Retaining Wall Block Calculator
Enter the wall length, how tall it will stand above the ground, and the block you plan to use. You'll get blocks per course, cap blocks, the gravel for the base and the drainage zone, and the drain pipe.
- Tested formulas
- Feet or metric
- Printable shopping list
2 What to buy
Updates as you type| Block face | 8" high × 1' 6" long |
|---|---|
| Courses | 6, building 4 ft including 6" buried |
| Blocks per course | 14 |
| Total block weight | 6,675 lb (3.3 tons) |
| Base: compacted gravel | 0.74 yd³ (about 1 ton) |
| Drainage stone behind the wall (12" wide) | 2.96 yd³ (about 4.1 tons) |
| Perforated drain pipe | optional on a wall this low (20 ft if you add one) |
On clay, or with a slope, driveway or structure above the wall, even short walls can need reinforcement. Allan Block's gravity limit drops to 1 ft 4 in for clay with a load above.
Shopping list
Quick answer
Blocks = courses × blocks per course. A 20 ft wall standing 3 ft tall in 8 × 18 inch blocks, with 6 inches buried, takes 6 courses of 14 blocks: 84 blocks, or 89 with 5% extra, plus 14 caps.
Retaining wall block sizes
| Block | Face (H × L) | Depth | Weight | Blocks per sq ft |
|---|---|---|---|---|
| Allan Block AB Classic | 8 × 18" | 12" | 75 lb | 1 |
| Allan Block AB Lite Stone | 4 × 18" | 12" | 35 lb | 2 |
| Allan Block AB Jumbo Jr | 8 × 9" | 9.5" | 35 lb | 2 |
| Anchor / Belgard Diamond Pro | 8 × 18" | 12" | 73 lb | 1 |
| Versa-Lok Standard | 6 × 16" | 12" | 82 lb | 1.5 |
| Pavestone RockWall Small | 4 × 11.65" | 6.75" | n/a | 3.09 |
Face sizes and weights from each maker's data. Blocks per sq ft of wall face. Use the calculator above for your exact size.
How the block count works
Add the buried depth to the exposed height and divide by the block's face height to get the number of courses, rounding up. Divide the wall length by the face length for blocks per course. Multiply the two, then add a few percent for blocks you cut or break at corners and ends.
Worked example: a 20 ft wall, 3 ft tall
- Allan Block AB Classic has an 8 × 18 inch face. With 6 inches buried, the wall is 42 inches of block: 42 ÷ 8 = 5.25, so 6 courses.
- 240 inches of wall ÷ 18 = 13.3, so 14 blocks per course.
- 6 × 14 = 84 blocks, plus 5% = 89 blocks, about 3.3 tons.
- Whole courses round the wall up: 6 courses is 48 inches of block, so it stands about 3½ ft above ground before the cap. Trim the top course or bury more if you need to stay under a height limit.
- Caps: 14. Base gravel: 1 ton. Drainage stone: 4.1 tons.
How to build it right
- Dig a trench about 24 inches wide. Allan Block and Anchor both use a 2 ft wide base trench for their full-size blocks.
- Lay 6 inches of compacted gravel. Anchor calls for 3/4 inch minus crushed stone with fines, and the ICC-ES report for compacted crushed stone. Allan Block uses its "wall rock". Compact it in layers and level it carefully: the first course sets the line for the whole wall.
- Bury the first course. The rules differ by brand. Allan Block asks for about 1 inch per foot of wall height on home walls. Versa-Lok asks for at least 6 inches, and Anchor's installation guide says 8 inches or 10% of the wall height, whichever is more. Use your block's rule.
- Fill behind the wall with clean stone. Allan Block and Anchor call for a drainage zone at least 12 inches wide behind the blocks, filled as you go up.
- Add a drain pipe on taller walls. Anchor recommends a 4 inch perforated pipe on walls over 3 ft. Allan Block requires one over 4 ft or where drainage is poor. Both want an outlet at least every 50 ft.
How tall can you go?
Each block has a maximum height as a plain gravity wall, and it shrinks on clay, slopes or anything heavy near the top. Anchor lists 4 ft for Diamond Pro. Allan Block's 6° blocks range from 1 ft 4 in (clay with a load above) to 4 ft 7 in (silty sand, level ground). Above about 4 ft, Anchor and Versa-Lok say to have the wall engineered, usually with geogrid. Many cities need a permit for walls over 3 or 4 ft, so check before you dig.
Frequently asked questions
How many blocks do I need for a retaining wall?
For 8 × 18 inch blocks, count about 1 block per square foot of wall face, including the buried course. Smaller blocks take more: a 6 × 16 inch block works out to about 1.5 per square foot, and a 4 × 12 inch block to 3.
How deep should the first course be buried?
It depends on the block and the wall height. Allan Block asks for about 1 inch per foot of height on home walls, Versa-Lok asks for at least 6 inches, and Anchor's installation guide says 8 inches or 10% of the height, whichever is more.
Do I need a drain pipe behind a retaining wall?
Anchor recommends one on walls over 3 ft. Allan Block requires one over 4 ft, on reinforced walls, or where water collects. On every wall, keep the 12 inch zone of clean stone behind the blocks.
How tall can a retaining wall be without an engineer?
Around 4 ft of exposed height at most, and less on clay, slopes or with a driveway above. Anchor and Versa-Lok both say to engineer walls over 4 ft, and many cities require a permit from 3 or 4 ft.
Last reviewed 2026-10-23. See how we calculate for our sources and assumptions.