Retaining Wall Calculator
Estimate blocks, drainage gravel, and material cost for concrete block, stone, and timber retaining walls.
Your Retaining Wall Estimate
- Enter wall length — total run of the wall in feet. For L-shaped or terraced configurations, calculate each section separately and add.
- Select wall height — measured from finished grade at the base to the top of the wall. Walls over 4 feet typically require engineering review.
- Adjust block density — standard concrete retaining blocks weigh about 130 lb/ft3. Natural stone varies — adjust for your material.
- Set waste factor — 10% covers a straight wall with a few corners. Increase to 15-20% for curved walls where many blocks need to be cut.
- Plan drainage gravel — the drainage layer result is critical — always install a perforated drain pipe at the base of the gravel layer.
Why drainage is the most important part of any retaining wall
Retaining walls fail for one reason more than any other: trapped water. When rain saturates the soil behind the wall, hydrostatic pressure builds rapidly — far more force than the soil's weight alone. A 3-foot wall without drainage can experience hundreds of pounds per square foot of lateral pressure after a heavy rain. Even well-built walls eventually fail under this load if the water has nowhere to go.
The fix is straightforward: 12 inches of crushed gravel directly behind the wall for the full height, plus a 4-inch perforated pipe at the base that daylights to a lower grade. That single detail is what separates walls that last 30 years from ones that lean and crack in five. Plan this into your material budget before you price the blocks.
Block count by wall height and length
| Wall height | 10 ft long | 20 ft long | 50 ft long | 100 ft long |
|---|---|---|---|---|
| 1 ft (2 courses) | 22 | 44 | 110 | 220 |
| 2 ft (4 courses) | 44 | 88 | 220 | 440 |
| 3 ft (6 courses) | 66 | 132 | 330 | 660 |
| 4 ft (8 courses) | 88 | 176 | 440 | 880 |
| 5 ft (10 courses) | 110 | 220 | 550 | 1,100 |
Includes 10% waste. Based on standard 12"x6" face blocks (2 blocks per square foot of wall face). Add cap blocks separately — 1 per linear foot of wall top.
Retaining wall material comparison
Material choice affects cost, lifespan, drainage performance, and whether you need professional installation:
| Material | Cost per sq ft (face) | Lifespan | DIY-friendly | Best for |
|---|---|---|---|---|
| Segmental concrete block | $15-$30 | 50+ years | Yes | Most residential walls |
| Natural stone (dry-stack) | $25-$45 | 75+ years | Moderate | Aesthetic feature walls |
| Pressure-treated timber | $10-$20 | 15-25 yrs | Yes | Garden beds, walls under 2 ft |
| Poured concrete | $20-$40 | 50+ years | No | Engineered walls, commercial |
| Gabion baskets | $15-$35 | 50+ years | Yes | Rustic look, excellent drainage |
Retaining wall calculation formulas
Block count and drainage material calculations:
Block count = Wall face area x 2 (for standard 12"x6" blocks)
Cap blocks = 1 per linear foot of wall length
Drainage gravel = Length x Height x 1 ft wide / 27 (cubic yards)
Base gravel layer = Length x 1.5 ft x 0.5 ft / 27 (cubic yards)
With waste = Block count x (1 + Waste %)
The base course should sit 6 inches below finished grade, on 6 inches of compacted crushed gravel. Each course steps back 3/4" to 1" from the previous (the built-in batter on most segmental blocks handles this automatically). Check local building codes — walls over 3 to 4 feet require permits in most jurisdictions.
Retaining wall installation cost by material (2025-2026)
| Material | Material per sq ft face | Installed per sq ft | 20-ft x 3-ft wall |
|---|---|---|---|
| Segmental concrete block | $4-$10 | $15-$35 | $1,000-$2,400 installed |
| Natural stone (dry-stacked) | $8-$20 | $20-$45 | $1,500-$3,300 installed |
| Pressure-treated timber | $3-$8 | $10-$25 | $700-$1,700 installed |
| Gabion basket (wire and rock) | $5-$12 | $15-$35 | $1,000-$2,400 installed |
| Poured concrete (engineered) | $8-$18 | $20-$45 | $1,500-$3,300 installed |
Installed cost includes excavation, gravel base, drainage layer, perforated drain pipe, and labor. A 3-ft wall 20 ft long has 60 sq ft of face area. Walls over 4 ft typically require engineering review adding $500-$2,000 in design fees. Northeast and California run 25-40% above these figures.
Related Calculators
Retaining walls need gravel, concrete, and fill — browse the full construction calculator collection for all related tools.
Frequently Asked Questions
How many retaining wall blocks do I need per square foot?
Standard 12"x6" retaining wall blocks cover 0.5 square feet each, so you need 2 blocks per square foot of wall face. For a 20-foot wall that is 3 feet tall: 20 x 3 = 60 sq ft x 2 = 120 blocks, plus 10% waste = 132 blocks total. Add cap blocks separately at 1 per linear foot of wall top (20 cap blocks for a 20-foot wall).
Do I need a permit for a retaining wall?
Most jurisdictions require permits for retaining walls over 3 to 4 feet tall. Some areas require permits for any wall that retains soil. Check with your local building department before starting — unpermitted walls are sometimes cited during home sales and may need removal. Walls over 4 feet almost universally require a structural engineer's stamp and inspections.
How much gravel do I need behind a retaining wall?
Plan for a drainage layer at least 12 inches wide for the full height of the wall, plus a 6-inch compacted base layer. For a 20-foot wall at 3 feet tall: 20 x 3 x 1 = 60 cubic feet = 2.2 cubic yards of drainage gravel, plus a perforated 4-inch drain pipe running the full length at the base of the gravel, daylighting to a lower point.
What causes a retaining wall to fail?
The most common cause is inadequate drainage — hydrostatic pressure from saturated soil behind the wall pushes it forward. Other causes include insufficient footing depth (especially in frost-prone areas), using undersized block for the wall height, skipping the batter (backward lean), and building on unstable or expansive clay. Walls that skip engineering review on anything over 3 feet are at highest risk.
How far back should a retaining wall lean?
Segmental concrete block walls should batter (lean backward) at roughly 1 inch per foot of wall height. A 3-foot wall should lean back 3 inches total at the top compared to the base. Most interlocking block systems build this batter into the block geometry automatically — each course naturally steps back the right amount as you stack. Dry-stacked stone requires you to manually set the lean on each course.