How to use this retaining wall calculator
- Measure the length of the wall.
- Enter the exposed height; one buried course is added automatically.
- Enter the face dimensions of your block and cap.
- Walls over about 3–4 ft usually need engineering and geogrid; check local rules.
Example
A 40 ft long, 3 ft high wall with 12 × 4 in blocks needs 400 blocks and 40 caps, plus 1.48 cu yd of base gravel and 4.94 cu yd of drainage gravel.
What to know before you buy
Segmental retaining wall blocks
Most DIY retaining walls use interlocking concrete blocks with a lip or pins that set each course slightly back. Common face sizes are 12 × 4 in, 16 × 6 in and 18 × 6 in. Check the manufacturer's maximum height for walls built without reinforcement.
Drainage is essential
Water pressure is the main reason retaining walls fail. A zone of clean crushed stone behind the wall and a perforated drain pipe at the base let water escape. Separate the gravel from the soil with filter fabric so it does not clog.
Blocks for a 12 × 4 in block wall (1 buried course)
| Wall | Wall blocks | Cap blocks |
|---|---|---|
| 20 ft × 1 ft | 80 | 20 |
| 20 ft × 2 ft | 140 | 20 |
| 30 ft × 2 ft | 210 | 30 |
| 40 ft × 3 ft | 400 | 40 |
| 50 ft × 3 ft | 500 | 50 |
Formula
Blocks = (length ÷ block length) × (height ÷ block height + 1 buried course). Base trench = length × 24 in × 6 in. Drainage zone = length × 12 in × wall height.
Tips from the pros
- Level the first course carefully: every error multiplies as the wall gets taller.
- Install a perforated drain pipe at the base of the drainage gravel.
- Use construction adhesive to secure the cap blocks.
Common mistakes to avoid
- Backfilling with soil instead of gravel directly behind the wall.
- Building walls over 3–4 ft without engineering and geogrid reinforcement.
Frequently asked questions
How deep should the first course be buried?
About 1 inch for every 8 inches of wall height, and at least half a block (typically one full course).
Do I need drainage gravel behind a retaining wall?
Yes. At least 12 inches of clean gravel behind the wall with a perforated drain pipe prevents water pressure from pushing the wall over.
How many blocks for a retaining wall?
Divide the wall length by the block length, and the height by the block height, adding one buried course. A 30 ft × 2 ft wall of 12 × 4 in blocks needs about 210 blocks.
How tall can a retaining wall be without engineering?
Many areas require engineering or a permit above 3–4 ft; check local rules.