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 |
Block systems and setback
Segmental retaining wall blocks come in two locking styles. Lipped blocks have a cast ridge on the underside that hooks over the back of the course below, which sets the setback automatically and speeds the build. Pin systems use fiberglass or plastic pins through preformed holes, and many of them offer two hole positions so you can choose a near-vertical face or a steeper batter. Pinned systems cost a little more and give more control.
Setback is the slight backward lean of each course, typically somewhere between a half inch and one and a quarter inches per course depending on the block. That lean is structural rather than cosmetic: it directs the weight of the wall back into the retained soil instead of letting it push outward. Never shim a wall vertical to match a fence line or a house wall, because doing so removes the resistance the design depends on.
Cap blocks finish the top course and are usually glued down with a flexible concrete adhesive rather than mortar, which lets the wall flex slightly without cracking the caps off. Buy caps by linear foot of wall rather than by square foot of face, and remember that corners and curves consume extra because caps have to be cut to close the miter.
The base and the drainage behind it
The buried base course does most of the work. Excavate below grade far enough to bury a full course, or about a tenth of the wall height plus the depth of the leveling pad, and build on 4 to 6 inches of compacted crusher run. Level that first course obsessively in both directions: every error is multiplied by every course above it, and there is no fixing it once the wall is up.
Behind the blocks, 12 inches of clean crushed stone plus a perforated drain pipe at the base is what keeps the wall standing. Saturated soil is dramatically heavier than dry soil and exerts hydrostatic pressure that these walls are not designed to resist. Most failed segmental walls did not fail because the blocks were wrong; they failed because water had nowhere to go. Wrap the stone in filter fabric so soil fines cannot migrate in and clog it, and daylight the pipe to somewhere lower.
Height limits, geogrid, and permits
Most block manufacturers and most jurisdictions treat about 3 to 4 feet as the limit for an unreinforced gravity wall built by a homeowner. Above that, the wall needs geogrid, a stiff mesh laid in horizontal layers extending back into the retained soil at specified intervals, which turns the soil mass itself into part of the structure. Geogrid layout is engineering, not a rule of thumb.
Many building departments require a permit and an engineered design above 4 feet, and some set the threshold at 3. A wall retaining a slope, carrying a driveway or structure above it, or built in tiers has additional requirements regardless of height, because the load is no longer just the soil. Check locally before ordering block: it is a short phone call, and rebuilding a failed wall costs several times what the permit and drawing would have.
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.