About this calculator
This calculator estimates the materials for a low garden retaining wall built from dry-stacked segmental blocks: the blocks themselves (including the buried courses below ground), the capping units along the top, the free-draining gravel behind the wall, the perforated drain pipe at its heel, and the geotextile that keeps soil out of the gravel.
It is a quantity take-off only. It does not check whether the wall will stand: sliding, overturning, bearing capacity, setback/batter, geogrid reinforcement and surcharge loads are not calculated. Higher walls must be designed by a structural or geotechnical engineer.
Formula
All dimensions are converted to whole millimetres, then:
courses above ground = ceil(visible height / block height)
total courses = courses above ground + buried courses
exposed height = courses above ground × block height
total block height = total courses × block height
half block = block length / 2
whole-block-start courses (1st, 3rd, 5th …) = ceil(total courses / 2)
offset courses (2nd, 4th, 6th …) = total courses − whole-block-start courses
whole-block-start course = ceil(wall length / block length)
offset course: half block + whole blocks + end piece
whole blocks = floor((wall length − half block) / block length)
end piece = (wall length − half block) − whole blocks × block length
end piece = 0 → nothing more
end piece = half block → a second half block
any other end piece → one more whole block, cut to length
half blocks = half blocks per offset course × offset courses
blocks cut in two = ceil(half blocks / 2)
blocks (net) = whole-block-start courses × blocks per whole-block-start course
+ offset courses × whole blocks per offset course
+ blocks cut in two
waste blocks = ceil(net × waste % / 100)
total blocks = net + waste blocks
capping (net) = ceil(wall length / capping unit length)
capping waste = ceil(capping net × waste % / 100)
gravel zone height = total block height − soil cap
gravel volume = wall length × gravel zone width × gravel zone height
drain pipe length = wall length
coils/lengths = ceil(pipe length / coil length)
geotextile (net) = wall length × (gravel zone height + gravel zone width)
geotextile overlap = ceil(net × overlap % / 100)
The blocks are laid in running bond. Courses are counted from the bottom, buried courses included. The 1st, 3rd, 5th … course starts with a whole block at the left end. The 2nd, 4th, 6th … course is offset by half a block: it starts with a half block, continues with whole blocks and ends with whatever length remains. Each half block is cut from a whole block, and one cut block gives two halves, so the halves from all offset courses are pooled. Any other cut piece (the shorter last block of a course) is cut from its own whole block; its offcut is not counted as usable anywhere else. The net count is therefore complete even at 0 % waste; the waste percentage is added on top for breakage and spares.
The visible height is rounded up to whole courses, so the wall as built can be slightly taller than the height you entered; the results show both. The gravel zone runs from the base of the lowest (buried) course up to the top of the blocks, less the optional soil cap. The geotextile runs up the back of the gravel zone (against the retained soil) and folds over its top.
Worked example
Defaults: wall 6 m long, 600 mm visible, 1 buried course, block 400 × 150 × 300 mm, 5 % waste, capping unit 400 mm, gravel zone 300 mm wide, no soil cap, 50 m pipe coils, 10 % geotextile overlap.
courses above ground = ceil(600 / 150) = 4; total = 4 + 1 = 5
total block height = 5 × 150 = 750 mm
courses 1, 3, 5 start with a whole block: 3 courses × ceil(6000 / 400) = 3 × 15 = 45
courses 2, 4 are offset: 200 (half) + 14 × 400 + 200 (half) = 6000 mm
→ 2 courses × 14 whole blocks = 28, plus 2 × 2 = 4 half blocks
half blocks: 4 / 2 = 2 blocks cut in two
blocks (net) = 45 + 28 + 2 = 75; waste = ceil(75 × 5 % = 3.75) = 4; total = 79
capping = ceil(6000 / 400) = 15; waste = ceil(0.75) = 1; total = 16
gravel = 6000 × 300 × 750 mm = 1,350,000,000 mm³ = 1.35 m³
drain pipe = 6 m → 1 coil of 50 m
geotextile = 6000 × (750 + 300) = 6.3 m² + 10 % = 6.93 m²
FAQ
Does this calculator check whether my wall is strong enough? No. It only counts materials. Stability (sliding, overturning, bearing, reinforcement) is not calculated at all. If the wall is above the engineering-review height you set, the calculator shows a notice that the wall needs engineering design.
Where do the half blocks come from? They are cut from whole blocks, two halves from each block, and those blocks are already in the net count. If the wall length is not a whole number of blocks, the last block of a course is cut to length from a whole block too; the offcut is not reused in the count. You can often reuse offcuts on site, so the count errs on the safe side.
Where does the 1000 mm engineering-review height come from? It is only a default, not a building-code value. The height above which a retaining wall needs engineering design depends on local rules, the soil, loads above the wall (a driveway, a building, a slope) and the block system. Set it to whatever applies to you. It is compared against the exposed height as built (whole courses above ground, capping excluded).
Why are buried courses counted? Segmental block walls are normally started below the front ground level so the base course is embedded. Those courses need blocks too, and the gravel zone starts from their base.
Is the gravel volume loose or compacted? It is the geometric volume of the zone behind the wall. No loose-to-compacted factor is applied. Ask your supplier how much to order for that volume.
Does the drain pipe length include the outlet? No. It is one run along the full wall length at the heel. Add the length needed to carry the water to its outlet.
Assumptions and limits
No standard is cited. Retaining-wall design is covered by geotechnical design standards such as EN 1997 (Eurocode 7), but this calculator performs none of that design, so citing one would imply a check that is not made. The quantities themselves are plain geometry.
These values are estimates, not normative figures, and you can change each of them: the 5 % breakage and spare allowance (applied to blocks and capping units alike, on top of the net count), the 300 mm gravel zone width, the 50 m pipe coil length, the 10 % geotextile overlap allowance and the 1000 mm engineering-review height.
The wall is straight, with no corners, curves, steps in the base or steps in the top. The blocks are laid in half-block running bond, with the 2nd, 4th, 6th … course from the bottom offset. Half blocks are paired, two from one cut block; no other offcut is reused. Special end or corner units are not modelled. The levelling pad or base under the first course, backfill soil, adhesive for the capping and any geogrid are not included. The drawings are schematic: the drain pipe is drawn at a fixed size, and on long walls the elevation shows a representative joint pattern rather than every block.