Retaining Wall Calculator
Enter the wall's length and exposed height and choose a block size. You'll get blocks, caps, buried courses, base gravel, drainage stone and drain pipe.
Enter the wall length, height and block size.
On this page
How to use the retaining wall calculator
- Measure the wall length along its face. For a curved wall, measure along the curve with a garden hose or tape.
- Enter the exposed height — the part above finished grade on the low side. The calculator adds the buried course for you.
- Pick the closest block size or enter the exact face length, height and depth from the product label.
- Read the shopping list: blocks, caps, base gravel for the trench, drainage stone for behind the wall and drain pipe.
How the calculation works
Worked example: a wall 20 ft long and 24 in high with 16 × 6 × 10 in blocks
- Buried depth: 6 in (a 2 ft wall needs the 6-inch minimum) → 1 buried course
- Exposed courses: 24 ÷ 6 = 4, so 5 courses in total
- Blocks per course: 240 in ÷ 16 in = 15 → 75 blocks, plus 5% = 79 blocks
- Caps: 240 ÷ 16 = 15, plus 5% = 16 caps
- Base gravel: 20 × 1.25 ft × 0.5 ft = 12.5 ft³ ≈ 0.46 yd³; drainage stone: 20 × 1 × 2.5 ft = 50 ft³ ≈ 1.85 yd³
How deep to bury the first course
Segmental block manufacturers generally call for the first course to be buried at least 6 inches, or 1 inch for every foot of wall height, whichever is greater. The buried course keeps the base of the wall from kicking out and gives it a stable start below any surface erosion. The calculator applies the same rule and rounds up to whole courses.
Base and drainage
- Base trench: typically about one and a half to two times the block depth wide, with at least 6 inches of compacted crushed stone. Level the first course carefully — every error is multiplied up the wall.
- Drainage stone: fill at least 12 inches behind the blocks with clean crushed stone (no fines) so water can drain instead of building pressure against the wall.
- Drain pipe: a perforated pipe at the base of the drainage stone, vented to daylight, is required by manufacturers for taller walls and poorly drained sites — and cheap insurance for any wall.
- Filter fabric between the drainage stone and the soil behind keeps fines from clogging the stone.
Know the limits. Walls taller than about 4 feet, walls holding back a slope, a driveway or a building, and terraced walls close together usually need an engineered design with geogrid reinforcement — and often a permit. Check your local building department before you start.
Common block sizes
| Type | Typical face (L × H) | Blocks per sq ft of face | Typical use |
|---|---|---|---|
| Small garden wall block | 12 × 4 in | 3.0 | Flower bed borders and low walls up to about 2 ft |
| Medium retaining block | 16 × 6 in | 1.5 | Garden and landscape walls |
| Large retaining block | 18 × 8 in | 1.0 | Taller landscape walls, reinforced walls |
Sizes vary by brand; use the exact dimensions from the product for a precise count.
Frequently asked questions
How many blocks do I need for a 3 ft tall, 30 ft long wall?
With 16 × 6 in blocks: 6 exposed courses plus 1 buried course is 7 courses; 30 ft needs 23 blocks per course (22.5 rounded up) — 161 blocks, or 170 with 5% extra.
Do I need caps?
Caps aren't structural, but they finish the wall, protect the top course and give you a place to sit. Glue them down with a concrete landscape adhesive.
Can I build a retaining wall without gravel?
Not one that lasts. The compacted gravel base keeps the wall level as the ground moves, and the drainage stone behind it relieves water pressure — the main reason small walls lean and fail.
Does a curved wall need more blocks?
Measure along the face of the curve and the count stays about the same. Tight curves may need a few blocks trimmed, so keep the extra at 5–10%.
Sources
- Segmental retaining wall manufacturers' installation guides (for example Allan Block) — buried course, base and drainage recommendations.
- Your local building code sets the height at which walls need permits or engineering.