Retaining Wall Calculator
- Updated
- Imperial and metric
- Free, no sign-up
Results
79 wall blocks in 5 courses and 21 caps
How to use the retaining wall calculator
- Measure the total length of the wall along its face. Add straight runs and curves together.
- Enter the exposed height: from finished grade in front of the wall to the top of the blocks, not counting caps.
- Choose your block's face size, or enter a custom length and height from the manufacturer's specs.
- Set the number of buried courses. Most installation guides bury at least one course below grade.
- Check the base trench, base depth and drainage zone, then add prices per block and cap for a cost estimate.
Retaining wall formula
Blocks = Wall length ÷ Block length × Courses × (1 + waste %)
Caps = Wall length ÷ Cap length × (1 + waste %)
Base (ft³) = Wall length × Trench width × Base depth × (1 + compaction %)
Drainage stone (ft³) = Wall length × Zone width × Height of all courses × (1 + 5%)
Walls are built in whole courses, so the exposed height is rounded up: a 2 ft 3 in wall in 6 in blocks takes 5 courses above grade, not 4.5. Segmental blocks are stacked dry, without mortar, so the face size is the laying size. (Mortared concrete block is different; use the concrete block calculator for that.) Weights use 1.5 tons per cubic yard for crusher run and 1.4 for clean crushed stone.
Worked example: a 24 ft garden wall
A wall 24 ft long with 2 ft showing, in 16 × 6 in blocks, has 4 courses above grade plus 1 buried: 5 courses of 18 blocks. That is 90 blocks, or 95 with 5% extra. At 12 in per cap it needs 26 caps, including waste.
The 24 in wide, 6 in deep base takes 27.6 ft³ of crusher run with 15% for compaction, about 1.53 tons, in a trench about 12 in deep. A 12 in drainage zone behind the full 2.5 ft of block is 2.33 yd³ of clean stone, about 3.27 tons. Add 24 ft of perforated pipe, plus the run to its outlet, and 60 ft² of filter fabric.
Blocks per 10 ft (3 m) of wall
With one buried course and no waste. Courses are rounded up, so the top of the wall may sit slightly above the height in the column.
| Block face | 1 ft exposed | 2 ft exposed | 3 ft exposed |
|---|---|---|---|
| 12 × 4 in (small garden wall block) | 40 (4 courses) | 70 (7 courses) | 100 (10 courses) |
| 16 × 6 in | 23 (3 courses) | 38 (5 courses) | 53 (7 courses) |
| 18 × 8 in | 20 (3 courses) | 27 (4 courses) | 40 (6 courses) |
| Block face (per 3 m) | 0.3 m exposed | 0.6 m exposed | 0.9 m exposed |
|---|---|---|---|
| 300 × 100 mm | 40 (4 courses) | 70 (7 courses) | 100 (10 courses) |
| 400 × 150 mm | 23 (3 courses) | 38 (5 courses) | 53 (7 courses) |
| 450 × 200 mm | 20 (3 courses) | 27 (4 courses) | 40 (6 courses) |
Gravel, pipe and fabric per 10 ft of wall
For 16 × 6 in block with one buried course, a 24 × 6 in base and a 12 in drainage zone. Perforated pipe is 10 ft for every 10 ft of wall.
| Exposed height | Base gravel | Drainage stone | Filter fabric |
|---|---|---|---|
| 1 ft | 0.64 tons | 0.82 tons (0.58 yd³) | 15 ft² |
| 2 ft | 0.64 tons | 1.36 tons (0.97 yd³) | 25 ft² |
| 3 ft | 0.64 tons | 1.91 tons (1.36 yd³) | 35 ft² |
Why the base and drainage matter
The base is a levelling pad of compacted crusher run (road base). The first course must be level along its length, because any error grows as the wall rises.
Water is the main enemy of a retaining wall. Saturated soil is heavier and pushes harder on the blocks, and in cold climates it can freeze. A zone of clean, free-draining stone directly behind the blocks, a perforated pipe at the bottom that drains to daylight or another outlet, and filter fabric between the stone and the soil let water escape without clogging the stone.
Tips for building a block retaining wall
- Check the rules first. Many block manufacturers and local codes require engineering, geogrid reinforcement or a permit for walls over about 3–4 ft (1–1.2 m), and sometimes for lower walls with a slope, driveway or building above them. Ask your building department before you dig.
- Call before you dig. In the US, call 811 to have buried utility lines marked before digging the trench.
- Follow the block's setback. Most segmental blocks have a lip or pins that set each course back slightly, so the wall leans into the slope. Corners, curves and steps may need corner blocks or extra cuts, so allow more waste.
- Fill hollow cores if required. Some blocks have cores that are filled with drainage stone. The calculator does not include that volume; get it from the manufacturer's specs.
- Fix the caps. Caps are usually bonded with a masonry or concrete adhesive made for the purpose.
- Planning a patio above the wall? Size it with the paver calculator, and order bulk stone with the gravel calculator.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block length to get blocks per course, then multiply by the number of courses, including the buried ones. A 20 ft wall with 3 ft showing, in 16 × 6 in blocks, has 6 courses above grade plus 1 buried, at 15 blocks per course: 105 blocks, or 111 with 5% extra.
How deep should the first course of a retaining wall be buried?
Most block manufacturers call for at least one full course below grade, and more for taller walls or walls at the bottom of a slope. The installation guide for your block gives the exact depth, so check it before you dig. Below the buried course sits a compacted gravel base, commonly about 6 inches (15 cm) deep.
What gravel do I need for a retaining wall?
Two kinds. The base is compactable crusher run (road base), typically 6 inches deep in a trench about 24 inches wide. Behind the wall, clean crushed stone such as #57 lets water drain. For a 20 ft wall with 3.5 ft of block (3 ft exposed plus one 6 in course buried), the base takes about 1.3 tons including 15% for compaction, and a 12 inch drainage zone about 3.8 tons.
Does a retaining wall need drainage?
Yes. Water trapped behind a wall adds pressure and is a common cause of bulging and failure. A typical detail is a 12 inch (30 cm) zone of clean stone behind the blocks, a perforated drain pipe at the bottom that drains to daylight, and filter fabric between the stone and the soil to keep it from clogging.
When does a retaining wall need a permit or an engineer?
Rules vary by location. Many block manufacturers and local codes require engineering, geogrid reinforcement or a permit for walls over about 3 to 4 ft (1–1.2 m), and sometimes for lower walls that hold back a slope, a driveway or a building. Check with your local building department and the block manufacturer before you build.