Retaining Wall Calculator
Blocks, caps, and gravel for a segmental retaining wall.
Walls over 3 to 4 ft usually need engineering and a permit.
You need
134 blocks
Includes the extra percentage for the buried first course and cut blocks.
- Cap units (12 in)
- 20 caps
- Wall face area
- 40 sq ft
- Base trench gravel
- 0.37 cu yd
- Drainage backfill
- 1.48 cu yd
Estimates only. Verify quantities before ordering.
How this is calculated
Blocks = wall face area / block face area, plus waste, rounded up. Caps assume 12 in units, one per foot of wall. Base gravel fills a 12 in wide x 6 in deep trench; drainage backfill assumes a 12 in wide column of gravel the full height of the wall.
Enter the wall's length and exposed height, pick your block size, and this calculator counts the blocks and caps plus the two gravel quantities every segmental wall needs: a compacted base under the first course and free-draining backfill behind the wall. Blocks are counted by face area, so the math works for any running-bond pattern.
One course of blocks should also be buried below grade for every 8 inches of exposed height (minimum one course), which the waste factor helps cover on short walls. Anything over 3 to 4 feet tall, or supporting a slope or driveway above it, moves out of DIY territory: most codes require an engineered design and a permit at that point.
Blocks for common wall sizes
Rows use 12 x 4 in blocks with 10% extra for the buried course and cuts.
| Wall size | Blocks | Caps | Backfill |
|---|---|---|---|
| 10 ft long x 1.5 ft tall | 50 blocks | 10 caps | 0.56 cu yd |
| 20 ft x 2 ft | 134 blocks | 20 caps | 1.48 cu yd |
| 30 ft x 2 ft | 200 blocks | 30 caps | 2.22 cu yd |
| 40 ft x 3 ft | 400 blocks | 40 caps | 4.44 cu yd |
| 50 ft x 3 ft | 500 blocks | 50 caps | 5.56 cu yd |
Common questions
How many retaining wall blocks per square foot?
Divide by the block's face area: the common 12 x 4 inch garden wall block covers a third of a square foot, so it takes three per square foot of wall face. A 16 x 6 inch block covers two thirds of a square foot, about 1.5 per square foot.
Does the first course really need to be buried?
Yes. Bury at least one full course, or about 10% of the wall height for taller walls, on a compacted gravel base. The buried course locks the wall's toe so soil pressure cannot kick the bottom forward, and skipping it is the most common cause of leaning garden walls.
What goes behind a retaining wall?
A column of clean crushed gravel at least 12 inches deep, ideally with a perforated drain pipe at the base sloped to daylight. Water, not soil, pushes walls over: saturated backfill roughly doubles the pressure, and the gravel plus pipe give it a way out.
How tall can I build without an engineer?
Most manufacturers rate their standard blocks for gravity walls up to 3 or 4 feet in good conditions, and many codes require permits and engineering above 4 feet measured from the bottom of the footing. Slopes, driveways, or fences above the wall lower that threshold, so check locally before building tall.