How this instrument works
A segmental retaining wall is built up in horizontal courses of interlocking concrete blocks, so its total block count comes down to how many blocks fit across each course's length, times how many courses stack up to the wall's total height. This instrument deliberately asks for your block's actual face dimensions rather than assuming a standard size, since real segmental retaining wall units vary far more in size than, say, a standard CMU block — using your product's real dimensions is what makes the count accurate.
Blocks per course comes from ceil(wall length in inches ÷ block face length), and number of courses from ceil(wall height in inches ÷ block face height); multiplying the two gives the total block count. Alongside that, the instrument estimates base gravel volume from a compacted crushed-stone footing beneath the wall — a typical 12 inch wide, 6 inch deep base is the built-in default, but real installation specs vary by manufacturer and wall height, so check your block system's own installation guide and adjust those two fields to match.
This is a materials-quantity calculator only — block and base-gravel counts from user-entered dimensions — not a soil-mechanics or earth-pressure structural design tool. It doesn't check overturning, sliding, drainage adequacy, or reinforcement (geogrid) requirements, all of which a real design has to account for. Many jurisdictions require a permit and, above roughly 3 to 4 feet in height, an engineer's stamped design — a widely repeated general rule of thumb rather than one universal code figure, since actual thresholds vary by location. This is a preliminary, educational estimate only; always confirm permit and engineering requirements with your local building department before building anything beyond a low garden wall.
- Enter Wall length (ft) and Wall height (ft) — the wall's overall footprint length and its finished height above grade.
- Enter Block face length (in) and Block face height (in) — your specific block's actual exposed face dimensions, from its product spec sheet.
- Enter Base gravel width (in) and Base gravel depth (in) — check your block system's installation guide; the built-in defaults are typical, not universal.
- Read Total blocks and Base gravel needed (cubic yards) beneath the inputs.
- For any wall approaching roughly 3 to 4 feet or supporting a slope, driveway, or structure above it, confirm permit and engineering requirements with your local building department before building.
Worked example — a 40 ft, 3 ft-tall segmental wall
Enter 40 into Wall length (ft), 3 into Wall height (ft), and 18 and 6 into Block face length (in) and Block face height (in) — an 18×6 inch segmental block face, a common size for this style of unit. Blocks per course: ceil((40×12) ÷ 18) = ceil(26.67) = 27. Number of courses: ceil((3×12) ÷ 6) = 6.
Total blocks reads 162, that is, 27 blocks per course times 6 courses stacked to reach 3 ft. With base gravel defaults of 12 in wide and 6 in deep, Base gravel needed (cubic yards) reads 0.7407 cy — a typical compacted crushed-stone footing running the wall's full 40 ft length, still worth confirming against the specific block manufacturer's own installation spec before ordering.
Questions
Do I need a permit or an engineer for a retaining wall?
Often, once the wall exceeds roughly 3 to 4 feet in height — a commonly repeated rule of thumb across DIY and contractor retaining-wall guides, not a single universal code figure, since actual permit and engineering thresholds vary by jurisdiction. Walls that support a slope, driveway, structure, or significant surcharge load above them can also trigger engineering requirements at a lower height. Always check with your local building department before starting one of meaningful height.
Why enter my block's actual dimensions instead of picking a standard size?
Because segmental retaining wall units vary widely in size between manufacturers and product lines — far more than something standardized like a typical concrete masonry unit — so a single 'standard' block size baked into the calculator would give a wrong count for most real products. Entering your specific block's face length and height, straight from its spec sheet, is what makes the block count accurate to the product you're actually buying.
What is the base gravel layer for, and why does it matter?
A compacted crushed-stone base beneath the first course provides drainage and a stable, level foundation for the wall to sit on — skipping or under-building it is one of the most common causes of a structure settling or leaning over time. The 12 inch wide, 6 inch deep default here is a typical figure, but real installation specs vary by block manufacturer and wall height, so check your specific product's install guide.
Does this calculator check whether my wall will actually hold back the soil?
No. This is a materials-quantity calculator for block and base-gravel counts only — it does not perform any soil-mechanics or earth-pressure analysis, and doesn't check overturning, sliding, or whether geogrid reinforcement is needed behind it. Those are genuine structural engineering questions that depend on soil type, wall height, and what's behind the wall, and should be evaluated by a licensed engineer for anything beyond a low garden wall.
Does 'wall height' include the buried base course?
This instrument treats Wall height (ft) as its finished, visible height above grade — the portion someone standing in front of it would actually see. It doesn't separately add courses buried below grade for embedment, a detail real designs typically include (commonly burying roughly one course per several feet of exposed height); factor that in separately if your design calls for below-grade embedment courses.