Segmental retaining wall (SRW) blocks stack without mortar, so a wall is simply courses of blocks: the number of blocks per course times the number of courses. The parts people forget are the course buried below grade, the cap units on top, the compacted base pad and the drainage stone behind the wall. This retaining wall block calculator includes all of them. Enter the wall length, exposed height and block dimensions and it returns the blocks, caps and the cubic yards of base and drainage aggregate.
The defaults follow common guidance in the Concrete Masonry & Hardscapes Association (CMHA) Segmental Retaining Walls Best Practices Guide and manufacturer installation guides: a compacted leveling pad at least 6 inches thick that extends about 6 inches in front of and behind the block, a first course buried at least 6 inches below grade, and a 12-inch-wide zone of free-draining stone behind the wall. For patios that meet the wall, use the paver calculator; for backfill and the soil you remove, the cubic yard calculator handles the volume.
How to use the retaining wall block calculator
- Measure the wall along its face. For curves, measure along the front edge; for corners, add the runs together.
- Enter the exposed height above finished grade. The calculator adds a buried course automatically if you choose to bury the first course.
- Enter the block's face length and height and its depth from the manufacturer's data. Common garden blocks are 12 inches long × 4 inches high; larger wall blocks are often 16–18 inches long × 6–8 inches high.
- Enter the cap length if the wall will be capped (enter 0 for no caps).
- Keep 5% waste for broken blocks and cuts at ends, corners and steps.
Formula
⌈ ⌉ means round up. The buried depth is at least 6 inches, so one course of 6–8 inch block or two courses of 4 inch block.
Worked example
A 30 ft garden wall, 3 ft exposed, using 16 × 6 inch blocks that are 12 inches deep, with the first course buried, 16 inch caps and 5% waste.
- Exposed courses: 36 ÷ 6 = 6; buried: 6 ÷ 6 = 1; total 7 courses (3.5 ft of block).
- Blocks per course: 360 ÷ 16 = 22.5, rounded up to 23.
- Blocks: 7 × 23 = 161 × 1.05 = 169.05, so 170 blocks.
- Caps: 360 ÷ 16 = 22.5 × 1.05 = 23.6, so 24 caps.
- Base pad: 30 × 2 ft × 0.5 ft = 30 ft³ = 1.11 yd³. Drainage stone: 30 × 1 × 3.5 = 105 ft³ = 3.89 yd³.
When a retaining wall needs an engineer
Under the International Residential Code (Section R404.4), retaining walls that are not laterally supported at the top and that retain more than 48 inches of unbalanced fill must be designed to resist overturning, sliding and soil pressure, which in practice means an engineered design. Walls supporting a driveway, a slope above, a building or another wall (a surcharge) may need engineering at lower heights. Taller segmental walls typically use layers of geogrid reinforcement extending back into the soil. Many municipalities also require a permit above a set height, so check locally before you buy materials.
Base and drainage materials
The leveling pad is usually compacted crushed stone (sometimes called dense-graded aggregate or road base), leveled carefully because every course above follows it. Behind the wall, clean angular stone such as ¾ inch crushed stone lets water drain; a perforated drain pipe at the base, outlet to daylight, is commonly used for taller walls or poorly draining soils. Convert the cubic yards to tons in the gravel calculator (about 1.4 tons per cubic yard).
Caveats and assumptions
- Block sizes vary by brand and line. Use the manufacturer's exact face dimensions, and note that some systems use multi-size units sold by face area.
- The cap course adds height. If your exposed height must include the cap, subtract the cap thickness first.
- Stepped bases on sloping ground and curved walls change the counts; add extra for steps and tight curves.
- This estimate does not include geogrid, drain pipe, filter fabric or adhesive for caps.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block length (round up) to get blocks per course, multiply by the number of courses including the buried course, and add about 5%. A 30 ft wall 3 ft high with 16 × 6 inch blocks needs about 170 blocks.
How deep should the first course be buried?
CMHA and manufacturer guides commonly call for at least 6 inches below finished grade, and more for taller walls or walls on slopes. Follow your block maker's guide.
How much gravel do I need for a retaining wall base?
A 6 inch compacted pad about 2 ft wide for a 12 inch deep block: roughly 1 ft³ per running foot, or 1.11 yd³ for a 30 ft wall.
Do retaining walls need drainage?
Yes. A zone of clean stone behind the wall relieves water pressure, which is a leading cause of wall failure. Taller walls usually add a perforated drain pipe at the base.
How tall can a retaining wall be without an engineer?
The IRC requires a design for walls retaining more than 48 inches of unbalanced fill, and local rules may be stricter. Surcharges from slopes, driveways or structures can trigger engineering at lower heights.
Sources
- CMHA — Segmental Retaining Walls Best Practices Guide — leveling pad, embedment and drainage guidance
- CMHA — Segmental Retaining Wall Sample Specifications — base and drainage aggregate specifications
- ICC — 2021 International Residential Code, Chapter 4 (R404.4 Retaining walls) — design requirement over 48 in of unbalanced fill
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.