Retaining Wall Calculator
Segmental block count, courses and cap units for a straight retaining wall.
A retaining wall is mostly hidden cost. The blocks are the visible part and usually the cheapest line on the invoice — base material, drainage gravel, filter fabric and the labour to excavate a level trench routinely cost more than the wall itself, and none of them appear in a block count.
The count itself is straightforward once you stop thinking in square feet and start thinking in courses. Blocks are laid one course at a time, so the number you need is units per course multiplied by the number of courses, rounded up, plus breakage. Block dimensions vary a lot between manufacturers, and a 12x4 block needs three times as many units per square foot as an 18x10.
How this calculator works
Every field is editable and the result updates as you type — nothing is submitted anywhere, all the maths runs in your browser. That matters for a number you are about to spend money on: you can see exactly which input moved the answer, and by how much.
What this calculation assumes
Assumes a straight wall of uniform height on level ground, with the block dimensions taken from the manufacturer sheet. Curves, steps, corners and any wall above three feet needing geogrid and engineering are outside this count.
Treat the result as an order-of-magnitude check rather than a final quantity. Product packaging changes, regional labour rates vary by a factor of two, and no calculator has seen your actual site.
Frequently asked questions
How tall can I build before I need an engineer?
Most segmental systems are rated to about three feet above grade without engineering, and four feet is the usual point where a permit and stamped drawings are required. Above that, or with any surcharge load such as a driveway behind the wall, you need a designed wall with geogrid reinforcement.
Do I really need drainage gravel?
Yes. Backfill without drainage becomes a saturated soil column that pushes outward with the full weight of the water it holds, and that is what tips walls over. A gravel drainage column behind the blocks plus filter fabric to keep soil out of it is what lets water escape instead of loading the wall.
Why is the first course buried?
Burying one course sets the wall below the frost line and below grade so soil cannot be washed out from under the toe. On a three foot wall that means excavating closer to four feet, which is why trench depth is the part of the job people underestimate most often.
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