Segmental retaining walls are built from interlocking concrete blocks stacked dry, with the first course buried for stability and clean gravel behind the wall so water can drain. Walls over about 4 ft (1.2 m) usually need geogrid reinforcement and an engineer’s design. Caps glued on top finish the wall.
How to use the Retaining Wall Calculator
- Wall length
- Exposed wall height
- Block size (face length × height)
- Buried courses
- Cap the wall
- Drainage gravel zone depth (behind wall)
- Price per block
- Price per cap
- Gravel price
- Drain pipe price
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the block size, enter the wall length and exposed height, how many courses to bury, whether to cap the wall and the depth of the drainage gravel zone. The calculator counts blocks and caps, the gravel volume and weight, the drain pipe length and the total cost.
Tips for an accurate estimate
- Sketch the job and split irregular areas into rectangles, circles and triangles before you measure.
- Add a waste allowance – typically 5–10% for simple layouts and 15–20% for diagonal, curved or heavily cut work.
- Check the coverage or yield printed on the product, because sizes and densities vary between manufacturers.
- Buy all units of a finished material (tile, brick, siding) from the same batch so colours match, and keep a few spares for repairs.
- Confirm local prices – quarry, lumber and steel prices vary widely by region and season.
Common mistakes to avoid
- Ordering exactly the calculated amount with no allowance for breakage, offcuts and settlement.
- Forgetting to subtract doors, windows and other openings – or subtracting small ones that still need cutting around.
- Confusing US short tons (2,000 lb) with metric tonnes (1,000 kg) when comparing quotes.
Inputs explained
| Field | What to enter |
|---|---|
| Wall length | Units: ft, m, in, yd, cm |
| Exposed wall height | Units: ft, m, in, yd, cm |
| Block size (face length × height) | Choose one: 12 × 4 in (garden wall block), 16 × 6 in, 18 × 8 in (large SRW), 12 × 6 in … |
| Buried courses | A number |
| Cap the wall | Choose one: No, Yes |
| Drainage gravel zone depth (behind wall) | Units: in, cm, mm, ft, m |
| Price per block | Units: /piece |
| Price per cap | Units: /piece |
| Gravel price | Units: /US ton, /t, /lb, /kg |
| Drain pipe price | Units: /ft, /m, /yd |
What each result means
- Courses = ceil([Exposed wall height]÷[Block height] – 0.000001) + [Buried courses]
- Blocks per course = ceil([Wall length]÷[Block length] – 0.000001)
- Wall blocks = [Courses]×[Blocks per course]
- Cap units = [Cap the wall]×[Blocks per course]
- Drainage gravel = [Wall length]×[Courses]×[Block height]×[Drainage gravel zone depth (behind wall)]
- Gravel weight = [Drainage gravel]×1522
- Total cost = [Wall blocks]×[Price per block] + [Cap units]×[Price per cap] + [Gravel weight]×[Gravel price] + [Wall length]×[Drain pipe price]
Example values
How the main result changes when Wall length changes and everything else stays at the example values above:
| Wall length (ft) | Wall blocks |
|---|---|
| 20 | 200 |
| 30 | 300 |
| 40 | 400 |
| 50 | 500 |
| 60 | 600 |
| 80 | 800 |
Formula
Courses = ceil(H ÷ block height) + buried coursesBlocks = courses × ceil(L ÷ block length)Caps = ceil(L ÷ cap length)Gravel = L × wall height incl. buried × gravel zone depthWorked example
Inputs — Wall length: 40 ft; Exposed wall height: 3 ft; Block size (face length × height): 12 × 4 in (garden wall block); Buried courses: 1; Cap the wall: Yes; Drainage gravel zone depth (behind wall): 12 in; Price per block: $3.5 /piece; Price per cap: $3 /piece; Gravel price: $40 /US ton; Drain pipe price: $1 /ft.
Results — Courses = 10; Blocks per course = 40; Wall blocks = 400; Cap units = 40; Drainage gravel = 4.94 yd³; Gravel weight = 6.33 US ton; Total cost = $1,813.37.
Frequently asked questions
How deep should the first course be buried?
At least 6 in or one-eighth of the wall height, whichever is more – usually one full block.
How much gravel goes behind a retaining wall?
A zone 12 in deep behind the blocks for the full height, plus gravel inside hollow cores.
Do I need a permit for a retaining wall?
Many places require a permit and engineered design for walls over 3–4 ft or walls supporting a slope, driveway or structure.
What formula does the Retaining Wall Calculator use?
Courses = ceil(H ÷ block height) + buried courses.