How to measure for a retaining wall
Measure the length of the wall in feet along the face you are building, then decide on the exposed height in inches — the part of the wall that shows above finished grade. Note the dimensions of the block you plan to use: most segmental retaining wall blocks are about 12 inches long, 4 to 6 inches high, and 8 to 12 inches deep. Enter those numbers above and the calculator returns the courses, blocks per course, total wall blocks, cap blocks, base gravel, and an estimated cost.
Typical wall heights
- Garden borders & raised beds: 8–18 inches
- Terraced planting beds: 18–30 inches
- Freestanding seat walls: about 18 inches
- Taller walls: anything over about 3–4 feet usually needs engineering, geogrid reinforcement, and a permit — check local code first.
How the courses and blocks math works
A block wall is built up in horizontal rows called courses, each made of blocks laid end to end:
- Courses = wall height ÷ block height, rounded up to a full row
- Blocks per course = (wall length × 12) ÷ block length, rounded up
- Total wall blocks = courses × blocks per course, plus a waste factor
- Cap blocks = one finished row across the top (equal to the blocks per course)
Because partial blocks and partial courses always round up, a slightly longer or taller wall can push you into another full row — confirm your block size before ordering.
Base preparation
A retaining wall is only as good as the pad under it. Excavate a level trench and add about 6 inches of compacted crushed gravel roughly twice the block depth wide, which is what the base-gravel figure above estimates. Compact it in lifts and check it dead level before setting the first course — every course above copies the first. Bury the bottom course about one inch of wall height for every foot of exposed height so the base stays locked in.
Common mistakes to avoid
- Skipping the buried course. The first course should sit below grade; counting only the exposed height under-orders blocks. Add the buried height to the wall height before calculating.
- Forgetting drainage. Backfill behind the wall with free-draining gravel and a perforated pipe so water pressure does not push the wall out.
- Ignoring the waste factor. Corners, curves, and cut end pieces create scrap — add 5–10%. The calculator builds this in.
- Setting caps without adhesive. The top course is not pinned, so bond it with landscape block adhesive.
Estimating cost
Block prices vary widely by style, texture, and region, and cap blocks usually cost a little more than standard wall units. The figure shown is an estimate of material cost only — wall blocks, caps, and base gravel — and does not include delivery, geogrid, drainage pipe, backfill, labor, or equipment rental. Use the regional adjustment and edit the unit price to match a local quote before you buy.