Retaining Wall Block Calculator
✓ Your estimate
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Wall Sq Ft
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Block Courses
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Blocks Needed
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Cap Blocks
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Base Gravel (yd)
⚠ This is a material estimate only. Retaining walls over 4 ft may require engineering and permits. Always verify local code requirements before building.
⚠ Important: Structural Disclaimer
This calculator estimates material quantities only. Retaining walls taller than 2 feet, walls on slopes, walls near structures or property lines, and walls holding back saturated soil can be structural elements. Walls over 4 feet typically require engineering review and a building permit. This tool is not a substitute for professional engineering or structural design advice.
What the Result Means
The block count covers the main wall face. Cap blocks are the finished top course -- include them if your block system uses a separate cap piece. Base gravel is the compacted foundation beneath the first course. For drainage gravel behind the wall, use the gravel calculator and estimate 6-12 inches of crushed stone along the full wall height and length. For topsoil to backfill and finish behind the wall, see the topsoil calculator. All outdoor tools are at the outdoor calculators page.
How the Calculation Works
Wall Face Area = Wall Length x Wall Height (sq ft)
Block Face Area = Block Length x Block Height (sq ft)
Block Count = Wall Face Area / Block Face Area x (1 + Waste %)
Courses = Wall Height (in) / Block Height (in)
Cap Blocks = Wall Length (in) / Block Length (in)
Base Gravel (cu yd) = Wall Length x Base Width (ft) x Base Depth (ft) / 27
Worked Example
Example: 20 ft wall x 2 ft high, 16 in x 6 in blocks, 10% waste
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Wall face: 20 x 2 = 40 sq ft
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Block face: (16/12) x (6/12) = 1.333 x 0.5 = 0.667 sq ft per block
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Base blocks: 40 / 0.667 = 60 blocks
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With 10% waste: 60 x 1.10 = 66 blocks
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Courses: (2 ft x 12) / 6 in = 4 courses
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Cap blocks: (20 ft x 12) / 16 in = 15 cap blocks
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Base gravel (6 in deep, 12 in wide): 20 x 1 x 0.5 / 27 = 0.37 cubic yards
Ordering Notes
Assumptions Used by This Calculator
- Wall face is treated as a simple rectangle (length x height).
- Block dimensions are based on visible face size (length x height), not total block depth.
- Waste accounts for cuts, breakage, and layout irregularities.
- Base gravel volume is estimated from entered depth and width at wall length.
- Drainage gravel behind the wall is not automatically calculated -- estimate separately using the gravel calculator.
- Taller walls, structural retaining walls, and walls near buildings or driveways require engineering and permits.
- This calculator is for material estimating only, not structural design.
Frequently Asked Questions
How many retaining wall blocks do I need?
Divide the wall face area (length x height) by the face area of one block (block length x block height). For a 20 ft wall that is 2 ft high using 16 x 6 inch blocks, you need about 60 blocks plus 10% waste -- about 66 blocks total.
How much gravel goes under a retaining wall?
A standard base for a garden-scale retaining wall is 4-6 inches of compacted gravel at least 12 inches wide. Taller walls need a wider, deeper base. Use the gravel calculator to estimate base material volume.
Do retaining walls need drainage gravel?
Yes -- drainage behind the wall is critical. Without it, hydrostatic pressure from saturated soil can topple even well-built walls. Place 6-12 inches of crushed stone directly behind the wall, with drainage fabric between the gravel and soil.
How many cap blocks do I need?
Cap blocks run along the top course of the wall. You need one cap block for every block length along the wall. For a 20 ft wall with 16-inch cap blocks, that is about 15 cap blocks. Use the cap block option in the calculator above.
How much waste should I add for retaining wall blocks?
For a straight wall, 5-10% is standard. For curved walls, corners, or complex patterns, add 10-15%. Blocks at corners and ends often require cuts.
When does a retaining wall need engineering?
Most municipalities require a permit and engineering review for retaining walls over 4 feet in height. Walls near property lines, driveways, slopes, or buildings may need review at lower heights. Always check local code before building.
Can I build a retaining wall myself?
For walls under 2 feet high on level ground away from structures, DIY is generally feasible with segmental concrete blocks. Above that height, consult a contractor or engineer. This calculator is for material estimating only and does not provide structural guidance.
Disclaimer: This calculator provides material estimates only. Retaining walls can be structural elements subject to soil pressure, drainage, and load conditions not captured by this tool. Walls over 4 feet, near buildings, or on slopes require professional engineering review. Verify all quantities with your block supplier and consult your local building department before construction. See all outdoor calculators.