Retaining Wall Block Calculator
Retaining walls hold back soil to create level terracing or define landscape beds. This calculator uses standard 12×4-inch and larger retaining wall block formats (Allan Block, Versa-Lok, and generic), accounting for buried base courses (below-grade rows for stability). Enter wall length, exposed height, and block type to get total block count, total course count, gravel base, drainage stone, and material cost. Gravity retaining walls up to 4 feet tall can be built as DIY projects with standard blocks; walls over 4 feet require a structural engineer and in most jurisdictions, a building permit. Drainage behind the wall is not optional — hydrostatic water pressure is the primary cause of retaining wall failure. Always install a 4-inch perforated drain pipe along the base course with clean crushed stone backfill.
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How to Use This Calculator
- 1Enter your measurements in the fields above. Use the units shown next to each field (ft, in, etc.).
- 2Click Calculate to get your material estimate instantly.
- 3Review the highlighted result — it shows the key quantity you need to order.
- 4Use Copy Results to save your estimate, or bookmark the URL — your inputs are encoded in the link.
Pro tip: Always add 10% to your material order to account for waste, cuts, and defects.
Frequently Asked Questions
How many retaining wall blocks do I need for a 20-ft wall, 3 ft tall?
Using standard 12×4-inch blocks: 3 ft exposed + 1 buried course = 10 courses high. Each course needs 20 blocks for 20 linear feet. That's 200 blocks plus 5% waste = 210 blocks.
Does a retaining wall need drainage?
Yes — hydrostatic pressure from water-saturated soil is the primary cause of retaining wall failure. Always install a 4" perforated drain pipe behind the base course and backfill with clean crushed stone, not soil. Weep holes every 4–6 feet also help.
Do I need a permit for a retaining wall?
Most jurisdictions require a permit for retaining walls over 4 feet tall (measured from the bottom of the footing). Some areas require permits for walls over 2 feet. Even if a permit is not required, walls over 3 feet tall should be designed to resist the soil pressure load behind them — consult a structural engineer for anything over 4 feet.
What is the batter (setback) on retaining wall blocks?
Most interlocking concrete retaining wall blocks have a built-in batter — a slight backward lean into the hillside per course, typically 1/2" to 1" per course depending on the product. This lean (batter) uses gravity to help the wall resist the soil load. Follow the manufacturer's installation guidelines for the specific block system you're using.