Retaining wall block calculator

The count works in courses (horizontal rows of block) from the exposed height, plus the buried courses below grade — at least 6 in or 10% of the wall’s height, whichever is more — then multiplies by the blocks per course.

How the block count works

Segmental retaining wall blocks are dry-stacked in courses. The exposed height is measured from the finished ground in front to the top of the cap, so the cap comes off first: the rest divided by the block height (rounded up) is the courses above grade. Then come the buried courses. Belgard’s and Allan Block’s guides both want at least 6 in of block below grade, and Allan Block adds 1 in for every foot of wall height; the calculator uses at least 6 in or 10% of the total block height, whichever is more, and rounds up to whole courses. Each course needs the wall length divided by the block’s face length.

blocks per course = wall length ÷ block face length (round up)
courses above grade = (exposed height − cap height) ÷ block height (round up)
buried courses = max(6 in, 10% of total block height) ÷ block height (round up)
blocks = (courses above + buried) × blocks per course × (1 + extra)
caps = wall length ÷ cap length · adhesive ≈ one 29 oz tube per 60 ft

Two worked examples: a small garden terrace, How many blocks for a 20 ft retaining wall, 2 ft high, and a longer wall right at the height where permits start, How many blocks for a 40 ft retaining wall, 3 ft high.

The parts people forget

• Leveling pad: a trench filled with compacted crushed stone that the first course sits on. Belgard’s guide calls for at least 6 in of compacted base extending 6 in in front of and behind the block, and Allan Block’s worksheet uses a pad 24 in wide by 6 in deep. The calculator uses the block depth plus 12 in, 6 in deep, and applies the same compaction model as the paver calculator: crusher run packs down about 25%.
• Trench depth: the 6 in pad plus the buried courses. With 4 in tall blocks and two buried courses, that is a 14 in trench.
• Drainage aggregate: clean crushed stone (3/4 in, often sold as #57) at least 12 in wide behind the blocks — both Belgard and Allan Block specify 12 in. It stops 6–8 in below the top; Allan Block finishes with 8 in of low-permeability fill, so surface water runs off instead of pouring into the stone.
• Drain pipe: perforated pipe at the back of the pad, daylighted or tied to a drain. Allan Block’s worksheet counts about the wall length; add the run from the wall to each outlet. Allan Block requires it for gravity walls over 4 ft and for poorly drained sites; on a small wall it is cheap insurance.
• Corners: each 90° outside corner usually takes a corner block in every course. The calculator counts them separately and doesn’t take them off the wall block count; the spare blocks cover the cuts.
• Cap adhesive: Allan Block’s estimating worksheet allows one 29 oz tube per 60 ft of capped wall — about a third of that for a 10 oz caulk tube. Buy a spare.
• Geogrid: over about 3 ft of exposed wall, many blocks need geogrid layered into the backfill. Allan Block’s manual says grid typically runs at least 60% of the total wall height back and sits on about 16 in centers; the calculator gives a rough square-footage (and rolls, once you enter the roll size) from that, but the real layout comes from the maker’s chart or an engineer.
• Hollow-core blocks: units like Allan Block are filled with wall rock as you build. Allan Block’s estimating worksheet counts the cores plus 12 in behind the wall as 1.4 ft of rock over the wall face, so the cores alone take about 0.4 ft; pick Hollow-core under More options to add it.
• Filter fabric: Belgard’s typical section puts filter fabric between the drainage stone and the soil behind it; the calculator sizes it up the back of the stone and over its top. Allan Block’s spec doesn’t suggest fabric with clay soils, where it can clog, so check your maker’s guide.

Curved walls

The blocks step back a little each course, so on a curve each course has a different length. On an outside curve (bulging toward you) the courses get shorter as you go up — Allan Block’s radius chart shows a 4 ft tall wall that ends at a 4 ft radius on top needs about 5 ft 2 in at the base. On an inside curve they get longer. Count blocks per course along the longest course: the base course of an outside curve, the top course of an inside curve.