Retaining Wall Block Calculator
Count wall blocks, cap blocks and drainage gravel for a segmental retaining wall.
Calculator
| Cap blocks, 12 in (+5%) | 21 caps |
|---|---|
| Drainage gravel, 12 in deep behind the wall (+10%) | 2.44 yd³ |
How it is calculated
Wall face = 20 × 3 = 60 sq ft. A 12×4 in block covers 1/3 sq ft, so 3 blocks per sq ft: 180 blocks; +5% = 189. Caps: 21.
About this calculator
A segmental retaining wall is built from interlocking blocks stacked with a slight setback. A block with a 12 × 4 inch face covers one third of a square foot, so a wall needs three blocks per square foot, plus cap blocks along the top.
Drainage behind the wall is critical. The calculator estimates a foot of drainage gravel behind the wall. Tall walls, walls holding back a slope that supports a building, and walls near property lines often need engineering and permits.
Worked examples
| Project | Wall blocks, 12×4 in face (+5%) | Rough cost |
|---|---|---|
| a 10 ft long, 2 ft high retaining wall | 63 blocks | $126 – $441 |
| a 20 ft long, 3 ft high retaining wall | 189 blocks | $378 – $1,323 |
| a 30 ft long, 4 ft high retaining wall | 378 blocks | $756 – $2,646 |
| a 50 ft long, 5 ft high retaining wall | 788 blocks | $1,576 – $5,516 |
Buying notes
- Different block systems have different face sizes; check yours against the 12×4 inch assumed here.
- The first course is usually buried below grade on a compacted base, so it adds to your height.
- Geogrid reinforcement, drainage pipe and fabric are not included.
Quick answers by size
Click a size for the exact quantity and cost.
How to measure and use this calculator
- Measure the wall's length and height.
- Read the wall blocks, cap blocks and drainage gravel.
- Bury the first course and compact the base.
- Check local rules; tall walls usually need an engineer.
Common mistakes
- Skipping drainage behind the wall.
- Building a tall wall without reinforcement.
- Not compacting the base.
Tips
- Walls over about 3–4 feet usually need engineering and geogrid; check local rules.
- Bury the first course below grade and set it level on compacted base.
- Always provide drainage behind the wall.
Frequently asked questions
How many retaining wall blocks do I need per square foot?
With a 12×4 inch face you need 3 blocks per square foot; other block sizes change the number.
How tall can I build a retaining wall without a permit?
Rules vary. Many areas require a permit or engineering above a height of about 3 to 4 feet. Check with your local building office.
Why does a retaining wall need drainage?
Water pressure behind the wall can push it over. Gravel and a drain pipe let the water escape.