Retaining Wall Calculator

Count the courses (wall height ÷ block height) and the blocks per course (wall length ÷ block width), then multiply and add waste. A 20 ft long, 3 ft 4 in high wall (6 × 1 m) in 18 × 8 in (450 × 200 mm) blocks is 5 courses of about 14 blocks — roughly 70, so order about 74 with 5%. The calculator does the courses, per-course count and waste for you.

A compacted gravel leveling pad under the first course, usually about 6 in (150 mm) deep in a trench roughly twice the block depth wide (about 24 in / 600 mm). Volume = length × trench width × depth: a 20 ft wall works out to around 0.7 yd³ (0.5 m³). The calculator sizes it from your base width and depth.

Bury the bottom course about one block below grade (roughly 10% of the wall height) on top of the compacted gravel pad, so the wall is anchored and can’t kick out at the bottom. On soft ground use a deeper, wider pad. Include that buried course in the wall height you enter.

Caps finish the top course and are usually glued on — one row along the wall length, which the calculator adds when “With caps” is on. Behind the wall you also need free-draining gravel backfill and a perforated drain pipe so water escapes instead of building up pressure; taller walls need geogrid reinforcement too.

As a rule of thumb, walls over about 3–4 ft (1–1.2 m), walls holding up a slope, or walls carrying a surcharge (a driveway, pool or structure above) should be designed by an engineer and often need a permit. This calculator estimates blocks and base for a straightforward garden wall — check local rules before you build higher.