This retaining wall calculator estimates segmental wall blocks, cap blocks and drainage gravel for a landscape retaining wall from its length, height and block size.
Structure honesty: one full course is buried below grade (counted here) on a compacted gravel leveling pad, and drainage gravel (12″ column behind the wall) with perforated pipe is what keeps landscape walls from leaning — hydrostatic pressure, not soil weight, kills most DIY walls. Walls over ~4 ft, terraces and anything holding a slope near structures generally need engineering and geogrid; many jurisdictions require permits above 3–4 ft.
Blocks = Courses × Per-Course, Plus the Buried One Everyone Forgets
Segmental walls are counting problems wearing landscaping clothes: length ÷ block face for courses, exposed height ÷ block height for rows — plus the below-grade course and leveling pad that separate a wall from a pile. Concrete-block math for structural CMU is our Concrete Block Calculator. Part of our Construction Calculators collection.
Worked Example (Verified)
- 10 ft × 2 ft high, 12×4″ blocks (defaults): per course 10, visible courses 6, +1 buried = 7 → 70 × 1.10 → 77 blocks; caps 11; drainage gravel ≈ 0.93 yd³
- Height discipline: a 3-ft wall in 6″-tall blocks needs 6 visible courses + buried — block height choice halves or doubles your stacking labor at the same wall size
- Curves and corners consume the 10% waste allowance genuinely (cuts concentrate there) — straight runs may return a few blocks; plan cuts before ordering adhesive for caps
- The gravel math surprises everyone: a 20-ft, 3-ft wall wants ~2.6 yd³ of drainage stone + pad — order it with the blocks, not after
Build Notes That Outlive the Estimate
- The first course is the wall: level it obsessively (it’s the only course you can level), on 6″ of compacted angular gravel.
- Backfill drainage stone in lifts as you stack, with landscape fabric separating stone from soil (fines migration clogs drainage silently).
- Setback (batter) is built into most block lips — trust the lip; vertical faces above ~3 ft want engineering review.
- Cap adhesive is construction adhesive rated for concrete; caps overhang ~½–1″ for the finished shadow line.
Related Tools
- Concrete Block Calculator — CMU counts for structural walls
- French Drain Calculator — the drainage behind and below
- Construction Calculators — the full collection
Frequently Asked Questions
How many blocks do I need for a retaining wall?
Per course: wall length ÷ block face length. Courses: visible height ÷ block height, plus one buried course. Multiply, add 10% for cuts and breakage. A 10-ft, 2-ft wall with 12×4″ blocks: 10 per course × 7 courses × 1.10 ≈ 77 blocks.
Why bury one course of blocks?
The buried course anchors the wall against sliding and undermining, and keeps the base below frost and erosion lines. It’s the most commonly skipped structural element in DIY walls — and the first one inspectors and manufacturers ask about.
How much gravel goes behind a retaining wall?
A drainage column at least 12″ wide behind the wall for its full height (plus the leveling pad below), roughly length × 1 ft × (height + 0.5) ÷ 27 in cubic yards — about 0.9 yd³ for the defaults here. Use clean, angular ¾” stone, wrapped in fabric, with a perforated drain pipe daylighted at the ends.
When does a retaining wall need an engineer?
Rules of thumb: exposed height over ~4 ft, walls supporting slopes, driveways or structures, tiered walls close together, or poor soils. Many codes require permits above 3–4 ft regardless. Geogrid reinforcement and drainage design are engineering territory — this calculator estimates materials for landscape-scale walls only.
Do I need cap blocks?
Structurally they’re optional; practically they finish the wall, lock the top course, and shed water. Count = length ÷ cap length + 10%. Trapezoid caps exist for curves; straight caps on curves mean many cuts — budget the waste.
Is this retaining wall calculator free?
Yes — free, instant, no sign-up.