Retaining Wall Block Calculator
Work out exactly how many blocks, cap units and how much drainage gravel a small garden or landscape wall needs, with the standard 5% waste allowance built in. Add your own prices for a full cost estimate — nothing here uses a stale national average.
How this calculator works
The calculator works out how many courses of block stack to the target height, how many blocks span the wall's length in each course, then multiplies the two and adds 5% waste for cuts and breakage:
courses = ceil(wall_height / block_height) blocks_per_course = ceil(wall_length / block_length) total_blocks = ceil(courses × blocks_per_course × 1.05) // 5% waste cap_units = ceil(wall_length / cap_length) drainage_gravel_cuyd = (wall_length × wall_height × 1ft_behind_wall) / 27
The gravel figure is a simple baseline assuming a 1 ft-thick zone of crushed gravel behind the full height of the wall — a common rule of thumb for small walls, not a substitute for an engineered drainage design on anything larger.
Worked example
A 20 ft long, 3 ft tall garden wall in standard 16 in × 6 in block, with 16 in caps:
- Courses: ceil(3 ÷ 0.5) = 6 courses (6 in block height = 0.5 ft).
- Blocks per course: ceil(20 ÷ 1.33) = 15 blocks (16 in block length ≈ 1.33 ft).
- Total blocks: ceil(6 × 15 × 1.05) = 95 blocks.
- Cap units: ceil(20 ÷ 1.33) = 15 caps.
- Drainage gravel: (20 × 3 × 1) ÷ 27 ≈ 2.2 cu yd.
At $3.25 per block, $5.50 per cap, and $45 per cubic yard of gravel, that example totals 95 × $3.25 + 15 × $5.50 + 2.2 × $45 ≈$491.
Blocks per square foot by size
| Block size | Face area | Approx. blocks / sq ft |
|---|---|---|
| 16 in × 6 in (standard) | 0.67 sq ft | ~1.5 |
| 18 in × 6 in (large) | 0.75 sq ft | ~1.3 |
| 12 in × 4 in (small) | 0.33 sq ft | ~3.0 |
Measuring your wall
- Wall length — the visible face length, measured straight along the run, or per straight segment if the wall curves or steps.
- Wall height — the exposed face height above finished grade, not including buried base courses.
- Block size — check your product's spec sheet; sizes vary by manufacturer even within a nominal category.
- Base prep — this calculator doesn't include base gravel below the first course, which is a separate, essential layer for any wall.
How much does a retaining wall cost?
Rather than quoting a national average that's stale the day it's published, this calculator prices the job from what you actually enter. Give a price per block, per cap unit, and per cubic yard of drainage gravel — each is itemized and totaled. The estimate doesn't include base gravel, drain pipe, geogrid, excavation, or installation labor.
Frequently asked questions
How many blocks per square foot of wall face?
Divide 144 by the block's length times height in inches to get blocks per square foot. A standard 16 in × 6 in block covers about 0.67 square feet, or roughly 1.5 blocks per square foot — smaller blocks need more pieces per square foot, larger blocks need fewer.
Do I need drainage gravel behind a retaining wall?
Yes, essentially always. A gravel backfill zone behind the wall, typically about a foot thick, lets water drain instead of building up hydrostatic pressure that can crack or push over the wall. Most designs also add a perforated drain pipe at its base.
At what height does a retaining wall need an engineer?
It varies by jurisdiction, but roughly 3 to 4 feet of exposed height is a commonly cited threshold. Above that, engineered design, geogrid reinforcement, and often a permit are typically required — check your local building department, and treat this calculator as covering small garden walls only.
What's a "cap unit" and do I need one?
A cap unit is the flat finishing block on top of the last course, covering the rough top of standard block and helping shed water. It's optional on purely utilitarian walls but standard on any wall meant to be seen.