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Retaining Wall Calculator

Count retaining-wall blocks, caps, and the base and drainage gravel for a block wall.

  • retaining wall
  • blocks
  • segmental
  • gravel
  • drainage
  • caps
  • landscaping

About Retaining Wall Calculator

A segmental block retaining wall is built from three things in quantity: the blocks themselves, the cap units that finish the top, and the gravel that levels the base and drains the back. This calculator produces a take-off of all four from the wall's length and height and your block size — how many blocks per course, how many courses, the total wall and cap count, and the cubic metres of base and drainage gravel.

It's a material estimator, not a structural design tool, and that distinction matters: the load a retaining wall holds back is substantial, and walls above about a metre, or any wall holding up a slope, driveway, or structure, generally need an engineer, reinforcement, and a permit. What this gives you is the shopping list once the design is settled — and a clear reminder of the drainage that every retaining wall depends on.

How to use

Pick metric or imperial and enter the wall's length and height, then the length and height of a single block — 300 × 200 mm is a common segmental size. Set the base pad's depth and width (the levelling trench is wider than the block) and the width of the free-draining gravel zone behind the wall.

The headline gives the total wall blocks, broken into courses and blocks-per-course, with the cap count alongside and the base and drainage gravel volumes in cubic metres and yards. Order around 5–10% extra on blocks for cuts and the odd breakage. Above all, treat the output as quantities only: the tool flags walls over a metre because those need engineered design, and it reminds you that drainage — gravel, a drain pipe, and often a geotextile — is what keeps a wall standing.

Frequently asked questions

  • How many blocks do I need for a retaining wall?

    Divide the wall height by the block height to get the number of courses, and the wall length by the block length to get blocks per course, rounding both up; multiply them for the total. A 10 m × 1 m wall in 300 × 200 mm blocks is 5 courses of 34 blocks, so 170 blocks plus a row of 34 caps. The calculator works this out for your block size and adds the cap course.

  • Why does a retaining wall need gravel behind it?

    Water is what destroys retaining walls. Soil behind the wall holds water, and saturated soil is both heavier and exerts hydrostatic pressure that pushes the wall over. A zone of free-draining gravel behind the blocks, with a perforated drain pipe at the base to carry water away, relieves that pressure. A geotextile between the gravel and the soil stops fines washing in and clogging it. Skipping the drainage is the most common cause of failure.

  • How much base gravel do I need under the wall?

    The base is a compacted levelling pad in a trench wider than the block — commonly around 150 mm deep and 600 mm wide for a low wall. Its volume is the wall length times that width times that depth: a 10 m wall needs about 0.9 m³. The calculator computes it from the base depth and width you enter; firm, level, well-compacted base is what keeps the courses true.

  • Do I need an engineer or a permit for a retaining wall?

    Often, yes. Many jurisdictions require engineered design and a permit for walls above roughly 1 m, and for any wall retaining a slope, a surcharge load like a driveway, or a structure — regardless of height. Those walls usually need geogrid reinforcement tied back into the soil. This tool counts materials only and does not check structural safety, so confirm the requirements with your local building department before building.

  • How much extra should I order for cuts and waste?

    Around 5–10% on the block count covers the cuts at corners, curves, and the ends of courses, plus the occasional cracked unit. Curved or stepped walls and any with lots of corners sit at the higher end. Gravel is cheap insurance too — order a little over, since compaction and an uneven trench bottom always absorb more than the bare calculation suggests.

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