Curved Retaining Wall Calculator
Lay out a curved block wall properly: the base-course radius to swing your string line from, worked backwards from the radius you want at the finished top, a check against your block's published minimum radius, and blocks and caps counted around each course's own arc — for inside and outside curves.
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A curved wall does not finish where you laid it out
Every segmental block steps back a fixed amount on each course — that is what gives a wall its batter. On a straight wall the setback just leans the face into the hill. On a curve it does something else: it walks the wall toward or away from the centre of the circle, so the radius changes with every course. An outside curve tightens as it rises. Lay the base course at the radius you want to see, and the top will finish tighter than you drew — possibly tighter than the block can physically turn, at which point you are sawing every unit on the top courses. The fix is to plan backwards from the top, which is what this calculator does.
How each number is worked out
- Courses: exposed height plus the buried bottom course, divided by the course height and rounded up. The buried course counts — it is a real course and it is where your layout line goes.
- Coning: a wall of n courses has n−1 joints between them, so the face travels the setback times n−1 from base to top. On an outside curve that comes off the radius; on an inside curve it adds to it.
- Base-course radius: if you specify the radius at the top, the base is the top radius plus the coning distance for an outside curve, minus it for an inside curve. This is the number you actually swing the string line at, so it leads the results.
- Governing course: the tightest course is the one that has to clear the block's minimum radius. Outside curves are tightest at the top, inside curves at the bottom — the check follows the curve direction automatically.
- Blocks per course: units butt tight at the face, so each course takes its own arc length — its radius times the sweep angle — divided by the block face width, rounded up. Because the radius moves, an outside curve needs slightly fewer blocks each course going up and an inside curve slightly more.
- Caps: counted around the top course arc at the cap width. On a tight curve caps have to be trimmed to close the joints, which the waste percentage is there to cover.
- Exposed face: the average of the base and top arcs times the exposed height — the area the per-square-foot figure is measured against.
Where the minimum radius numbers come from
Minimum radius is a property of the unit, not a rule of thumb, and the makers publish it. Versa-Lok gives 8 ft as the minimum outside radius to the face of a Standard unit with nothing cut, and recommends setting the top-course target a few inches above that — 8 ft 2 in — to allow for creep or a course added later; for inside curves it gives a recommended minimum of 6 ft measured at the bottom of the wall, noting that tighter concave curves are structurally fine but look ragged. Its published setback is 3/4 in per course. Allan Block quotes a tightest radius of 4 ft at the top of the wall with full-size units, and 2.5 ft with half-width units, and describes the same coning effect — tightest at the top, so the base course needs the larger radius. The default here is the Versa-Lok Standard set because it is the one system whose face width, course height, setback and both curve minimums are all published together. For any other block, put your own spec-sheet numbers in the detail panel; if you have no published minimum, leave that field blank and the check will say so instead of inventing a verdict.
Sanity check on the method: Versa-Lok publishes a worked table of base-course radii for a 8 ft 2 in top radius at wall heights from 0.5 to 4 ft. Running those heights through this calculator reproduces every row of that table exactly — 8 ft 7-1/4 in at 4 ft tall, 8 ft 5-3/4 in at 3 ft, and so on down to 8 ft 2 in for a single course.
This tool handles the curve geometry and the block count around it. For the rest of the wall — base gravel, drain rock, pipe, fabric and geogrid — run the same wall through the full block takeoff, size reinforcement in the geogrid calculator and drainage in the drainage calculator. To compare block systems by face size and price before you commit to one, use the block comparison. Block dimensions and prices here use the same nominal figures as that tool so the numbers stay comparable.
Curve notes that save cutting
- Plan from the top, lay from the bottom. Decide the radius you want to see at the finished top, back-plan the base, then swing the string line. Doing it the other way round is what puts you on the saw.
- Add a margin for creep. Versa-Lok's own guidance is to aim a couple of inches above the published minimum rather than exactly at it, so that a course added later does not push you under.
- Outside curves squeeze at the back, inside curves fan at the back. Faces stay tight either way — that is why block counts follow the face arc, and why the gaps that open behind an inside curve have to be filled with gravel rather than left hollow.
- Half units turn tighter. If the curve will not make the minimum, a half-width unit roughly halves the radius you can turn before switching to cutting.
- Start at corners, adjust in the middle. Where a curve meets a straight run or a corner, build the corner first and make up the difference away from it, with any partial units at least 4 in wide.
Frequently asked questions
Why is the base course radius different from the radius I want?
Because every course steps back. On an outside curve each setback moves the wall toward the centre of the circle, so the radius gets tighter the higher you go — the wall cones inward. If you lay the base course at the radius you want to see at the top, the top will finish tighter than you planned. You have to back-plan: start the base course wider by the setback times the number of joints below the top.
What is the minimum radius for a retaining wall block?
It depends on the unit, and it is published by the maker. Versa-Lok gives 8 ft to the face for an outside curve with no units cut, and recommends planning a couple of inches over that for creep; for an inside curve it gives a recommended minimum of 6 ft measured at the bottom of the wall. Allan Block quotes a tightest top-of-wall radius of 4 ft with full-size units and 2.5 ft with half units. Read yours off the spec sheet — the field here is editable.
Does the minimum radius apply at the top or the bottom of the wall?
At whichever end the curve is tightest, and that flips with the curve. An outside (convex) curve tightens as it rises, so the top course governs. An inside (concave) curve opens up as it rises, so the bottom course governs. That is exactly why the published numbers are quoted where they are — an outside minimum at the top of the wall, an inside minimum at the bottom.
How many blocks does a curved wall need?
Count each course around its own arc, not around one length. Blocks sit tight at the face, so a course needs its arc length divided by the block face width. Because the radius changes course by course, an outside curve needs slightly fewer blocks as it rises and an inside curve slightly more. Estimating every course from the base measurement is the reason inside curves run short at the top.
Can I build a curve tighter than the minimum radius?
Yes, by cutting units, and on inside curves it is normal — Versa-Lok notes a tighter concave curve is structurally fine but the face looks ragged. On outside curves you either saw units down or switch to a half-width unit, which is why the half-block minimum is so much smaller. Plan for the saw time and the extra waste rather than discovering it on the top course.
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.
- VERSA-LOK — Technical Bulletin 3: Curves and Corners (PDF) — The ¾ in per-course setback that moves the radius as the wall gets taller, the 8 ft minimum convex (outside) radius, the 6 ft minimum concave (inside) radius, and the worked example that back-calculates the base-course radius from a finished top-course radius.
- VERSA-LOK — Standard unit specifications — VERSA-LOK Standard unit dimensions — 16 in wide × 6 in high × 12 in deep (406.4 × 152.4 × 304.8 mm).
- Allan Block — AB Collection retaining wall blocks — AB Classic unit dimensions — 18 in × 8 in × 12 in (460 × 200 × 300 mm), 6° setback.