Retaining Wall Materials
Height is the exposed wall above grade, not counting the cap.
Concrete
Retaining Wall Calculator
Quick answer: blocks = (wall length ÷ block length) × (courses above grade + 1 buried course). A 40 ft wall, 3 ft high, in 16 × 6 in block takes 30 blocks per course × 7 courses = 210 blocks (221 with 5% extra), plus 30 caps and about 2 tons of base gravel.
Plan a segmental block retaining wall: wall blocks, cap blocks, the compacted gravel leveling pad, and the drainage stone behind the wall. Built for small, gravity-style landscape walls.
Wall section
How to calculate retaining wall blocks
Segmental retaining wall blocks are laid in running courses, so the count is blocks per course times the number of courses. The base and drainage materials are simple rectangular volumes along the length of the wall.
⌈L ÷ block length⌉ × (⌈H ÷ block height⌉ + 1)
= wall blocks (L and H in inches); caps = ⌈L ÷ cap length⌉
- Blocks per course: wall length in inches ÷ block face length, rounded up. 40 ft = 480 in ÷ 16 in = 30 blocks.
- Courses: exposed height ÷ block height, rounded up, plus one buried course. 36 in ÷ 6 in = 6, plus 1 = 7.
- Wall blocks: blocks per course × courses, then add about 5% for cuts at ends, corners, and curves.
- Caps: wall length ÷ cap length, rounded up.
- Base gravel: length × trench width × base depth. Divide cubic feet by 27 for cubic yards, and multiply by about 1.4 for tons of crushed stone.
- Drainage stone: length × zone width × total height of the block courses.
Worked example
A 30 ft garden wall stands 30 in above grade, built from 16 × 6 in blocks with 16 in caps, on a 24 × 6 in base with a 12 in drainage zone.
- Blocks per course: 360 ÷ 16 = 22.5 → 23; courses: 30 ÷ 6 = 5 + 1 buried = 6
- Wall blocks: 23 × 6 = 138 → 145 with 5% extra; caps: 23 → 25 with 5% extra
- Base: 30 × 2 × 0.5 = 30 ft³ = 1.11 yd³ ≈ 1.56 tons
- Drainage: 30 × 1 × 3 ft (6 courses × 6 in) = 90 ft³ = 3.33 yd³
Block counts by face size
Face sizes are nominal; check your block's actual length and height on the product sheet.
| Block face | Blocks per ft² of face | Blocks per 10 ft course | Courses per ft of height | Blocks per 10 ft of 3 ft wall* |
|---|---|---|---|---|
| 12 × 4 in | 3.00 | 10.0 | 3.0 | 100 |
| 16 × 6 in | 1.50 | 7.5 | 2.0 | 52.5 |
| 18 × 6 in | 1.33 | 6.7 | 2.0 | 46.7 |
| 16 × 8 in | 1.13 | 7.5 | 1.5 | 45 |
*36 in exposed plus one buried course, before extra for cuts.
Retaining wall planning tips
- Know when it stops being a DIY wall. IRC R404.4 requires an engineered design for retaining walls that are not supported at the top and hold back more than 48 in of unbalanced fill. Slopes, driveways, or structures above the wall can trigger engineering at lower heights — check local code.
- Geogrid is part of the design, not an add-on. Each block system has a maximum gravity-wall height; above it, layers of geogrid extend back into the compacted fill. Use the manufacturer's charts or an engineer for grid length and spacing.
- The base decides whether the wall stays straight. Use crushed angular gravel (not pea gravel or rounded rock), compact it in lifts, and level the first course carefully; errors in the base course multiply up the wall.
- Give water a way out. Clean crushed stone behind the wall and a perforated drain pipe at the base, daylighted or connected to a drain, keep water pressure from building up behind the blocks. Cap the drainage zone with a few inches of soil to slow surface water.
- Count the step-ups. On sloping ground the base steps up in block-height increments, and the buried course follows it. Estimate each section at its own height for an accurate count.
- Curves and corners use more. Tight curves and 90° corners need cut or special corner units. Keep at least 5% extra, more for walls with several turns.
Retaining wall block FAQ
How many retaining wall blocks do I need?
Divide the wall length in inches by the block length for blocks per course, then multiply by the number of courses, including one buried course. A 40 ft wall 3 ft high in 16 by 6 in block needs 30 blocks per course × 7 courses = 210 blocks, or about 221 with 5 percent extra.
How deep should the gravel base be for a retaining wall?
A compacted base of crushed angular gravel about 6 in deep and 24 in wide is a common minimum for small segmental walls. That works out to 1 cu ft of base per foot of wall, or about 1.5 cubic yards and 2 tons for a 40 ft wall.
How much of a retaining wall should be buried?
Block manufacturers typically call for at least one full course below grade, often stated as 6 in minimum or about one eighth to one tenth of the wall height, whichever is more. Taller walls and walls on slopes need deeper embedment; follow the installation guide for your block.
How tall can a retaining wall be without engineering?
Under the International Residential Code (R404.4), retaining walls that are not laterally supported at the top and retain more than 48 in of unbalanced fill must be designed by an engineer. Many local codes set their own limits and count surcharges such as driveways or slopes above the wall, so check with your building department.
How much drainage gravel goes behind a retaining wall?
A zone of clean crushed stone about 12 in wide behind the blocks, running the full height of the block courses, is common. That is 1 cu ft per foot of wall for every foot of height; a 40 ft wall with 3.5 ft of block courses needs about 5.2 cubic yards.
How we built this calculator: results use the standard volume, area, and coverage formulas shown on this page, with typical U.S. material densities, bag yields, and waste factors that you can override. Written and checked by the YardsCalculator editorial team. Last reviewed: October 2026.