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.
Blocks, gravel and cost by wall size
| Wall (exposed) | 12×6″ blocks | Caps | Backfill stone | Installed |
|---|---|---|---|---|
| 50 ft × 2 ft | 263 | 51 | 4.6 yd³ | $3,000–$6,000 |
| 50 ft × 3 ft | 368 | 51 | 6.5 yd³ | $4,500–$9,000 |
| 50 ft × 4 ft | 473 | 51 | 8.3 yd³ | $6,000–$12,000 |
| 100 ft × 3 ft | 735 | 101 | 13.0 yd³ | $9,000–$18,000 |
Installed range uses a typical $30–$60 per face sq ft planning figure for segmental block walls; walls 4 ft and up usually need engineering on top.
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 much gravel goes behind a retaining wall?
Plan a 12-inch-wide column of clean ¾-inch drainage stone for the full wall height, plus the block cores. Per 10 ft of a 3-ft wall (with a buried course) that is about 1.2 cubic yards, roughly 1.6 tons; this calculator shows the backfill in yards and tons together with the base trench gravel.
How deep should a 4 foot retaining wall be?
Bury about 1/8 of the wall height — 6 inches for a 4-ft wall, so at least one full course below grade — on top of a 6-inch compacted gravel leveling pad. That makes the trench roughly 12 inches deep. A wall retaining 4 ft or more usually needs an engineered design and geogrid under the IRC.
What is the 1/3 rule for retaining walls?
It is a rule of thumb for timber and post-supported walls: about one-third of each post’s length should be buried (a 6-ft post holds back about 4 ft). Segmental block walls follow the block maker’s guidance instead, typically burying 1/8 of the height (minimum one course).
How much does it cost to build a 100 ft retaining wall?
It depends mostly on height. A 100-ft wall 3 ft tall has 300 sq ft of face; at a typical installed $30–$60 per face sq ft that is about $9,000–$18,000, or roughly $3,000–$6,000 in DIY block, cap and gravel. Taller walls cost more per foot because they need geogrid and engineering.
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.