Retaining Wall Calculator
A complete material takeoff for a segmental block wall — blocks, base, drainage, geogrid and cost — not just a block count.
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What a retaining wall actually needs
Most block-count calculators stop at the wall itself. The parts that decide whether a wall lasts — the compacted base, the drainage stone, the pipe, the fabric and (on taller walls) the geogrid — are exactly the parts they skip. This calculator sizes the whole assembly so your material order matches what actually goes in the ground.
How the block and cap count works
- Courses: total wall height ÷ block height, rounded up. Total height is the exposed height plus the buried base course, so a 3 ft wall with an 8 in block buried one course is really 44 in — six 8 in courses.
- Blocks per course: wall length ÷ block face width, rounded up. End blocks get cut to length.
- Caps: wall length ÷ cap width. Caps sit on top of the last full course, glued down with concrete adhesive.
- Waste: a few percent covers cuts, corners and the occasional cracked block.
Preset dimensions for systems like Allan Block AB Classic (18 × 8 in), Versa-Lok Standard (16 × 6 in) and big-box blocks are nominal. Always confirm against your product's spec sheet and edit the dimensions to match — that is what makes the count exact. Switch to metric and the presets carry the makers' own published metric sizes — 460 × 200 × 300 mm for AB Classic, 406.4 × 152.4 × 304.8 mm for the VERSA-LOK Standard — rather than round numbers, because a 1 cm error on face width moves the block count by more than 2%.
Base, drainage and pipe
- Leveling pad: a compacted granular base, typically 6 in deep and a little wider than the block, gives the wall a flat, solid footing. Under-building the base is the top cause of a leaning wall.
- Drainage stone: a zone of clean ¾ in crushed stone behind the blocks — usually a foot wide — gives water a path down instead of pushing on the wall. Hollow-core systems also fill the block cores with the same stone.
- Perforated pipe: a 4 in perforated pipe runs along the base of the drainage zone and daylights out the side or to a drain every 50 ft or so. The estimate is the wall length plus a short run at each of those outlets, so it matches the drainage calculator for the same wall.
- Filter fabric: non-woven geotextile keeps native soil from clogging the drainage stone. The estimate wraps the back and bottom of the stone zone with a 10% overlap.
Gravel is sold by weight or volume. We show cubic yards (or m³) and tons (or tonnes) using about 1.4 tons per cubic yard (≈1,680 kg/m³, roughly 105 lb/ft³) — ask your supplier for their exact figure and round the order up. To size the drainage on its own — including hollow-block core fill, pipe outlets and a bulk-vs-bagged gravel comparison — use the retaining wall drainage calculator.
Geogrid: when a wall needs reinforcement
Geogrid is a structural grid laid between courses and extending back into the retained soil, tying the whole mass together. Whether you need it — and how many layers, how long, and how they're spaced — is an engineering decision that depends on height, soil type and what sits above the wall. The optional estimate here assumes a layer roughly every two courses, extending back about 60% of the wall height (minimum 4 ft), purely so you can budget. To lay the layers out course by course — spacing, length and rolls — use the dedicated geogrid calculator. Neither tool is a substitute for an engineered design.
DIY vs contractor cost
The calculator totals your material prices for the DIY number, and multiplies the wall's face area (length × exposed height) by an installed rate for the contractor number. Installed segmental walls commonly run $30–$60 per face square foot including base prep, drainage and labor — the range is wide because access, soil and wall height move it a lot. Put your own quotes in to compare honestly. For a full price breakdown by material, region and site conditions, use the retaining wall cost calculator.
Frequently asked questions
How many retaining wall blocks do I need?
Blocks per course is the wall length divided by the block's face width; the number of courses is the total height (exposed plus buried) divided by the block height. Multiply them and add a few percent for cuts. A 40 ft wall exposed 3 ft in an 18 × 8 in block with one course buried works out to six courses of 27 — about 170 blocks with waste.
Do I need geogrid?
It depends on height, soil and load, not a rule of thumb. Many walls under 3–4 ft on level ground use none; taller walls or walls holding a slope or driveway need engineered geogrid. Anything over 4 ft — or shorter with a load above — should be designed by an engineer, and the geogrid figure here is only for budgeting.
Why is drainage so important?
Water pressure, not weight, fails most walls. Clean drainage stone, a perforated pipe that daylights, and filter fabric let water out before it can push the wall over. It's the cheapest insurance in the whole project.
How much does a retaining wall cost?
DIY materials are often a few dollars per face square foot; contractor-installed walls run about $30–$60 per face square foot. A 120 sq ft wall face is therefore roughly $3,600–$7,200 installed. Enter local prices above for your real number.
Sources
The published rules and unit dimensions this page relies on, and the document each one is taken from. Manufacturers revise their literature — check your own product’s current spec sheet before you order.
- Allan Block — AB Collection retaining wall blocks — AB Classic preset dimensions — 18 in × 8 in × 12 in (460 × 200 × 300 mm).
- VERSA-LOK — Standard unit specifications — VERSA-LOK Standard preset dimensions — 16 in × 6 in × 12 in (406.4 × 152.4 × 304.8 mm).
- Allan Block — Retaining Wall Specification Manual (PDF) — The 12 in (300 mm) minimum wall rock behind the facing, the 4 in (100 mm) drain pipe, and base thickness of 4 in (100 mm) under 4 ft (1.2 m) walls and 6 in (150 mm) above that height.