below ground. A 20 ft wall 2 ft high in 12 × 4 in blocks needs 168 blocks with 5% spare.

How much gravel goes behind a retaining wall?

Nicolock's instructions allow for 12 in of drainage stone behind the wall for its full height, plus a 6 in levelling pad underneath. The calculator works out both.