Quick answer: Divide the wall's face area by one block's face coverage for the block count, add a cap course, and plan ~1 ton of gravel backfill per 30 sq ft. Walls over 4 ft usually need an engineer.
Calculate blocks, cap blocks, base gravel, and drainage backfill for segmental retaining wall (SRW) blocks — with cost estimate.
Segmental retaining wall (SRW) blocks stack in courses, so the estimate is simple multiplication:
This calculator also estimates your base trench gravel (6 in deep × 12 in wide under the full wall) and drainage backfill (12 in of clean crushed stone behind the full height of the wall) — the two materials DIYers most often forget to order.
Every properly built retaining wall buries its first course about 10% of the wall height, minimum one inch per foot of wall. A 3-ft wall should have its first course roughly 4 inches below grade, sitting on 6 inches of compacted crushed stone.
Retaining walls rarely fail from weak blocks — they fail from water pressure (hydrostatic pressure) building up behind them. Every wall over 2 feet needs:
Our calculator includes the backfill gravel tonnage automatically. Size the drain pipe run with the Linear Footage Calculator.
Also note: if the slope above the wall carries a driveway or structure (a "surcharge"), the engineering threshold drops.
A standard retaining wall block has a 12 in wide by 8 in high face, so 1.5 blocks cover each square foot of wall. Walls over 4 ft usually need engineering and a permit — check locally before ordering.
| Wall size | Face area | Blocks | Pallets (approx) |
|---|---|---|---|
| 10 ft × 2 ft | 20 sq ft | 30 | 1.3 pallets |
| 20 ft × 3 ft | 60 sq ft | 90 | 3.8 pallets |
| 30 ft × 3 ft | 90 sq ft | 135 | 5.6 pallets |
| 40 ft × 4 ft | 160 sq ft | 240 | 10 pallets |
A standard 12" × 4" face block covers 1/3 sq ft, so you need 3 blocks per square foot of wall face. A 16" × 6" block covers 2/3 sq ft (1.5 blocks per sq ft), and an 18" × 8" jumbo block covers 1 sq ft.
Plan a 12-inch wide column of clean crushed stone the full height of the wall, plus a 6-inch deep base trench. For a 20-ft long, 3-ft tall wall that's roughly 2.6 cubic yards (about 3.5 tons) including the base.
No — segmental (dry-stack) retaining wall blocks sit on compacted crushed stone, not concrete. A rigid concrete footing can actually crack with frost movement. Mortared or poured walls are a different system and do need footings.
DIY with standard SRW blocks: roughly $15–$25 per square foot of wall face in materials. Professionally installed: $25–$50+ per square foot depending on height, access, and block style.
Yes — step the base trench so each section of the first course stays level and buried. Never follow the slope with the courses; blocks must always stack dead level.
Building a wall, terrace, or garden bed? These tools pair well with the retaining wall calculator:
New to hardscaping? These guides walk through the fundamentals:
Every constant this calculator uses and where it comes from, so you can check it rather than take our word for it.
Local codes amend the model IRC and adopt different cycles — your building department governs. Manufacturers revise yields; where your product's data sheet disagrees with a default here, use the product's number. See our full methodology.