Free Landscape Calculator
Retaining Wall Calculator
Calculate exactly how many retaining wall blocks, cubic yards of gravel backfill and cost for your landscape retaining wall. Instant results with 5% waste factor included.
👤 Who is this for: DIY landscapers, contractors & hillside homeowners.
📊 Understanding Your Results
Blocks Needed — Total retaining wall blocks including 5% waste for cuts. Order full pallets for best pricing (60–100 blocks per pallet).
Wall Square Feet — Total face area of your wall. Used for material quantities and contractor quotes.
Gravel Backfill — Cubic yards of crushed stone needed behind the wall for drainage. Proper drainage is critical — walls without it fail from hydrostatic pressure.
How the Retaining Wall Calculator Works
Our calculator determines block count by dividing wall face area by individual block area, then adding 5% waste. The gravel backfill estimate is based on a standard 12-inch drainage layer — the minimum required for wall stability. Walls over 3 feet tall typically require permits.
The Retaining Wall Formula
Step 2: Blocks = (Face Area ÷ Block Face Area) × 1.05 waste
Step 3: Gravel Backfill (cu yds) = (Length × 1 ft depth × Height) ÷ 27
Step 4: Base course is buried — add 1 extra course to height
Example: 20 ft × 3 ft wall with 6″H × 12″L blocks = 60 sq ft ÷ 0.5 = 120 blocks × 1.05 = 126 blocks. Gravel: 2.2 cu yards.
Retaining Wall Height Guide
| Wall Height | Permit Needed | Engineer Needed | DIY Feasibility |
|---|---|---|---|
| Under 2 feet | Usually No | No | ✅ Easy DIY |
| 2–3 feet | Sometimes | No | ✅ Moderate DIY |
| 3–4 feet | Often Yes | Sometimes | ⚠️ Experienced DIY |
| Over 4 feet | Yes | Usually Yes | ❌ Hire contractor |
📝 Real Project Example
24 ft long × 3 ft high wall, standard 12×6 in blocks:
- Wall face: 24 × 3 = 72 sq ft
- Blocks: 72 ÷ 0.5 sq ft per block = 144 → 158 with 10% waste
- Base gravel: 24 × 1.5 ft wide × 0.5 ft deep = 18 cu ft = 0.67 cubic yards
- Drainage backfill (12 in): 24 × 3 × 1 = 72 cu ft = 2.67 cubic yards
- Cap units: 24 ÷ 1.5 = 16 caps
- Cost check: 158 blocks at ~$2.50 = ~$395 + caps + ~3.3 yd³ gravel
Water pressure — not the blocks — is what kills retaining walls. Never skip the gravel backfill and perforated drain behind the wall.
📚 Sources & Methodology
How we calculate. Step 1: wall face area = length × total height (including the buried base course). Step 2: blocks = face area ÷ block face area, plus 10% waste. Step 3: base gravel = length × trench width × 0.5 ft, converted to cubic yards. Step 4: drainage backfill = length × height × 1 ft behind the wall. Caps counted separately by wall length.
Cross-checked. Our 6-inch compacted base and 10%-burial guidance match professional installation references; walls above ~4 ft typically require an engineered design and permit.
Sources:
- OnlyCalculators — Retaining Wall Calculator: Blocks, Caps, Gravel & Cost (block-count methodology)
- Estimate The Job — Retaining Wall Calculator (6-inch base, burial and drainage guidance)
Last updated: October 2026.
Frequently Asked Questions
Common questions about retaining wall construction and materials.
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Bury one full block course below grade — never skip this step.
Set each course 0.5–1 inch back for proper batter.
Use gravel backfill only — never clay behind walls.
Install a 4″ perforated drain pipe at wall base.