How this retaining wall calculator works
A segmental wall is a grid of blocks, so the count is two rounded divisions — courses up the height, blocks along the length — plus the gravel that makes the wall actually last:
wall blocks = courses × blocks per course → × (1 + waste) · caps = one course
At the defaults — a 20 ft long, 3 ft high wall in 12 × 8 in block — that is 5 courses of 20, 105 wall blocks with waste plus a 20-block cap course, and about 4.25 tons of base and backfill gravel — roughly $731 in materials; the full chain is in the worked example below.
The parts of a segmental retaining wall
A dry-stacked block wall is more than the blocks you see — it is a drainage system with a face on it. The pieces:
- Wall blocks: the interlocking units — a lip or pin sets each course back for batter. Common faces are 12 × 8 in and 18 × 6 in.
- Cap blocks: the finished top course, glued on with masonry adhesive; they shed water and hide the cores.
- Leveling pad: 4–6 in of compacted crushed stone under the base course — the foundation the whole wall rides on.
- Drainage stone: clean #57 backfilled behind the wall, with a perforated pipe to daylight — priced on the gravel calculator.
- Geogrid: soil-reinforcement layers tied into the wall on anything tall or surcharged — engineer-specified.
Base, burial, drainage, and how high you can go
Most failed walls fail at the bottom or from water, not from the blocks. The rules that keep one standing:
- Bury the base: one inch of block below grade per foot of height, at least one full course — set the wall height in the calculator to include it.
- Level the pad: 4–6 in of compacted stone, dead flat; the first course sets every course above it.
- Drain it: clean stone and a perforated pipe behind the wall — water pressure is what pushes walls over.
- Batter: the built-in setback leans the wall into the slope a little each course.
- Height limit: 3–4 ft is the usual no-engineer ceiling; taller, or with a slope or driveway above, needs geogrid and a design.
Blocks per square foot, and a whole wall by size
Table 1 is the nominal block count per square foot of wall face by block size; Table 2 runs the standard 12 × 8 in block out to common walls — courses, blocks, caps and gravel.
| Block face | Blocks / ft² |
|---|---|
| 12 × 8 in | 1.50 |
| 18 × 6 in | 1.33 |
| 16 × 6 in | 1.50 |
| 12 × 4 in | 3.00 |
| Wall | Courses | Blocks | Caps | Gravel |
|---|---|---|---|---|
| 10 × 3 | 5 | 53 | 10 | 2.25 t |
| 20 × 3 | 5 | 105 | 20 | 4.25 t |
| 30 × 3 | 5 | 158 | 30 | 6.25 t |
| 20 × 4 | 6 | 126 | 20 | 5.25 t |
How to measure for a retaining wall
- Measure the wall length along the face; for a curved or stepped wall, pace the run and total the segments.
- Set the total height — exposed height plus the buried base course, from the build guide.
- Pick the block face so the course and per-course counts are right for your unit.
- Leave the cap course on unless you’re coping the top another way, and set 5% waste for cuts on curves and ends.
- Read blocks, caps and gravel, then add pipe, fabric, adhesive and any geogrid from the exclusions below.
Worked example: a 20 × 3 ft retaining wall
The default, worked line by line — 20 ft long, 3 ft high, 12 × 8 in block, cap course, 6 in base pad, 5% waste:
wall blocks = 5 × 20 = 100 → ×1.05 = 105 + 20 caps
gravel = (pad 0.74 + backfill 2.22) = 2.96 yd³ × 1.4 = 4.25 t
cost = $420 + $120 + $191 = $731
Go one foot taller to 4 ft and it is a sixth course — 126 blocks and 5.25 tons of gravel, and you are at the height where local code usually starts asking for an engineer. Notice the gravel is a real line: the drainage stone behind a wall routinely costs more than people budget, because a wall without it is a wall on borrowed time.
What this retaining wall estimate doesn’t include
The blocks, caps and gravel are the bulk of the material, but a wall that lasts needs more:
- Drain pipe and fabric. A perforated pipe to daylight and filter fabric between soil and drainage stone.
- Cap adhesive. Masonry construction adhesive to set the cap course.
- Geogrid. Soil-reinforcement grid for tall or surcharged walls — sized by an engineer.
- Excavation and compaction. Digging the base trench and compacting the pad and backfill in lifts.
- Base and drainage stone haul. Delivery of the crushed stone, priced by the load.
- Engineering and permits. Required over the local height threshold — this sizes a standard wall, it isn’t a design.
Retaining wall FAQ
How many blocks do I need for a retaining wall?
Courses = wall height ÷ block height; blocks per course = wall length ÷ block length, both rounded up. A 20 ft long, 3 ft high wall in standard 12 × 8 in block is 5 courses of 20 — 100 wall blocks, 105 with 5% waste, plus a 20-block cap course. That is about $540 in block and cap at the default prices.
How high can I build a retaining wall without an engineer?
Most jurisdictions let a homeowner build a segmental wall up to 3 to 4 ft of exposed height without a permit or engineered design; taller walls, or walls with a slope or surcharge above them, need geogrid reinforcement and an engineer. Always check local code — the threshold is often exactly 4 ft.
How much gravel do I need for a retaining wall?
Two places: a compacted leveling pad under the wall and a drainage chimney of clean stone behind it. The default 20 × 3 ft wall works out to about 2.96 cubic yards — 4.25 tons — with a 6 in base pad and a 12 in drainage zone. The calculator sizes both from the wall dimensions.
How many retaining wall blocks for a 30 foot wall?
A 30 ft long wall at 3 ft high in 12 × 8 in block is 5 courses of 30 — 150 wall blocks, 158 with 5% waste, plus a 30-block cap course, and about 6.25 tons of base and backfill gravel. Longer walls scale the per-course count; taller ones add courses.
Do you need drainage behind a retaining wall?
Yes — water building up behind a wall is what pushes it over. Backfill the first foot behind the blocks with clean #57 drainage stone (never soil), run a perforated drain pipe along the base to daylight, and separate the stone from soil with filter fabric. The gravel figure here covers the stone, not the pipe.
How deep should the base of a retaining wall be?
Bury the bottom course: a rule of thumb is one inch of buried block for every foot of wall height, and at least one full course below grade. Under that sits a leveling pad of 4 to 6 in of compacted crushed stone, dead flat — the base course sets the whole wall, so it is worth the time.
How much does a retaining wall cost?
The default 20 × 3 ft wall is about $731 in materials — block, caps and gravel — at the prices shown. Installed by a contractor, segmental block walls commonly run $25 to $60 per square foot of wall face in 2026, so labor, drainage pipe and any geogrid are most of the delivered cost.
Sources and standards
Block counts are geometry; the wall rules come from manufacturer specs and local code:
- NCMA / SRW manufacturer specs: block face sizes, batter setback and the base-burial rule of thumb.
- IRC / local code: the exposed-height threshold (often 4 ft) above which a permit and engineered design are required.
- Drainage practice: clean stone backfill, perforated pipe to daylight, and filter fabric behind the wall.
- The engineer of record: tall or surcharged walls are designed with geogrid — this is a planning tool, not a spec.
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