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Retaining Wall Calculator

Estimate blocks, cap blocks, and gravel for retaining walls.

Last updated: June 13, 2026

Calculator

Wall Length

feet

Wall Height

feet

Block Face Size

Results

Wall Blocks Needed270

Add 10% waste: 297

Cap Blocks30

For top course

Gravel Backfill3.9 cubic yards

For drainage behind wall

Wall Area90 sq ft

How to Use This Calculator

1Enter the wall length in feet.
2Enter the wall height in feet.
3Enter the block height in inches.
4View block count, courses, base material, and drainage stone needed.

Formula Explanation

Retaining wall blocks are calculated by courses (rows) and blocks per row. The number of courses is determined by dividing wall height by block height.

Courses = (Wall Height × 12) ÷ Block Height Blocks Per Course = Wall Length × 12 ÷ Block Length Total Blocks = Courses × Blocks Per Course

Variables:

Wall HeightDesired wall height in feet

Wall LengthTotal wall length in feet

Block HeightIndividual block height in inches

Worked Examples

20 ft long, 3 ft high, 6" blocks

1. Courses = (3 × 12) ÷ 6 = 6 courses
2. Blocks per course = (20 × 12) ÷ 12 = 20
3. Total = 6 × 20 = 120 blocks

Result: 120 blocks, 6 courses, ~0.5 tons base gravel

Why Use This Calculator

Complete material list including drainage stone.
Accounts for base course buried below grade.
Calculates landscape fabric and drain pipe needs.
Prevents retaining wall failure from poor planning.

336 × 280 Large Rectangle

300 × 250 Medium Rectangle

Tips & Best Practices

Bury the first course below grade for stability.
Use 6 inches of compacted gravel base.
Install drainage stone and perforated pipe behind the wall.
Walls over 4 feet typically require engineering approval.

Common Mistakes to Avoid

❌ Skipping the drainage behind the wall.

✅ Water pressure is the #1 cause of retaining wall failure. Always install drainage gravel and perforated pipe behind the wall.

❌ Building over 4 feet without engineering.

✅ Walls over 4 feet tall need engineered design. Failure can cause property damage or injury.

Frequently Asked Questions

How deep should the base be for a retaining wall?

Use a 6-inch compacted gravel base. The first course of blocks should be partially or fully buried below grade for stability.

Do retaining walls need drainage?

Yes, always. Install 12 inches of drainage gravel behind the wall and a perforated drain pipe at the base. Without drainage, water pressure will push the wall over.

How tall can I build a retaining wall without a permit?

Most jurisdictions allow retaining walls up to 4 feet without a permit or engineering. Check your local codes — some areas require permits for any height.

336 × 280 Large Rectangle

300 × 250 Medium Rectangle

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<iframe src="https://redicalc.com/embed/construction/retaining-wall-calculator" width="100%" height="600" style="border:none;border-radius:12px;box-shadow:0 1px 3px rgba(0,0,0,0.1);" title="Retaining Wall Calculator — RediCalc.com" loading="lazy" allowtransparency="true"></iframe>

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Disclaimer: This calculator is provided for informational and educational purposes only. Results are estimates and should not be used as a substitute for professional advice. RediCalc.com makes no guarantees regarding the accuracy, completeness, or suitability of any calculations. Always consult a qualified professional before making financial, health, construction, or other important decisions.

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