Retaining Wall Calculator
Segmental wall blocks, caps and the stone behind them — base trench, drainage column, tons, on one page.
Retaining Wall Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Straight walls; curves and corners enter as length.
- No geogrid, adhesive or pipe pricing on this page.
- Surcharges (slopes, drives above) are not modeled.
In short: A 24 × 3 ft wall in 16×6 blocks: 18 per course × 6 courses = 108 → 119 ordered at 10%, 18 caps. Stone: 0.888889 yd³ base + 2.666667 yd³ drain = 5.155556 tons of crusher run.
Formula
blocks/course = ⌈L × 12 ÷ face⌉; courses = ⌈H × 12 ÷ block height⌉
base yd³ = L × 2 × 0.5 ÷ 27
drain yd³ = L × H ÷ 27 (12-in column)
tons = (base + drain) × 1.45
[('the face', 'blocks × courses — no waste yet'), ('12-in column', 'free-draining stone behind the wall'), ('1.45 t/yd³', 'crusher run, the gravel page’s density'), ('~4 ft', 'the common engineered-design threshold')]
Worked Example
- Grid the face: blocks per course × courses.
- Apply waste, then add caps as one course.
- Trench the base: twice the depth wide, 6 in compacted.
- Column the drain: L × H ÷ 27 behind the face.
- Convert stone to tons and check the height gate.
The 24 × 3 wall in 16×6: 18 × 6 = 108 → 119 ordered, 18 caps. Base 0.888889 yd³, drain 2.666667 yd³ (L×H÷27 exact) — 3.555556 yd³ × 1.45 = 5.155556 tons of stone. Embedment: 3 in of the first course below grade.
Strengths & Limits Of This Model
Where this engine is strong
- Drainage stone quantified, not forgotten
- Embedment and caps wired to the module
- Height gate honest, non-prescriptive
Where it stops
- No geogrid or pipe take-off
- Single straight run per computation
Practical Use Cases
Garden walls
Block and stone for the 4-and-under class.
Terraces
Tiered walls computed run by run.
Drainage orders
The stone column, priced in tons.
Cap orders
One course of caps, counted once.
Permit talks
Quantities in hand before the counter.
Methodology & Editorial Standards
The face grids on the entered block module (ceiling to whole blocks and courses); waste applies to the grid, caps order as one course. Base stone follows the twice-the-depth × 6-in trench rule; drainage the exact L×H÷27 column; both convert at 1.45 t/yd³ crusher run. The height gate is written non-prescriptively — jurisdictions differ; the page prices materials and says so. Heights over 50 ft are refused.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Retaining Wall Calculator — 10 Expert FAQs
10 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many retaining wall blocks do I need?
Blocks per course × courses: ⌈L×12 ÷ face⌉ × ⌈H×12 ÷ height⌉, then 5-15% waste. A 20-ft × 3-ft wall of 8-in blocks is 30 × 4.5 → 5 courses ≈ 150 + cuts.
How deep should the base trench be?
About 6 inches of compacted crusher run under a first course buried at least half its height — a 12-inch dig for a 6-inch block. Width runs twice the block’s depth so the face has room to work.
How much gravel goes behind a retaining wall?
A free-draining column about 12 inches deep behind the face: L × H ÷ 27 cubic yards, plus the base. Our 24 × 3 example: 2.666667 yd³ behind, 0.888889 under — 5.16 tons together.
How high can I build without an engineer?
Most jurisdictions draw the line at 3-4 feet — past it, permits and an engineered design (geogrid, heavier units, deeper base) are the norm. Walls also carry surcharges: a driveway above the wall changes the question. Check locally; this page prices materials.
Do retaining walls need drainage pipe?
Behind anything over knee height, yes — a perforated pipe at the base of the stone column, fabric-wrapped or socked, pitched to daylight. The stone carries the water to the pipe; the pipe carries it away.
Why bury the first course?
Embedment is ballast: half the first block below grade keeps the stack from kicking out at the toe, and it costs one course of digging. Add a course of burial where the ground slopes in front.
Do I need geogrid?
For taller walls, or any wall carrying a slope or load above it, geogrid layers tie the face into the backfill — that is engineered territory, priced by the design, not by this page. Under 4 feet with honest drainage, segmental units usually stand alone.
How many cap blocks do I order?
One per top-course position — the same count as blocks per course. Caps are thinner, smoother units that shed rain off the top; they are bonded with construction adhesive, not mortar.
Can soil be used as backfill right against the wall?
No — soil holds water, and held water is the pressure that fells walls. Free-draining crushed stone against the block, fabric between stone and soil, pipe at the bottom. Gravel, never dirt, behind the face.
Should the wall lean back?
A slight batter — many systems step each course back about half an inch — tips the weight into the hill. Follow the system’s lugs or pins; the count on this page does not change, the set-out does.