Retaining Wall Calculator
Enter your dimensions for an instant answer — the exact quantity to buy (waste included), a material cost, and an estimated installed cost. Change any figure to update the result live.
| ITEM | QTY | PRICE RANGE |
|---|---|---|
| blocks | 126 | $315–$1,008 |
| Estimated material total | $315–$1,008 |
- Gravel base (6 in): ≈ 0.54 tons
How many blocks do I need for a retaining wall?
Enter the wall length and height in feet. The calculator works out the face area, divides it by the face area of one standard segmental block (about 0.5 sq ft), and adds 5% for cuts and breakage. It also estimates the compacted gravel base — 6 inches deep and a foot wider than the wall — as a companion line.
Results are whole blocks because that is how walls are bought; a typical retaining block faces about 4 in × 18 in. For walls above roughly 4 feet, most jurisdictions require an engineered design — our numbers cover the standard gravity-wall range.
Formula & methodology
blocks = (length ft × height ft) ÷ 0.5 ft² per block, + 5% waste
Face area divided by block face area, rounded up to whole blocks with a 5% waste allowance. The 6-inch gravel base is computed at 1.4 tons per cubic yard over a footprint one foot wider than the wall.
All constants are published on the material reference; the full model is on the methodology page.
20 ft long × 3 ft high: you need 120 blocks (buy 126 with waste), about $315–$1,008 in materials. See how we calculate this →
Popular sizes
Retaining Wall cost
Blocks run $2.50–$8 each nationally; installed walls are priced per face square foot and vary sharply by state.
retaining wall cost by state →Frequently asked questions
How many blocks for a retaining wall?
Divide the wall face area (length × height) by the face area of one block. A standard block faces about 0.5 sq ft, so a 20×3 ft wall needs roughly 120 blocks — buy about 126 with waste.
How much base gravel for a retaining wall?
Plan a 6-inch compacted gravel base one foot wider than the wall; the calculator estimates the tonnage automatically with every result.
Do I need a permit for a retaining wall?
Rules vary, but many US jurisdictions require an engineered design and permit once a wall exceeds 3–4 feet in height or supports a slope or driveway. Check your local building department before you build.
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