Retaining Wall Block Calculator
Dry-stacked segmental retaining-wall units (12 × 4″ face garden units or 18 × 6″ engineered units). Enter the exposed wall height; add one buried course for the base. Walls over 4 ft need an engineer and geogrid.
Order 284 blocks (4 cubes) — block ships in cubes of 90; corner and half blocks are ordered separately.
- Area
- 90 ft²
- Unit 12×4″
- 3.00 per ft²
- Net blocks
- 270
- Waste / cuts 5%
- + 14
- Blocks to buy
- 284
- Cubes90 per cube
- 4
- Mortar, 80 lb bags (type N/S)
- 23 bags
- Mason sand, 50 lb bags
- 20 bags
- Materials + delivery$2–$4 / block · Sep 2026
- $693 – $1,261
- Installed (with labor)typ. $1,986
- $1,413 – $3,061
Quantities follow the ticket. Links may earn us a commission at no cost to you; they never change a number.
What will this block (cmu) job cost?
See the installed price range for these quantities and get up to 3 quotes from local contractors. Free, no obligation.
How to calculate concrete block by hand
Blocks = Wall area (ft²) × 1.125 × (1 + waste) · Mortar bags = blocks ÷ 12.5
Worked example
- Wall area: 40 ft × 8 ft = 320 ft².
- Standard block face 8 × 16″ nominal = 0.889 ft² → 1.125 blocks per ft².
- Net: 320 × 1.125 = 360 blocks; with 5% waste, 378 → order 5 cubes (90 each) or 378 loose.
- Mortar: 378 ÷ 12.5 ≈ 31 × 80 lb bags; mason sand about 1 yd³ per 7 bags if mixing from bulk.
- Standard 8 × 8 × 16 block
- 1.125 per ft² · 37 lb
- Mortar per 100 block
- ≈ 8 × 80 lb bags
- Cube of block
- 90 pcs · ≈ 3,300 lb
- Courses per 8 ft wall
- 12
Questions crews ask
How many retaining wall blocks do I need?
A 12 × 4″ face unit covers ⅓ ft², so 3 per square foot of wall. A 30 ft wall 3 ft high is 90 ft² → 270 blocks, plus a buried course (30 more) and caps (30 at 12″). 18 × 6″ units cover 0.75 ft²: 1.33 per ft².
What base does a retaining wall need?
A trench 6″ deep plus the height of one block, with 6″ of compacted ¾″ minus, levelled, and the first course buried. Behind the wall: 12″ of clean drainage gravel with a perforated pipe at the base, and landscape fabric between gravel and soil.
How tall can a retaining wall be without an engineer?
Most codes allow up to 3–4 ft of exposed height for a gravity wall without a permit or engineering. Above that, or with a slope or driveway above it, the wall needs geogrid reinforcement and a stamped design.