A block retaining wall is a grid of courses: how many rows high, times how many blocks per row. Add the cap row and the gravel leveling pad and you have the order. Here is the method — plus the base, drainage and the point at which you need an engineer.
Counting the blocks
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%.
Retaining Wall Calculator
Enter the wall length, height and block size for the blocks, caps and base gravel to order.
The base gravel pad
A compacted gravel leveling pad goes 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).
Bury the first course
Set the bottom course about one block below grade (roughly 10% of the wall height) on the compacted pad, so the wall is anchored and can’t kick out at the bottom. On soft ground use a deeper, wider pad — and include that buried course in the wall height you enter.
Caps and drainage
Caps finish the top course and are usually glued on — one row along the wall length. Behind the wall you need free-draining gravel backfill and a perforated drain pipe so water escapes instead of building up pressure; taller walls need geogrid reinforcement too. Water is what pushes walls over, so the drainage is not optional.
When you need an engineer
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 estimates blocks and base for a straightforward garden wall — check local rules before you build higher.