Retaining Wall Calculator — blocks and courses

How it works

A segmental (stacked-block) retaining wall is built up in courses — horizontal rows of block. Count the rows from the height, the blocks in a row from the length, and multiply:

courses = ⌈height ÷ block-height⌉

blocks per course = ⌈length ÷ block-length⌉

wall blocks = courses × blocks per course

Heights and lengths are in feet and the block face is measured in inches, so each division converts to a whole number of blocks, rounded up because a partial block still means placing a whole one. Block face sizes vary by product — common segmental units are about 12 in long × 8 in high, but larger units run 18 in long — so both are inputs.

Most walls finish with a cap row of solid units along the top:

cap blocks = ⌈length ÷ cap-length⌉

Measure the exposed wall, and remember that a proper retaining wall also needs a buried base course, gravel backfill and drainage — this counts the block, not the excavation.

Worked example

A 20 ft long, 3 ft high garden wall built from 12 × 8 in blocks needs ⌈36 ÷ 8⌉ = 5 courses and ⌈240 ÷ 12⌉ = 20 blocks per course, so 5 × 20 = 100 wall blocks, plus 20 caps. A long, low 40 × 2 ft border takes 120 blocks, and a 30 × 4 ft terrace in 18 × 6 in units needs 160 blocks.

Frequently asked questions

How many blocks do I need for a retaining wall?

Divide the wall height by the block height for the number of courses, divide the length by the block length for blocks per course, and multiply: blocks = ⌈height ÷ block-height⌉ × ⌈length ÷ block-length⌉. A 20 ft × 3 ft wall of 12 × 8 in blocks needs 100 blocks.

What is a course in a retaining wall?

A course is one horizontal row of blocks. The wall's height divided by the block's face height gives the number of courses, rounded up. Each course holds the same number of blocks — the wall length divided by the block length.

Do I need cap blocks?

Most finished retaining walls use a row of cap (or coping) blocks along the top for a clean edge and to protect the wall. The cap count is the wall length divided by the cap length, rounded up. Caps are often glued down rather than pinned.

Does this include the base course and backfill?

No — it counts the visible wall block only. A sound retaining wall also needs a level, compacted base with a buried first course, free-draining gravel behind it and a drain pipe. Estimate the gravel and base separately.

What block size should I enter?

Use the face dimensions of your chosen block — its length and height as it shows on the wall — which the manufacturer lists. Standard segmental units are near 12 in long by 8 in high; larger landscape units are 18 in long. Bigger blocks mean fewer pieces.

How is this different from a concrete block calculator?

A concrete block calculator counts blocks by wall area and a blocks-per-square-foot figure, which suits a tall structural wall. This one counts by courses — rows up the height times blocks along the length — which matches how a long, low retaining wall is built and adds a cap row along the top.