How this planner works
- Courses. Visible height ÷ block height, rounded up. One more course is buried, or more if 10% of the height is taller than one block.
- Blocks. Round wall length ÷ block face length up for each course, multiply by the number of courses and add the cut allowance. Overall height includes the pad, buried and exposed blocks, and any cap.
- Leveling pad. A trench 6" wider than the block on each side, filled with 6" of compacted gravel (plus 15% for compaction).
- Drainage. A 12"-wide zone of clean gravel behind the wall for its full block height, a perforated pipe along the base, and fabric between the gravel and the soil behind it. For hollow blocks, enter the manufacturer’s gravel volume per block to add core fill separately. A zero entry excludes core fill.
- Caps and adhesive. Wall length ÷ cap length, plus extra; about one tube of adhesive per 15 ft of caps.
Reference guidance:Allan Block retaining wall installation guidance. Confirm material coverage and specifications with your supplier.
Worked example: a 20 ft wall, 2 ft tall
- Standard 6" × 16" blocks: 4 courses above ground plus 1 buried = 5 courses.
- 15 blocks per course × 5 courses, plus 5% = 79 blocks, about 2.8 tons to move.
- Trench: 24" wide and 12" deep. Leveling pad: 1.19 tons of gravel.
- Drainage gravel behind the wall: 2.59 tons, 30 ft of pipe and 90 sq ft of fabric. Any hollow-core fill is extra.
- Caps: 21, with 2 tubes of adhesive.
Frequently asked questions
How tall can I build a retaining wall myself?
Many homeowners build gravity block walls up to about 3 to 4 feet. Above that, many US building departments require a permit and a design from an engineer, and taller walls usually need geogrid reinforcement. Rules vary, so check with your local building department before you start, and follow the block maker's height limits.
How deep should the first course be buried?
A common rule is to bury the first course, or at least 10% of the wall's height, whichever is more. The planner uses that rule. The buried course sits on a compacted gravel leveling pad, usually about 6 inches deep.
Why does a retaining wall need gravel behind it?
Water trapped in the soil behind a wall pushes on it and is a common reason walls bulge or fail. A zone of clean, free-draining gravel about 12 inches wide behind the blocks lets water drain down to a perforated pipe at the base, which carries it away.
Do I need geogrid?
Geogrid is a mesh laid between courses and into the soil behind the wall to tie it back. Many block makers call for it above roughly 3 to 4 feet, or on lower walls that carry a slope, driveway or structure above them. Follow the installation guide for the block you buy.
Do I glue retaining wall blocks?
Most segmental wall blocks interlock with a lip or pins and are not glued, except for the top course and the caps, which are bonded with a concrete or landscape block adhesive so they cannot be knocked off.