Retaining Wall Calculator
Segmental block walls: courses, blocks, caps, base, drainage stone and pipe — and what it all costs before you load the truck.
What changes as a retaining wall gets taller
Soil pressure behind a wall grows with the square of its height — a 4-ft wall holds back roughly four times the push of a 2-ft wall, not twice. Drag the exposed height and watch the design change.
Rules of thumb from block manufacturers: bury about 1/8 of the wall height (at least one course), set the base on 6 inches of compacted gravel, and backfill with 12 inches of clean drainage stone around a perforated drain pipe. Most US building codes (e.g. IRC R404.4) require an engineered design for walls over 4 ft, and many localities set a lower threshold or add a surcharge rule for slopes and driveways above.
How the block count works
blocks = ⌈wall length ÷ block length⌉ × ⌈(height + buried) ÷ block height⌉ × (1 + waste)
Both counts round up — you can’t buy 0.4 of a course. The buried course is part of the wall: it keeps the base from kicking out and is always counted. Curved walls use roughly the same number of blocks along the centre line; walls with 90° corners need corner blocks, which most manufacturers sell separately.
Building it right
- Call 811 before you dig. Excavate a trench 24 in wide and deep enough for 6 in of base plus the buried course.
- Compact 6 in of crushed base in two lifts; screed ½ in of sand or fines to level the first course perfectly.
- Lay the first course level side to side and front to back — every error is multiplied as the wall rises.
- Fill block cores and a 12-in zone behind the wall with clean drainage stone; run perforated pipe at the base to daylight.
- Stagger vertical joints, sweep each course clean, and glue caps with concrete adhesive.
Questions people ask
How many retaining wall blocks do I need?
Divide the wall length by the block face length to get blocks per course, then multiply by the number of courses (total height including the buried course ÷ block height). A 40-ft wall, 3 ft tall exposed, in 12×6-inch-face blocks needs 40 blocks per course × 7 courses (6 exposed + 1 buried) = 280 blocks, plus 5% for cuts and breakage.
How many cap blocks do I need?
Divide the wall length in inches by the cap length. For 12-inch caps on a 40-ft wall that’s 40; for 16-inch caps, 30. Add a couple for curves and corners, and secure caps with masonry adhesive.
How much gravel do I need behind a retaining wall?
A base trench about 24 inches wide and 6 inches deep of compacted crushed base, plus a 12-inch-wide column of clean drainage stone (like #57) behind the blocks for the full wall height. For a 40-ft × 3.5-ft wall that’s about 2.2 tons of base and roughly 7 tons of drainage stone including the block cores.
How much does a retaining wall cost?
DIY segmental block walls run roughly $10–$20 per face square foot in materials. Professionally installed block walls commonly cost around $30–$60 per face sq ft, and engineered walls, natural stone or poured concrete can exceed $60–$100+. Regional prices vary widely.
How tall can I build a retaining wall without an engineer?
Many US jurisdictions follow the International Residential Code, which requires engineering for walls retaining more than 4 ft of unbalanced soil (measured from the bottom of the footing), or less if there is a surcharge like a slope, driveway or building above. Check with your local building department.