Retaining Wall Calculator
Estimate wall blocks, caps, drainage gravel, and cost for segmental concrete, stone, and timber retaining walls.
Total run of the wall. For L-shaped or terraced walls, calculate each section and add.
How to Use
- Enter wall length - the total run in feet; add sections separately for L-shaped or terraced walls.
- Select height - measured from finished grade at the base to the top; over 4 ft usually needs engineering.
- Pick block size - standard 12×6 segmental blocks are 2 per square foot of wall face.
- Set waste - 10% covers a straight wall with corners; 15–20% for curves where blocks are cut.
- Plan the gravel - the gravel result is the drainage layer plus base - never skip it, and add a perforated drain pipe.
A 20 foot long, 3 foot tall block wall takes about 132 standard 12x6 blocks (2 per square foot of face), 20 cap blocks, and roughly 2.8 cubic yards of gravel for the compacted base and the 12 inch drainage zone behind the wall. Bury the first course - about 10 percent of wall height - and keep unreinforced walls at 4 feet or under. Enter your wall size above.
What Holds a Wall Up
Why Drainage Decides Whether a Wall Lasts
Retaining walls fail for one reason more than any other: trapped water. When rain saturates the soil behind the wall, hydrostatic pressure builds fast - far more force than the soil weight alone. A 3-foot wall without drainage can see hundreds of pounds per square foot of lateral pressure after a storm, and even well-built walls eventually lean or crack under it.
The fix is simple: 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 separates walls that last 30 years from ones that fail in five. The gravel result here includes both the drainage layer and the compacted base trench - budget it before you price blocks. If the base course sits on a poured footing, size the mix with the concrete bag calculator.
Block Count by Wall Height and Length
| Wall height | 10 ft | 20 ft | 50 ft | 100 ft |
|---|---|---|---|---|
| 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 12×6 in face blocks (2 per sq ft of wall face). Cap blocks are counted separately at 1 per linear foot of wall top.
Retaining Wall Material Comparison
Material choice drives cost, lifespan, drainage, and whether the job is DIY-friendly:
| Material | Cost per sq ft (face) | Lifespan | DIY | Best for |
|---|---|---|---|---|
| Segmental concrete block | $15–$30 | 50+ yrs | Yes | Most residential walls |
| Natural stone (dry-stack) | $25–$45 | 75+ yrs | Moderate | Feature walls |
| Pressure-treated timber | $10–$20 | 15–25 yrs | Yes | Garden beds, walls under 2 ft |
| Poured concrete | $20–$40 | 50+ yrs | No | Engineered / commercial |
| Gabion baskets | $15–$35 | 50+ yrs | Yes | Rustic look, great drainage |
Retaining Wall Formulas
Block and gravel math:
Block count = Face area × blocks per sq ft
Cap blocks = 1 per linear foot of wall
Drainage gravel = Length × height × 1 ft wide ÷ 27 (cubic yards)
Base gravel = Length × 1.5 ft × 0.5 ft ÷ 27 (cubic yards)
With waste = Block count × (1 + waste %)
Set the base course 6 inches below finished grade on 6 inches of compacted gravel. Each course steps back about 1 inch (the block batter handles this). Walls over 3–4 ft require permits in most jurisdictions.
Design & Permit Note
- Segmental retaining wall design follows the concrete masonry industry guidance, which sets out the base, drainage, and reinforcement details this calculator assumes. The rule most codes share: an unreinforced wall over about 4 feet tall, measured from the bottom of the buried course, or any wall holding back a slope, driveway, or other surcharge, needs geogrid reinforcement and usually an engineer and a permit. Check your local threshold before building.NCMA - Segmental Retaining Wall Design Guidance
Next Steps
Blocks counted, line up the rest of the build:
Related Calculators
Retaining walls need gravel, concrete, and fill - browse the full construction calculator collection.
Frequently Asked Questions
How many retaining wall blocks do I need per square foot?
Standard 12×6-inch blocks cover half a square foot each, so you need 2 per square foot of wall face. A 20-foot wall 3 feet tall is 60 sq ft × 2 = 120 blocks, plus 10% waste = 132, plus about 20 cap blocks (one per linear foot of top). Larger block systems cover more face area, which is why the calculator includes a block-size selector.
Do I need a permit for a retaining wall?
Most jurisdictions require permits for walls over 3 to 4 feet; some require them for any wall that retains soil. Check with your building department first - unpermitted walls can be flagged during home sales and may need removal. Walls over 4 feet almost always require a structural engineer's stamp.
How much gravel do I need behind a retaining wall?
Plan a drainage layer at least 12 inches wide for the full wall height, plus a compacted base trench. The calculator combines both into the gravel result. For a 20-foot, 3-foot wall that is roughly 2.8 cubic yards. Always run a 4-inch perforated drain pipe at the base, daylighting to a lower point.
What causes a retaining wall to fail?
The leading cause is poor drainage - hydrostatic pressure from saturated soil pushes the wall forward. Other causes are shallow footings (especially in frost areas), undersized block for the height, skipping the backward batter, and building on unstable clay. Skipping engineering review on anything over 3 feet is the highest-risk shortcut.
How deep should I bury the first course?
Bury the bottom course about one-tenth of the finished wall height, and never less than one full block - so a 3 foot wall sits roughly 3 to 4 inches below the front grade, and a taller wall proportionally deeper. The buried course rests on the compacted gravel base and anchors the wall against sliding. In cold climates, excavate the base below the local frost line.
Do I need a drainage pipe behind a retaining wall?
Yes, for anything beyond short garden edging. Water building up behind the blocks is the number one cause of retaining wall failure - saturated soil pushes far harder than dry soil. A perforated pipe at the base of the drainage gravel, sloped to daylight or a drain, carries that water off before it can build pressure. Wrap the pipe and gravel in filter fabric so soil does not clog them.
How far back should a retaining wall lean?
Segmental block walls should batter back about 1 inch per foot of height, so a 3-foot wall leans back roughly 3 inches at the top. Most interlocking systems build this batter into the block geometry, stepping back automatically as you stack. Dry-stacked stone requires setting the lean manually on each course.