Retaining Wall Calculator
Building a retaining wall with concrete blocks or segmental retaining wall (SRW) units requires careful block counting. This calculator tells you how many blocks to buy, the wall's face area, and the number of courses (rows) needed to reach your desired height.
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Formula
Blocks = ceil(Wall Area / Coverage per Block); Rows = ceil(Height / 0.75)
Wall area equals length times height. Dividing by coverage per block gives the block count, rounded up. Rows are calculated assuming standard 6-inch-tall SRW blocks with a 3/4-inch setback per course, giving approximately 0.75 feet of height gain per row.
How to use the Retaining Wall Calculator
- 1
Enter your wall length (ft)
Value should be in ft.
- 2
Enter your wall height (ft)
Value should be in ft.
- 3
Enter your coverage per block (sq ft)
Value should be in sq ft/block.
- 4
Read your results instantly
Results update in real time as you type.
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Retaining Wall Height Limits
Segmental retaining walls up to 4 feet in exposed height generally do not require an engineer's stamp in most jurisdictions. Walls above 4 feet typically require engineering review, a building permit, and may need deadman anchors, geogrid reinforcement, or battered (angled) construction. Always check local codes before building any wall above 3 feet, as soil type and surcharge loads (driveways, buildings) affect the design.
Base Preparation
A proper base is the most important part of any retaining wall. Excavate 6–12 inches below grade and compact a 6-inch layer of crushed gravel. The first course of blocks must be partially buried (at least half the block height) and perfectly level. Poor base preparation is the leading cause of retaining wall failure. Also install drainage pipe (perforated 4-inch pipe wrapped in filter fabric) behind the wall to relieve hydrostatic pressure.
Tips & Insights
Batter the Wall Back
Standard SRW blocks are designed with a setback so each course leans slightly back into the hillside. Follow the manufacturer's recommended batter (typically 1 inch per foot of height) for stability.
Install Drainage Behind the Wall
Hydrostatic water pressure is the primary reason retaining walls fail. Always install perforated pipe wrapped in filter fabric at the base of the wall, draining to daylight at each end.
Cap the Wall
Use cap blocks on the top course. Cap blocks are glued down with construction adhesive and create a finished, professional appearance while preventing the top course from shifting.
Worked Examples
Garden Terrace Wall
75 blocks for a 2-foot tall, 25-foot long garden terracing wall.
Driveway Retention
157 blocks — order 160 to account for base course waste and any chips.
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Frequently Asked Questions
How many blocks do I need per square foot of retaining wall?
Standard 6-inch-tall SRW blocks (like Versa-Lok or Allan Block) typically cover about 0.67 sq ft per block, meaning you need about 1.5 blocks per square foot of wall face.
How deep should a retaining wall foundation be?
Bury the first course at least one-half block height (about 3 inches for standard blocks). For walls over 3 feet, bury one-tenth of the total wall height as a minimum, plus set it in compacted gravel.
Do retaining walls need drainage?
Yes, always. Without drainage, hydrostatic water pressure behind the wall builds up after rain and is the number one cause of retaining wall blowouts. Install perforated pipe and drainage gravel behind all retaining walls.
What is geogrid and when do I need it?
Geogrid is a mesh reinforcement laid horizontally between block courses and extending back into the hillside to anchor the wall. It's required for walls over 3–4 feet tall and is always required by block manufacturers for taller walls.
Can I build a retaining wall without a permit?
Many jurisdictions allow walls under 3–4 feet without a permit. Above that threshold, engineering and permits are typically required. Check with your local building department — rules vary significantly by municipality.
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