Retaining Wall Calculator

Mortared blocks are laid to a module rather than cut from stock, so the count is (Wall Length / (Block Length + 10mm joint)) rounded up, times (Wall Height / (Block Height + 10mm joint)) rounded up, plus a waste factor for cuts. The 10mm mortar joint is part of the module: a 440 x 215mm block coordinates to 450 x 225mm, which is why a standard wall works out at ≈10 blocks/m².

Railway sleepers work differently, because they are cut to length and the offcut serves the next course. We build the actual list of pieces the wall needs and pack it into as few sleepers as possible, largest piece first, then show you the cutting plan. A 3m wall three courses high takes 5 sleepers rather than the 7 that per-course rounding would suggest.

Where a wall is longer than one sleeper, the pieces are laid in a running bond: alternate courses start with a half sleeper so the vertical joints do not stack in a column, which in a wall holding back soil would be a line of weakness. A wall of 2.4m or less needs no stagger, because each course is a single piece with no joint in it. The staggering is built into the plan rather than added afterwards, because it sometimes needs an extra sleeper, and a plan that ignored it could leave you short.

No piece is cut shorter than 300mm. A stub that short sits unrestrained at the end of a course, in the position soil pressure is highest, so where the arithmetic would leave one we lengthen it by shortening the piece before it. Across every wall size this page accepts, that costs no extra timber. One practical note the plan does not model: a saw cut removes three or four millimetres, so on a job with many cuts leave the waste factor at 10% rather than dropping it to zero.

Mortar is calibrated to manufacturer coverage, not just joint geometry: a 25kg bag (~0.012 m³) lays about 12 100mm blocks with a 10mm joint. Because the bed and perpend joints run the full block depth, mortar scales with depth, so a 215mm-deep wall needs roughly 1.8 bags/m², about double a 100mm block.

Drainage includes a 150mm gravel layer behind the wall, perforated pipe at the base, and geotextile membrane to stop soil clogging the drainage. Backfill is calculated as a 300mm depth behind the wall face.