The block count is straightforward. What is not straightforward is that a retaining wall holds back tonnes of saturated soil, and the ones that fail almost never fail because of the blocks. Read the drainage section before you order anything.
Enter the wall face area and the block size. Height matters more than area here — check the height warning below.
Works in metric and US imperial — you choose at the start and can switch at any time. Every figure this calculator uses is published with its source, and the result comes with a printable order sheet. No account, no email, and nothing you type leaves your browser.
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Face area divided by the face area of one block, plus an allowance. What is not straightforward is that a retaining wall holds back tonnes of saturated soil, and the ones that fail almost never fail because of the blocks. Height matters more than area here, and the height that matters is the retained height — how much soil the wall holds back, measured from the base of the wall to the retained ground level — not how tall the wall looks.
A driveway, a parking space, a shed, or a slope continuing upward behind the wall is a surcharge: extra load the plain height does not account for. Surcharge is the single most common reason a wall that looked adequate on paper is not. If there is anything at all above the retained level, the wall needs designing rather than counting.
Batter the wall back into the slope — most segmental block systems set each course back slightly, and the lean is not decorative, it is how the system resists the soil. Compact the backfill in layers of 150–200 mm as you build, keeping heavy plant away from the back of the wall, because a compactor working too close can push a new wall out of line. And note that dry-stacked and mortared walls behave completely differently: segmental systems rely on setback, mass and friction; mortared masonry relies on its footing and reinforcement.
Lower than most people assume, and the trigger is the retained height plus any surcharge. A driveway, shed or upward slope behind the wall adds load that plain height does not account for. Local permit thresholds vary, so check before building — a retaining wall holds back tonnes of saturated soil and failures are dangerous.
How much soil the wall holds back, measured from the base of the wall to the retained ground level. It is the number that matters for permits and design, and it is not the same as the visible height of the wall.
That is batter, and it is structural rather than decorative. Most segmental block systems set each course back slightly, and the resulting lean is how the wall resists the pressure of the soil behind it.
Segmental retaining units commonly run 15–35 kg each, and some are heavier. On a wall of a few hundred blocks that is a serious amount of lifting, so it is worth checking the weight before ordering.