How the calculation works
Segmental retaining wall blocks are dry-stacked: no mortar, with a lip or pin setting each course slightly back. Counting them is a matter of courses and blocks per course, plus the courses you bury below ground.
- Courses above ground = exposed height ÷ block height, rounded up.
- Total courses = courses above ground + buried courses.
- Blocks per course = wall length ÷ block length, rounded up.
- Caps = one row along the top.
- Gravel = drainage zone behind the wall + a compacted base pad under it.
blocks = (⌈height ÷ block height⌉ + buried) × ⌈length ÷ block length⌉ × (1 + waste %)
drainage gravel (cu ft) = length × built height × zone width
Worked example
A wall 30 ft long with 2 ft showing, built from 6 × 16 in blocks with one buried course: 24 in ÷ 6 = 4 courses above ground, plus 1 buried makes 5. 360 in ÷ 16 = 22.5, so 23 blocks per course. 5 × 23 = 115 blocks, or 121 with 5 percent extra, plus 25 caps. The wall is built 2.5 ft tall, so a 12 in gravel zone behind it is 2.78 cubic yards. The 24 × 6 in base pad adds 1.11 cubic yards, for 3.89 cubic yards in all, roughly 5.4 tons.
Blocks per square foot of wall face
| Face size | Blocks per sq ft | Courses for 4 ft |
|---|---|---|
| 4 × 12 in | 3 | 12 |
| 6 × 16 in | 1.5 | 8 |
| 8 × 16 in | 1.125 | 6 |
| 8 × 18 in | 1 | 6 |
Small 4 in blocks are light and easy to handle, but they are meant for low garden borders. Larger, heavier units are more stable and are the usual choice for walls that hold back real soil. Each manufacturer publishes a maximum unreinforced height for its block; stay below it.
Height limits, permits and engineering
Soil is heavy, and wet soil is heavier. A retaining wall fails by sliding, tipping or bulging, and a failed wall can injure people and damage property. As a rule of thumb:
- Many building codes require a permit, and often an engineered design, for walls over about 4 ft (1.2 m), measured from the bottom of the footing or base.
- Many block systems limit gravity walls (no soil reinforcement) to about 3 ft (1 m) or less.
- Any wall holding back a slope, a driveway, a pool, a building or another wall needs engineering regardless of height.
- Terraced walls interact with each other unless they are spaced far apart. Get advice before stacking terraces.
Rules vary by city and county, so check local code before you buy materials.
Drainage is not optional
Most retaining wall failures trace back to water. Free-draining crushed stone behind the wall stops water pressure from building up. A perforated drain pipe at the base of the gravel zone, run to daylight, carries the water away. Many installers also place filter fabric between the gravel and the soil to keep fines from clogging the stone. The calculator's gravel figure covers the base pad and the drainage zone; the soil behind the zone is your own excavated backfill.
Building tips
- Call your utility-locating service before you dig.
- Get the first course perfectly level. Every error in the base shows up, larger, at the top.
- Compact the base pad in lifts and compact the backfill as each course goes up.
- Glue caps with a masonry adhesive made for landscape block.
For the depth of the trench and how much of the wall goes underground, see how deep a retaining wall base should be.
Frequently asked questions
How many retaining wall blocks do I need for a 30 ft wall?
With 6 × 16 in blocks, a 30 ft wall takes 23 blocks per course. At 2 ft exposed plus one buried course, that is 115 blocks, or 121 with 5 percent extra.
How tall can a retaining wall be without an engineer?
It depends on local code and the block system. Many codes require engineering and a permit above about 4 ft, and many block systems limit unreinforced walls to about 3 ft. Check before you build.
How much gravel goes behind a retaining wall?
A common rule of thumb is a zone of crushed stone about 12 in wide behind the wall for its full height, plus a compacted base pad of about 6 in under the first course.
Do I need caps on a retaining wall?
Caps are not structural on most walls, but they finish the top, shed water and protect the top course. Count one row along the full length.