How many retaining wall blocks do I need?
Divide the wall face area by the face area of one block: an 8 × 18 in block covers 1 sq ft, so blocks = length × height in feet.
blocks = wall length × total height ÷ block face area
Total height means the visible height plus the buried first row. For a 20 ft wall standing 3 ft above grade with 6 in buried: 20 × 3.5 = 70 sq ft of face, so 70 blocks of 8 × 18 in, or 70 × 1.5 = 105 blocks with a 6 × 16 in face. Curves and steps in the wall change the count, so add each straight run as its own wall in the calculator. Cap units are counted separately along the top, by the length of one cap.
| Unit | Face (height × length) | Face area | Units per sq ft | Source |
|---|---|---|---|---|
| Standard (e.g. AB Classic) | 8 × 18 in | about 1 sq ft | 1 | [1] |
| Half-height (e.g. AB Lite) | 4 × 18 in | 0.5 sq ft | 2 | [1] |
| Versa-Lok Standard | 6 × 16 in | 0.667 sq ft | 1.5 | [2] |
Allan Block lists about 11 standard blocks per square meter of face.[1] Other brands use different faces, so pick Custom block size and enter the face from the product sheet.
Retaining wall block calculator: blocks for common walls
A 30 ft wall 3 ft tall (including the buried row) takes 90 standard blocks or 135 with a 6 × 16 in face.
| Wall length | 2 ft, 8×18 | 2 ft, 6×16 | 3 ft, 8×18 | 3 ft, 6×16 | 4 ft, 8×18 | 4 ft, 6×16 |
|---|---|---|---|---|---|---|
| 10 ft | 20 | 30 | 30 | 45 | 40 | 60 |
| 20 ft | 40 | 60 | 60 | 90 | 80 | 120 |
| 30 ft | 60 | 90 | 90 | 135 | 120 | 180 |
| 50 ft | 100 | 150 | 150 | 225 | 200 | 300 |
Base gravel and drainage rock
A gravity wall sits on a compacted gravel base and has at least 12 inches of free-draining rock behind it.
- Walls 4 ft and taller: dig a trench at least 24 in wide and 12 in deep and place at least 6 in of compacted wall rock.[3]
- Shorter walls: an 18 in wide, 10 in deep trench with 4 in of wall rock is acceptable.[3]
- Behind the wall: fill the block cores and at least 12 in behind the wall with wall rock, compacting in lifts of 8 in or less.[3]
- Drain pipe: required for walls over 4 ft or in silty or clay soils.[3]
| Item | Size | Cubic feet | Cubic yards |
|---|---|---|---|
| Base, wall under 4 ft | 18 in wide × 4 in deep | 5 | 0.19 |
| Base, wall 4 ft and taller | 24 in wide × 6 in deep | 10 | 0.37 |
| Drainage rock, 3 ft wall | 12 in thick × 3 ft high | 30 | 1.11 |
| Drainage rock, 4 ft wall | 12 in thick × 4 ft high | 40 | 1.48 |
Turn the cubic yards into tons with the stone calculator, which handles crushed stone and crusher run, or size a longer base with the gravel calculator.
How deep to bury the first row
Bury at least 6 inches of block, and for walls over 6 ft add 1 inch of burial for every extra foot of height.[4][3]
The buried row still counts as wall face, which is why the calculator asks for the total height. Check the plans or the manufacturer's chart for slopes or loads above the wall, which call for more.
When does a retaining wall need an engineer?
The model residential code requires a designed wall when it is not supported at the top and retains more than 48 inches of unbalanced fill.[5]
Check before you buildLocal codes, slopes, driveways or buildings above the wall, and clay soils all lower the height a simple gravity wall can safely hold. Manufacturers state that final designs for construction must come from a local registered engineer. These numbers are for estimating materials only.
Common retaining wall mistakes
- Counting only the visible height: the buried row needs blocks too.
- Skipping the drainage rock: the 12 in of rock behind the wall relieves water pressure.[3]
- Thin base: under-built bases are where walls start to lean; follow the trench sizes above.[3]
Frequently asked questions
How many blocks do I need for a retaining wall?
Length × total height in feet for standard 8 × 18 in blocks (1 per sq ft). A 20 × 3.5 ft wall takes 70 blocks.
How much gravel do I need behind a retaining wall?
At least 12 inches of wall rock behind the blocks: about 1.1 cubic yards per 10 ft of a 3 ft wall.[3]
How deep should the first row of a retaining wall be buried?
At least 6 inches, plus 1 inch per foot of height above 6 ft.[4]
How tall can a retaining wall be without an engineer?
Under the model residential code, walls retaining more than 48 inches of unbalanced fill must be designed; local rules can be stricter.[5]