Retaining Wall Calculator
Blocks, caps, and the base trench gravel — the full order for a segmental block wall.
Common questions
How many retaining wall blocks do I need?
Wall length ÷ block length = blocks per course; wall height ÷ block height = courses; multiply. A 20 ft wall, 24" tall, in standard 12×4" blocks needs 120 blocks plus 20 caps.
What goes under a retaining wall?
A compacted gravel base: trench 12" wider than the block, 6" of crusher run compacted dead level, and bury the first course at least half its height. The base course is 90% of whether the wall stands straight in five years.
How tall can I build without an engineer?
Most jurisdictions allow up to 3–4 ft of gravity wall without a permit or engineering — above that, or with a slope or driveway above the wall, you need both. Check locally before digging.
Do retaining walls need drainage?
Anything above a couple of courses, yes: clean gravel backfill behind the blocks and a perforated drain pipe at the base. Water pressure, not soil, is what topples walls.