Retaining wall block calculator
Enter wall length and height, plus your block's face size — get block count, cap count and how many courses you'll lay.
Result
Wall blocks needed
How many blocks do I need?
Formula
wall ft² = length × height · blocks = wall ft² ÷ (block w × h ÷ 144) × 1.10
caps = (length × 12) ÷ block width
Example: a 20 ft wall, 3 ft tall, in standard 12 × 3 in modular blocks (0.25 ft² face): 60 ft² ÷ 0.25 = 240 → 264 blocks (12 courses) + 20 caps.
Common block sizes
| Block face (w × h) | ft² per block | Blocks per 10 ft² |
|---|---|---|
| 12 × 3 in (classic retaining) | 0.25 | 40 |
| 12 × 4 in | 0.33 | 30 |
| 11.5 × 8 in (mini wall) | 0.64 | 16 |
| 18 × 6 in (large format) | 0.75 | 13 |
The part that decides if the wall survives
- Base: 4–6 in of compacted crusher run (compute it in the paver base calculator), first course buried 10% of wall height.
- Drainage: gravel backfill + drain pipe behind every wall over 2 ft — without it, hydrostatic pressure pushes walls over. Gravel tonnage: gravel calculator.
- Setback: most systems lean each course back ~⅛–¼ in — wall kits' pin systems handle it; buy per the manufacturer's chart for your height.
- Permits: exposed walls over 4 ft (and sometimes over 36 in) need an engineer's design and a permit in most US jurisdictions.
Frequently asked questions
How many blocks for a 20 ft wall 3 ft high?
With 12×3 in blocks: 60 ft² ÷ 0.25 = 240 → 264 with waste, plus 20 caps and ~1.5 yd³ of base gravel.
How many cap blocks do I need?
Caps = wall length × 12 ÷ cap width, plus 1–2 spares. A 20 ft wall with 12 in caps: 20 caps.
How tall can a block retaining wall be without engineering?
Most codes and manufacturers draw the line at 4 ft exposed (some at 36 in). Above that — or with a slope/driveway above the wall — get an engineer.
Do I need gravel behind a retaining wall?
Yes for anything over ~2 ft: 12 in of drainage gravel plus a perforated pipe at the base. Water is what topples walls, not soil.
How much does a retaining wall cost?
DIY block walls run $15–30/ft² in materials; installed, $30–70/ft². The calculator's cost panel does materials either per block or per ft².