How Many Blocks Do I Need?
Blocks per course = wall length in inches ÷ block face length
Courses = (height above ground − cap height) ÷ block face height, plus buried courses
Multiply them, add about 5% for cuts and breakage, and round up.
Worked example. A 20 ft garden wall, 24 in high, built from 4 × 11.5 in blocks with a 3 in cap row:
- Blocks per course: 240 in ÷ 11.5 in = 20.9, so 21.
- Courses above ground: (24 − 3) ÷ 4 = 5.25, so 5 courses, which builds to 23 in with the cap.
- Buried courses: 1 (at least one full course, and at least 1 in per foot of height).
- Blocks: 6 courses × 21 = 126, plus 5% = 133 blocks.
- Caps: 20 ft ÷ 12 in caps = 20, plus 5% = 21 caps.
- Gravel: about 1 ton for the base and 2 tons for drainage, so order 3.5 tons.
| Block face | Blocks per sq ft | Courses per foot of height |
|---|---|---|
| 4 × 11.5 in | 3.13 | 3.0 |
| 6 × 16 in | 1.50 | 2.0 |
| 8 × 18 in | 1.00 | 1.5 |
The Parts of a Block Wall
Allan Block’s basic installation guide, which other manufacturers’ DIY instructions broadly match:
- Trench and base. Dig a trench about 24 in wide and compact a base of at least 6 in of crushed rock. Basalite’s instructions extend the base at least 6 in in front of and behind the first course.
- Buried course. Set the first course level on the base, below finished grade. Bury about 1 in of block per foot of wall height; in practice, bury at least the whole first course.
- Drainage zone. Fill at least 12 in behind the blocks with clean crushed stone as you build, the full height of the wall.
- Drain pipe. A perforated pipe at the back of the base, vented to daylight, is needed for taller walls and poorly draining soils.
- Caps. Glue the cap row down with masonry adhesive.
Know Your Height Limit
Small block walls are gravity walls: their own weight holds the soil back. Each block system has a maximum height for that:
- some small 4 in garden blocks are rated for 2 ft;
- Basalite’s gravity walls go up to 3 ft, including the buried course;
- Allan Block’s gravity walls go up to 4 ft.
Past that, walls need geogrid reinforcement and, per HomeGuide’s cost data, walls over 4 ft typically need a structural engineer. Any wall holding back a driveway, a building, or a steep slope needs engineering at any height. The calculator flags walls over 3 ft and over 4 ft.
Want the Cost?
The retaining wall cost calculator uses the same inputs and adds installation labor per square foot of wall face.
Frequently Asked Questions
How Many Retaining Wall Blocks Do I Need?
Divide the wall length by the block’s face length for blocks per course, and the height by the block’s face height for courses. Add at least one buried course, then about 5% for cuts. A 20 ft wall of 4 × 11.5 in blocks, 24 in high with caps, needs about 133 blocks and 21 caps.How Much of a Retaining Wall Should Be Buried?
Allan Block’s guide buries about 1 in of block per foot of wall height, on top of a 6 in compacted base. In practice, bury at least the whole first course.How Tall Can a Retaining Wall Be Without an Engineer?
It depends on the block. Manufacturer limits for gravity walls without reinforcement range from 2 ft (some small garden blocks) to 3 ft (Basalite) or 4 ft (Allan Block). Walls over 4 ft generally need an engineer’s design and often a permit.What Gravel Goes Behind a Retaining Wall?
Use clean crushed stone, about 12 in wide behind the blocks and the full height of the wall, so water drains instead of building up pressure. Base trenches use compactible gravel with fines.Sources
- Basic installation (Allan Block)
- Typical retaining wall installation instructions (Basalite)
- 4 in × 11.5 in wall block installation (Keystone, via Lowe's)
- Concrete retaining wall cost (HomeGuide, updated Dec 2025)
- Gravel prices (HomeGuide, updated Dec 2025)
Prices are national ballparks checked October 2026. Local prices vary.