Retaining Wall Block Calculator

How many blocks a segmental retaining wall takes — per course and total, with caps — plus the two quantities everyone forgets: the base gravel trench under it and the drainage backfill behind it, both in yards and tons.

Height above grade. Walls over ~3–4 ft typically need engineering and a permit.
Most systems lean back 3/4–1″ per course; shown as total batter.
5% straight runs; 10% with curves and steps.
Base trench & backfill
6″ compacted is the common figure.
Block depth + ~12″ total working room is typical guidance.
At least 12″ of clean stone directly behind the wall.

Structural disclaimer: block sizing, wall height, geogrid reinforcement and drainage design must follow your project plans, local code, and an engineer where required — this tool estimates quantities only. Walls above roughly 3–4 ft of exposed height commonly require engineering and a permit; check before you build. Densities are typical figures — confirm with your supplier.

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Ordering the stone? Both gravel figures here convert to a supplier order on the Gravel Calculator — road base/crusher run preset for the trench, crushed stone #57 for the drainage backfill.

How much gravel for a retaining wall base?

Wall length × trench width × 6″ of compacted depth — for a typical trench about a foot wider than the block, that’s roughly 0.3 cubic yards (about half a ton) of base gravel per 10 feet of wall.

base gravel yd³ = lengthft × (trench widthin ÷ 12) × (depthin ÷ 12) ÷ 27
tons ≈ yd³ × 1.55 (compacted road base)

The trench is dug wider than the block — common guidance is the block depth plus about 12″ of total working room — and filled with 6″ of well-graded crushed stone, compacted in lifts and screeded level. A 30 ft wall with a 20″-wide trench at 6″ needs about 0.93 yd³ ≈ 1.4 tons. The first course of block then sits partly buried in that trench, which is why the calculator adds a buried course by default.

How many blocks for a retaining wall?

blocks per course = wall length ÷ block length
courses = wall height ÷ block height (+ 1 buried course)
total = per course × courses × (1 + waste)

A 30 ft wall, 2 ft exposed, in 12×4″ garden blocks: 30 per course × 7 courses (6 exposed + 1 buried) = 210 blocks plus 5% waste — about 221, and 30 caps. Block face sizes here are generic presets; enter your actual unit’s dimensions for anything precise, since segmental systems vary.

Retaining wall backfill: the drainage stone behind the wall

Water, not soil, is what pushes walls over — so at least 12″ of clean, angular drainage stone goes directly behind the blocks for the full height, usually with a perforated drain pipe at the base and fabric between stone and soil. The volume is simply length × height × the 12″ zone:

backfill yd³ = lengthft × total heightft × (zone widthin ÷ 12) ÷ 27  ·  tons ≈ yd³ × 1.4

For the 30 ft × 2.33 ft (incl. buried course) example, that’s about 2.6 yd³ ≈ 3.6 tons of stone — often more material than the base itself, and the piece most first-time builders under-order.

Setback (batter) and what it does to height

Segmental blocks lean into the slope by design — a lip or pin steps each course back, commonly 3/4–1″ per course. The calculator shows the total batter so you can see how far the top of the wall lands behind the base; it doesn’t change block counts, but it matters for layout against property lines and fences.

When a wall needs an engineer

Most jurisdictions draw a line around 3–4 ft of exposed height (often 4 ft including the buried course), above which a wall needs engineered design — and walls with slopes, driveways or structures above them can need it at any height. Geogrid layers, where required, come from the engineered design and the wall system’s own documentation, which is why this calculator deliberately doesn’t guess at them. Treat everything here as a materials takeoff, not a design.

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Frequently asked questions

How many retaining wall blocks per square foot?

144 ÷ (block length × height in inches): a 12×4 block is 3 per ft² of wall face; an 18×6 is 1.33. Multiply by face area (including the buried course) and add waste.

How deep should the base trench be?

Common practice is 6″ of compacted gravel plus the buried first course — so the trench runs roughly 10–12″ deep for a typical block, deeper in frost-prone or soft ground. Your wall system’s documentation and local practice govern.

Do I really need to bury the first course?

Yes — setting the first course below grade locks the wall’s toe against sliding and is standard practice for segmental walls. The calculator includes it by default; untick it only for borders that retain nothing.

What stone goes behind a retaining wall?

Clean, angular crushed stone (a #57-type gradation drains well) in a zone at least 12″ wide for the wall’s full height, with a perforated drain pipe at the base daylighted away from the wall. Rounded pea gravel and native soil against the wall both trade away drainage.

How tall can I build without an engineer?

Many areas allow unreinforced segmental walls up to about 3–4 ft, but the threshold, and what counts toward it, varies by jurisdiction — and slopes or loads above the wall change everything. Check your local requirements before building; this tool only counts materials.

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