How to calculate retaining wall blocks
Rows = Exposed height (in) ÷ Block height (in), rounded up + buried rows
Total blocks = Blocks per row × Rows
Worked example
A wall 20 ft long and 2 ft high, built with 16 × 6 in blocks and one buried row:
- Blocks per row: 240 ÷ 16 = 15
- Rows: 24 ÷ 6 = 4, plus 1 buried = 5
- Blocks: 15 × 5 = 75, or 79 with 5% extra, plus 16 cap stones
Gravel for a retaining wall
Most failed retaining walls fail because of water, not weak blocks. Two gravel zones keep a wall standing:
- Base: a trench about 24 in wide filled with 6 in of compacted crushed gravel. We add 20% for compaction.
- Drainage zone: 12 in of clean crushed stone (#57) behind the wall, full height, with a perforated drain pipe at the bottom that lets water out.
Common block sizes
| Block | Face size | Blocks per sq ft of wall face | Typical use |
|---|---|---|---|
| Small garden wall | 12 × 4 in | 3 | Raised beds, edging, walls under 2 ft |
| Medium | 16 × 6 in | 1.5 | Most DIY walls up to 3–4 ft |
| Large | 18 × 8 in | 1 | Taller walls, terracing |
How many rows to bury
A common rule is to bury 1 in of block for every 8 in of wall height, with a minimum of one full row, or half a block for small units. The buried row locks the wall into the ground so it can't slide forward. Always check the block maker's installation guide, which overrides general rules.
When you need a permit
Many US jurisdictions require a permit, and often an engineer's design, for retaining walls over 3–4 ft or for walls holding back a driveway, slope or structure. Rules vary, so check with your local building department before you start.
Frequently asked questions
How many retaining wall blocks do I need per square foot?
It depends on block face size: 3 per sq ft for 12 × 4 in blocks, 1.5 for 16 × 6 in, and 1 for 18 × 8 in. Add rows that are buried below ground.
How much gravel do I need behind a retaining wall?
Plan 12 in of drainage stone behind the full height of the wall. For a 20 ft wall 2.5 ft tall in total, that is about 1.9 cubic yards, roughly 2.5 tons.
Do I need cap stones?
Caps aren't structural, but they finish the top, protect the blocks from water and give a neat edge. Glue them down with masonry adhesive.
How tall can I build a retaining wall myself?
Most DIY block walls are kept under 3–4 ft. Taller walls usually need geogrid reinforcement, engineering and a permit.
Last reviewed 2026-11-14. Material weights are typical industry averages and vary by supplier, moisture and compaction.