Retaining Wall Calculator
Wall blocks, caps, base gravel and drainage stone for a segmental block retaining wall, buried course included.
Wall blocks to buy
231
blocks
11 courses of 20 blocks, with 6 in buried below ground.
Layout
- Courses, including buried
- 11
- Blocks per course
- 20
- Buried below ground
- 6 in
- Cap blocks
- 20 caps
Base gravel (6 in, compacted)
- Base gravel
- 0.87 tons
- Base gravel
- 0.61 cu yd
Drainage gravel (12 in behind the wall)
- Drainage gravel
- 4.04 tons
- Drainage gravel
- 2.85 cu yd
- Perforated drain pipe
- – ft
Cost
- Blocks and caps
- –
How many retaining wall blocks do I need?
Divide the wall length by the block length to get blocks per course. Add the buried depth (at least 6 inches) to the height above ground and divide by the block height to get the number of courses. Multiply the two and add 5%. A 20 ft wall 3 ft high in 4 × 12 in blocks is 11 courses of 20: 231 blocks plus 20 caps.
Worked example: a 20 ft long, 3 ft high garden wall
- Buried course: bury at least 6 in (or 1 in per foot of height, if more). For a 3 ft wall that's 6 in.
- Courses: 36 in above ground + 6 in buried = 42 in ÷ 4 in blocks = 10.5, so 11 courses.
- Per course: 240 in ÷ 12 in = 20 blocks.
- Blocks: 11 × 20 = 220, plus 5% = 231 blocks, and 20 caps.
- Base: a trench 18 in wide with 6 in of compacted gravel: about 0.87 tons.
- Drainage: 12 in of gravel behind the wall, full height: about 4.04 tons.
Blocks per foot of wall
Blocks for each foot of wall length, per course, by block face size.
| Block face (height × length) | Blocks per foot, per course | Courses per foot of height |
|---|---|---|
| 4 × 12 in | 1 | 3 |
| 4 × 16 in | 0.75 | 3 |
| 6 × 16 in | 0.75 | 2 |
| 8 × 18 in | 0.67 | 1.5 |
Remember to add the buried course: at least 6 in below ground.
The parts of a block retaining wall
- Base trench: 18 in wide for walls under 4 ft, 24 in for taller walls, with at least 6 in of compacted gravel (wall rock).
- Buried course: the first course sits below ground: at least 6 in, or 1 in for every foot of wall height.
- Drainage gravel: 12 in of clean gravel directly behind the blocks, all the way up, so water drains instead of pushing on the wall.
- Drain pipe: a perforated pipe at the back of the base for walls 4 ft and over, or anywhere drainage is poor, vented to daylight.
- Caps: glued on top with masonry adhesive to finish the wall.
These rules come from Allan Block's installation guides. Other brands are similar, but always follow the instructions for the block you buy.
Before you build
- Check the height limit. Walls 4 ft and taller, or walls holding back a slope, a driveway or a building, usually need an engineered design and a permit.
- Measure the block's face, not its depth. The face is what you see from the front.
- Curves and corners need extra blocks and cuts. Raise the extra to 10%.
- Stepped walls on a slope: work out each step as a separate wall.
Frequently asked questions
How much of a retaining wall goes below ground?
At least 6 inches, or 1 inch for every foot of wall height if that is more (Allan Block). That is usually one buried course.
How much gravel do I need behind a retaining wall?
A 12 inch wide column of clean gravel behind the blocks for the full height. For a 20 ft wall standing 3.5 ft including the buried course, that is about 70 cu ft, or 2.6 cubic yards before extra.
How deep should the base be for a retaining wall?
At least 6 inches of compacted gravel in a trench 18 in wide for walls under 4 ft, or 24 in wide for taller walls.
Do I need a permit for a retaining wall?
Often for walls 4 ft or taller, or walls supporting a load such as a driveway or slope. Rules vary, so check with your local building department.
How many retaining wall blocks are in a pallet?
It depends on the block size and the manufacturer, so check the pallet label or ask the store. Our result tells you how many blocks to buy; round up to whole pallets only if the pallet price is better.
Related calculators
Paver CalculatorPavers, base gravel, bedding sand and edging for a patio or driveway.Gravel CalculatorTons, cubic yards and bags of gravel for a driveway, path or patio.Concrete Block CalculatorBlocks, mortar bags and core fill for any block wall, with cost.Landscape Rock CalculatorTons, cubic yards and bags of river rock or decorative stone for beds.
Sources and method
- Allan Block — basic installation — trench 18 in (under 4 ft) / 24 in; 6 in base; wall rock 12 in behind; drain pipe over 4 ft
- Allan Block — reinforced wall construction — buried block minimum 6 in or 1 in per ft of wall height
- Engineering ToolBox — densities of dirt, sand, gravel and other materials — gravel, dry 105 lb/ft³
Last checked October 12, 2026. How we build and test every calculator: methodology.