Retaining Wall Calculator
Blocks, caps, drainage gravel and base for a retaining wall.
About this tool
Enter the wall's length and exposed height, the block's length, height and depth, and the cap length. The tool works out the number of courses (including the buried one), blocks and caps to buy, the drainage gravel and base volumes, and the length of drain pipe.
How a block retaining wall is built
Segmental retaining walls are built from interlocking concrete blocks laid in courses, each stepped back slightly into the slope. The first course is buried below ground, at least one full course or about 10% of the wall height, to stop the base kicking out. So a 0.8 m wall in 150 mm high blocks needs 6 visible courses plus 1 buried, and with 300 mm long blocks a 10 m wall takes 34 blocks per course, about 250 blocks with 5% spare, plus 36 caps. Behind the wall goes a band of about 300 mm (12 in) of clean, free-draining gravel with a perforated drain pipe at the base, so water can't build up pressure. Below sits a compacted base of crushed stone about 150 mm deep. The tool estimates all of these. Honest limits: follow the block manufacturer's installation guide. Walls taller than about 1.2 m (4 ft), walls holding back slopes, driveways or buildings, and poor or wet soils need geogrid reinforcement and an engineer's design, and many areas require a permit. Not engineering advice. The paver calculator and the gravel calculator are related.
Frequently asked questions
How many retaining wall blocks do I need?
Blocks per course (length รท block length) times the number of courses, including the buried one, plus about 5% spare.
How deep should the first course be buried?
At least one full block course, or about 10% of the wall's height, whichever is more.
Do retaining walls need drainage?
Yes. A gravel backfill and perforated drain pipe stop water pressure building up behind the wall.
When does a retaining wall need an engineer?
Typically above about 1.2 m (4 ft) or when it supports slopes, driveways or structures; check local rules.
Is my data sent anywhere?
No. Everything is calculated in your browser, and nothing is stored or sent.