How many retaining wall blocks do I need?
Work out how many courses the wall has and how many blocks fit in each course, then multiply. Include the buried first course. Most segmental block manufacturers call for at least one course below grade.
Blocks = (wall height ÷ block height + buried courses) × (wall length ÷ block length)
A 30 ft wall standing 3 ft high with 6 × 16 in blocks: 36 ÷ 6 = 6 courses, plus 1 buried, is 7 courses. Each course takes 360 ÷ 16 = 22.5, so 23 blocks. 7 × 23 = 161 blocks, or 170 with 5% for breakage. Caps 16 in long: 24.
Round each course up, not the whole wall. Measure each straight run separately and give a run its own height if it steps down.
Retaining wall block sizes
| Face (height × length) | Blocks per sq ft of face | Typical use |
|---|---|---|
| 4 × 12 in | 3 | Garden edging and walls up to about 2 ft |
| 6 × 16 in | 1.5 | Most homeowner walls |
| 6 × 18 in | 1.33 | Walls with a deeper block |
| 8 × 18 in | 1 | Larger walls |
Sizes differ by brand. Enter your block's face height and length above and the count follows. Caps usually match the block length.
Base gravel, drainage stone and pipe
A segmental wall typically sits on a compacted gravel base about 6 inches deep in a trench about 24 inches wide. Behind the wall, about 12 inches of clean crushed stone runs the full height so water can drain. A 4 inch perforated pipe at the bottom of that stone carries water out the end. Filter fabric between the stone and the soil keeps the stone from clogging.
For the 30 ft example that's about 1.9 tons of base gravel, 6 tons of drainage stone and 40 ft of pipe, with 10 ft added for an outlet. Your block's install guide has the exact base and drainage figures; use them if they differ.
Retaining wall cost
Materials for the 30 ft × 3 ft example come to about $1,486 at the default prices, or about $14 per sq ft of face including the buried course. Add your own block prices for a real number.
Labor is usually the larger part. At a build rate of 4 sq ft of face per worker hour, including digging, base and backfill, the wall takes about 26 hours. With $65 an hour and 25% markup on materials, the calculator puts a hired-out price for this 3 foot retaining wall at about $3,025–$4,095, or roughly $34–$46 per sq ft of visible wall. Tight access, a slope above the wall, or poor soil all push it higher.
Walls over 4 ft need an engineer
This calculator counts materials. It doesn't design a wall. Walls over about 4 ft, and often shorter walls with a slope, driveway, building or other load above them, typically need an engineered design with geogrid and a building permit. The International Residential Code exempts walls up to 4 ft, measured from the bottom of the footing, that don't support a surcharge, but local rules differ. Check with your building department before you buy.
Pricing a retaining wall job
Hardscapers usually price walls per square foot of face, built up from block, stone, pipe and labor. Switch to contractor mode to set your markup and build rate, then send the ticket on a landscaping estimate. For mortared CMU walls, use the concrete block calculator.
Questions
How many retaining wall blocks do I need per square foot?
It depends on the face size. Blocks with a 6 × 16 in face take 1.5 per sq ft, 4 × 12 in blocks take 3, and 8 × 18 in blocks take 1.
How deep should the first course of a retaining wall be buried?
Typically at least one full course. Many manufacturers ask for more on taller walls, so follow your block's install guide.
How much gravel do I need behind a retaining wall?
Typically a 12 inch wide zone of clean crushed stone for the full height. For a 30 ft wall 3.5 ft tall including the buried course, that's about 6 tons with 10% extra.
Do I need a permit for a retaining wall?
Often not for walls under 4 ft with nothing above them, but walls over about 4 ft, or shorter walls holding back a slope, driveway or building, typically need an engineered design and a permit. Check your local code.