Retaining Wall Calculator

Calculate blocks, cap blocks, base gravel, and backfill gravel for your retaining wall. Includes warnings for walls that may require engineering approval and permits.

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Worked Example

A homeowner is building a 25 ft long, 3 ft tall retaining wall using standard 12″ × 4″ × 8″ blocks with cap blocks.

  1. 1 Wall dimensions: 25 ft long × 3 ft tall
  2. 2 Block size: Standard 12″ × 4″ × 8″
  3. 3 Courses: 36″ / 4″ = 9 courses
  4. 4 Blocks per course: 25 ft / 1 ft = 25 blocks
  5. 5 Base blocks: 9 × 25 = 225 blocks
  6. 6 Add 10% waste: 225 × 1.10 = 248 blocks
  7. 7 Cap blocks: 25 caps
  8. 8 Base gravel: ~1.1 cu yd
  9. 9 Backfill gravel: ~1.9 cu yd
  10. 10 Total gravel: ~3.0 cu yd
Order 248 wall blocks + 25 cap blocks + 3.0 cubic yards of gravel (1.1 for base, 1.9 for backfill).

How Retaining Wall Materials Are Calculated

Courses: Wall height divided by block height gives the number of horizontal rows (courses). Blocks per course: Wall length divided by block width. Total blocks = courses × blocks per course, plus your waste percentage.

Cap blocks: One row of caps across the top — wall length divided by block width.

Base gravel: The trench beneath the wall is typically the block depth plus 12 inches wide, filled to your base gravel depth. Volume = length × trench width × base depth.

Backfill gravel: The drainage layer behind the wall. Volume = wall length × backfill width × wall height. This gravel allows water to drain and reduces hydrostatic pressure against the wall.

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Important Building Notes

Walls over 4 feet typically require engineering approval and permits. Check with your local building department before starting construction. Unpermitted walls may need to be removed.

The first course should be buried below grade for stability. Set the first row of blocks 1–2 inches below the finished ground level on a compacted gravel base. This prevents the wall from sliding forward.

Always include drainage gravel behind the wall. Backfill with 3/4" crushed stone wrapped in filter fabric. Without proper drainage, water pressure builds up behind the wall and can cause failure, especially after heavy rain or during freeze-thaw cycles.

Frequently Asked Questions

When does a retaining wall need engineering? +
Most jurisdictions require engineered plans and permits for retaining walls over 4 feet tall (measured from the bottom of the footing to the top of the wall). Some areas set the threshold at 3 feet. Always check local building codes before starting — an unpermitted wall can result in fines or required removal.
Why should I bury the first course? +
Burying the first course of blocks below grade anchors the wall and prevents it from sliding forward under soil pressure. Typically, 1-2 inches of the first block should be below the finished ground level. This also hides the base gravel trench for a cleaner appearance.
What gravel goes behind a retaining wall? +
Use 3/4" crushed stone (angular, not rounded) for both the base and the backfill behind the wall. The angular edges lock together for stability and allow water to drain freely. Wrap the backfill gravel in filter fabric to prevent soil from clogging the drainage layer over time.
Do I need cap blocks? +
Cap blocks are strongly recommended. They provide a finished look, protect the top course from water infiltration, and are typically glued in place with construction adhesive to prevent shifting. Most retaining wall block manufacturers sell matching cap blocks.
How do I calculate geogrid? +
Geogrid is typically required for walls over 3-4 feet tall. It is placed between courses and extends back into the retained soil to increase stability. The spacing and length depend on wall height and soil conditions — consult an engineer for specific geogrid requirements on taller walls.

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Disclaimer: This calculator provides estimates for planning purposes only. Actual material requirements depend on site conditions, compaction, grading, and local building codes. Always verify measurements on-site and consult with your material supplier before purchasing.
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