Retaining Wall Calculator
Calculate blocks, courses, gravel base, and backfill for a segmental retaining wall. Enter the wall length and height plus your block dimensions to size every component. For walls over about 4 feet, check whether your jurisdiction requires an engineered design.
Retaining Wall Diagram
How the Formula Works
Calculate block courses
Courses = ⌈Wall Height (in) ÷ Block Height (in)⌉ Always round up — a partial course still requires a full row of blocks
Calculate blocks per course
Blocks per course = ⌈Wall Length (in) ÷ Block Length (in)⌉ Apply waste factor
Total Blocks = ⌈Courses × Blocks per course × 1.10⌉ 10% waste accounts for cuts, breakage, and alignment adjustments
Calculate gravel base volume
Gravel (ft³) = Wall Length × Block Depth × Gravel Base Depth The gravel base trench runs the full length of the wall at the block depth width
Calculate backfill volume
Backfill (ft³) = Wall Length × Wall Height × Backfill Depth Gravel backfill fills the zone directly behind the wall
Frequently Asked Questions
Divide your wall height by the block height to get the number of courses, then divide wall length by block length to get blocks per course. Multiply and add 10% for waste: Total blocks = ⌈(Wall Height ÷ Block Height) × (Wall Length ÷ Block Length) × 1.10⌉. For a 20 ft × 3 ft wall using standard 12 × 6 in blocks, that is (36/6) × (240/12) × 1.10 = 6 × 20 × 1.10 = 133 blocks.
A retaining wall requires a compacted gravel base at least 6 inches (150 mm) deep. The gravel trench should be as wide as the block depth and run the full length of the wall. For a 20 ft wall with 6-in deep blocks and 6-in gravel base, you need 20 × 0.5 × 0.5 = 5 cubic feet (about 0.19 yd³) of compacted gravel. Use the calculator above for your exact dimensions.
Backfill volume depends on your wall height and how far behind the wall you are filling. The formula is: Backfill (ft³) = Wall Length × Wall Height × Backfill Depth. Use angular crushed stone (#57 stone) or gravel for the drainage zone directly behind the wall — never use clay or topsoil, which traps water and increases hydrostatic pressure.
The most common retaining wall block dimensions are 12 in long × 6 in high × 6 in deep (300 × 150 × 150 mm). Popular brands like Allan Block use 6 × 6 × 12 in dimensions. Versa-Lok standard blocks are 16 × 6 × 12 in. Always check the actual dimensions of the specific block you are purchasing before calculating.
Retaining wall blocks cost $2–$10 each depending on size and style. A basic 20 ft × 3 ft wall with 130 blocks at $5 each is $650 in block material alone. Add $50–$150 for gravel base, $50–$100 for drainage fabric, and professional installation runs $15–$40 per sq ft. Use the cost estimator in the calculator above by entering your price per block.
The gravel base must be at least 6 inches (150 mm) deep and fully compacted. For walls over 3 feet tall, increase base depth to 8–12 inches and bury the first course of blocks below grade. The rule of thumb is to bury 1 inch of base course for every foot of finished wall height — so a 4-foot wall needs a 4-inch buried first course on top of the gravel base.
Add 10% waste to your block count to account for cuts at corners and ends, breakage during handling, and blocks used to adjust courses for level. For walls with many corners or curves, increase the waste factor to 15%. This calculator automatically applies a 10% waste factor to the total block count.
No — skipping the gravel base is the most common cause of retaining wall failure. The gravel base provides a level, compacted surface for the first course, allows water to drain away from the base, and distributes wall load across the soil. Without it, the wall will shift, settle unevenly, and can collapse under hydrostatic pressure from trapped water.