Retaining Wall Calculator
Enter wall length and height. We'll size the blocks plus the base gravel and drainage gravel that go behind it.
Advanced options
- Wall area
- 0 sq ft
- Block courses (rows)
- 0
- Blocks per course
- 0
- Cap blocks
- 0
- Base gravel
- 0 cu yd
- Drainage gravel
- 0 cu yd
- Landscape fabric
- 0 sq ft
- Estimated cost (blocks only)
- —
How the retaining wall calculator works
A 20-ft long, 3-ft tall retaining wall takes 189 standard 12″×4″ landscape blocks plus 20 cap blocks, 0.74 cubic yards of base gravel, and 2.22 cubic yards of drainage gravel. Three feet of height is 9 courses of 4-inch block, and 20 feet of length is 20 blocks per course — 180 blocks, or 189 with a 5% waste factor. Add 60 sq ft of landscape fabric and a run of 4-inch perforated drain pipe at the base.
The math: block count = courses (rows) × blocks per course. Courses = wall height ÷ block height. Per-course count = wall length ÷ block length. The first (buried) course is one full course, even though only the top of it is visible.
Base gravel sits in a 4–8" deep × 12–24" wide trench under the first course. It's compacted and screeded level. Without a proper base, walls fail within a few seasons.
Drainage gravel goes behind the wall, in the 6–18" of space between the wall and the soil it's holding back. Combined with weep holes or a French drain at the base, it prevents water buildup that would push the wall over.
Block types
- 12"×4" landscape block — the box-store standard (Anchor Diamond Pro, Pavestone, etc.). Locking lip on top to engage the next course. Good for walls up to ~3 ft tall (1 m). 6 blocks per face per square foot.
- 16"×6" landscape block — bigger profile, more mass. Walls to ~4 ft (1.2 m). Pricier but fewer blocks needed.
- CMU 16"×8" — structural concrete masonry unit. For walls over 4 ft and engineered designs (with rebar + footing). Requires permits in most jurisdictions.
Wall height limits without engineering
Most US municipalities allow DIY-built retaining walls up to 3–4 ft (1–1.2 m) without a permit or engineering. Anything taller usually requires a structural engineer's stamp + permit + inspections. Always check your local code before starting.
Common questions
How thick does the base gravel need to be?
4 in / 10 cm minimum for a 2 ft wall, 6 in / 15 cm for a 3 ft wall, 8 in / 20 cm for a 4 ft wall. Compact in 2–3 inch lifts with a plate compactor. Clay soils need extra base; sandy soils can go thinner.
What's "geogrid" — do I need it?
Geogrid is a polymer mesh laid horizontally between block courses, extending back into the soil. Required by manufacturers for walls over ~3 ft to anchor the wall against tipping. Each course needs its own geogrid layer. If your wall is 4+ courses, plan for geogrid.
How does cost compare to other retaining options?
Landscape block walls: $25–$45/sq ft installed. Poured concrete: $30–$60/sq ft. Boulder walls: $20–$50/sq ft. Pressure-treated timber: $15–$30/sq ft (5–10 year life). Block is the most popular DIY option for the longevity-to-cost ratio.
For the gravel that goes around the wall, see the gravel calculator — sand for joints, see sand.
Planning the full project?
These calculators usually get run in the same afternoon.