Retaining Wall Calculator
Wall blocks, courses, and cap blocks for a segmental retaining wall.
Input sheet
DIY at your own risk. Calcora's calculators and guides are general estimates and information only — not professional, engineering, legal, or safety advice. Always verify local building codes and permit requirements, and hire a licensed pro for electrical, gas, plumbing, structural, or any work you're not fully comfortable doing yourself.
Segmental retaining wall blocks are priced each and stacked in courses, so the estimate is really two counts: blocks per course along the length, and courses up the height. This calculator gives both, plus the cap blocks that finish the top.
How it works
Blocks per course = wall length ÷ block length, rounded up. Courses = wall height ÷ block height. Total is the product; cap blocks match one course.
Blocks per course is the wall length divided by one block's face length, rounded up — a 20 ft wall with 12 in blocks takes 20 per course. Courses are the wall height divided by block height; a 3 ft wall of 4 in blocks stacks 9 courses. The total is the product, and caps match one course.
The count above grade is not the whole wall. The first course should be buried in a compacted gravel base — roughly one course below grade for short walls — so a wall that shows 3 ft may be built 3 ft 4 in. Add a course to the height if you are burying one, and remember most block systems step each course back slightly into the slope.
Blocks per course = wall length ÷ block length (rounded up). Courses = wall height ÷ block height (rounded up). Total blocks = per course × courses; cap blocks = one course.
Worked examples
A 20 ft garden wall, 3 ft tall, using 12 × 4 in blocks. → 180 wall blocks plus 20 caps.
240 ÷ 12 = 20 blocks per course; 36 ÷ 4 = 9 courses; 20 × 9 = 180. Caps match the 20-block course.
A 40 ft terrace edge, 2 ft tall, same block. → 240 wall blocks plus 40 caps.
480 ÷ 12 = 40 per course; 24 ÷ 4 = 6 courses; 40 × 6 = 240.
Tips & gotchas
- Spend your effort on the base: 6 in of compacted gravel, dead level, with the first course buried. Nearly every leaning block wall failed at the base, not the blocks.
- Backfill behind the wall with clean gravel and add a perforated drain pipe at the base — trapped water is what pushes walls over.
- Anything over 4 ft tall (including the buried course) typically needs engineering and a permit; many block systems also require geogrid reinforcement past 2–3 ft.
FAQ
Do I need a gravel base?
Yes — bury the first course in a compacted gravel trench about 6 in deep and one block deep. The base does more for wall life than anything above it.
Do I need to glue the cap blocks?
Yes — caps sit above the interlocking lips, so a bead of concrete adhesive is what keeps them from walking off the top course.
How much gravel goes behind the wall?
Plan on about 1 cubic foot of drainage gravel per square foot of wall face — a 12 in-wide gravel column from the base to near the top.
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