How Many Retaining Wall Blocks Do I Need?
Blocks per course = Wall length ÷ Block face length → Total blocks = Blocks per course × Number of courses
Worked example: A 20 ft long, 3 ft tall wall using a standard SRW block (18” face length × 6” height): 20 ft = 240 in ÷ 18 in = 13.3 → 14 blocks per course. Height 3 ft = 36 in ÷ 6 in/course = 6 courses. Total: 14 × 6 = 84 blocks, plus roughly 5–10% waste for cuts at wall ends = ~90–93 blocks to order.
Circular and Curved Retaining Walls
Circular or curved walls (common for tree rings, raised planters, and fire pit surrounds) use the wall’s radius rather than a straight length to find the course circumference:
Blocks per course (circular) = (2 × π × Radius) ÷ Block face length
Most segmental retaining wall blocks have a slightly trapezoidal face specifically to allow this kind of curved coursing — but every block profile has a minimum radius it can curve to before gaps open up on the outer face; check your specific block manufacturer’s minimum-radius spec for tight curves.
Worked example: A circular tree well with a 4 ft radius, 2 courses tall, using an 18” face block: circumference = 2 × π × 4 = 25.1 ft = 301 in ÷ 18 in ≈ 17 blocks per course × 2 courses = 34 blocks.
Base Prep and Reinforcement
- Base trench: dig below grade and fill with 4–6 inches of compacted crushed gravel as a level, well-draining foundation — this is the single most common point of failure on DIY retaining walls when skipped or done shallow.
- Buried course: bury the first course of block partially or fully below finished grade (commonly half the block height) for stability.
- Geogrid reinforcement: for walls generally taller than 3–4 feet, or walls holding back a slope above the wall (surcharge load), horizontal geogrid fabric is embedded in the backfill at intervals to prevent the wall from tipping — this threshold varies by local code and soil type, so confirm with your block manufacturer’s engineering tables or a local engineer for anything beyond a low garden wall.
Retaining Wall Block vs. Cinder Block (CMU)
Purpose-built segmental retaining wall (SRW) block is manufactured specifically for gravity retaining walls — it’s solid or has interlocking pins/lips, has a textured face, and is designed to be dry-stacked without mortar. Standard concrete cinder block (CMU) is a general masonry unit, hollow-core, and not engineered for the same lateral soil pressure without additional reinforcement (rebar and grout-filled cores) — cinder block retaining walls are sometimes built for very short garden-edge applications but generally need more engineering consideration than purpose-built SRW block for anything beyond a low edge.
Frequently Asked Questions
How many retaining wall blocks do I need?
Divide your wall’s length by one block’s face length to get blocks per course, then multiply by the number of courses (wall height ÷ block height). Add 5–10% extra for cuts at wall ends and corners.
How do I calculate blocks for a circular or curved retaining wall?
Use the wall’s radius to find the circumference (2 × π × radius), then divide by your block’s face length the same way as a straight wall. Most segmental retaining wall blocks have a slightly tapered face designed for curved coursing, but check your block’s minimum radius for tight curves.
How many retaining wall blocks do I need for a fire pit or tree ring?
Use the Circular/Curved Wall mode above with your fire pit or tree ring’s radius — the math is identical to a curved retaining wall, just typically at a smaller radius and shorter height (often just 1–2 courses).
Do I need a permit for a retaining wall?
Many jurisdictions require a permit and engineered design for walls taller than about 3–4 feet, or any wall supporting a slope, driveway, or structure above it — always check your local building code before building a wall beyond a low garden-edge height.
What’s the difference between retaining wall block and cinder block (CMU)?
Purpose-built segmental retaining wall block is designed specifically for gravity retaining applications, with interlocking pins or lips and a textured face for dry-stacking. Standard cinder block (CMU) is a general masonry unit not engineered for the same lateral soil pressure without added reinforcement.
Do I need geogrid for my retaining wall?
Geogrid reinforcement is typically recommended for walls taller than roughly 3–4 feet, or any wall holding back a slope above it. Check your specific block manufacturer’s engineering tables, since the exact height threshold varies by soil type and local code.
How much base gravel do I need for a retaining wall?
Plan for a 4–6 inch compacted crushed gravel base trench along the full length and width of the wall’s footprint — this is the most common point of failure on DIY walls when skipped or built too shallow.
Can I use this instead of Allan Block, Home Depot, or Lowe’s calculator?
Yes, as an independent, brand-agnostic estimate to sanity-check any manufacturer’s or retailer’s tool. Block dimensions vary slightly between manufacturers, so always confirm your final order against the exact block you’re buying.
How many cap blocks do I need for a retaining wall?
Divide your wall’s total linear length by your cap block’s coverage length (caps are sold by linear foot of coverage, often slightly different from the wall block’s own face length) and round up.
How tall can a retaining wall be without an engineer?
This varies significantly by jurisdiction, but many codes set the threshold around 3–4 feet, above which an engineered design and permit are commonly required. Always confirm with your local building department rather than relying on a general rule of thumb.
Get free quotes from local landscaping & masonry contractors
Taller walls and surcharge loads need more than a block count. Get a contractor or engineer involved before you excavate.