Retaining Wall Calculator: 99 Blocks for a 20×4 ft Wall

What does retaining wall needed 99 blocks mean?

It means ninety-nine blocks build a 20 ft by 4 ft wall once cutting waste is allowed for. The wall itself is 80 square feet, or 11520 square inches, since a square foot holds 144 square inches; at 128 square inches per 16 in by 8 in block the exact count is 90 blocks, and the 10% waste allowance takes it to 99 blocks, since 90 × 1.10 is exactly 99. The waste blocks are not spare stock exactly — they are the blocks the wall consumes in cuts and breakage, so a wall ordered without them runs short at the last course.

Output · formula · meaning

Three things to check before you trust the number

First, check the block size you are actually buying: 16 in by 8 in is the standard retaining block, but a 12 in long block takes this wall to 132 blocks with waste, and more joints too. Second, measure the exposed face, not the footing: buried courses are counted separately, and a wall measured with its footing over-orders. Third, judge the waste honestly: a straight wall fits 10%, but a stepped or curved wall can want 15%, since every step is blocks consumed. Cap blocks, base gravel and drainage pipe are counted separately, never as waste.

Worked example

20 ft long, 4 ft high, 16 in by 8 in blocks, 10% waste. Wall area: 20 × 4 = 80 sq ft, or 11520 sq in. Block area: 16 × 8 = 128 sq in. Blocks exact: 11520 ÷ 128 = 90. With waste: 90 × 1.10 = 99 blocks. In practice that means buying 99 blocks, and expecting the last blocks to go on the cuts and the breakage. If the same wall used 12 in long blocks instead, the need would rise to 132 blocks with waste, but the wall built would be identical.

Every branch, checked

With a 30 ft long wall instead of 20 ft, the area rises to 120 sq ft and the need to 149 blocks with waste. With a 5 ft high wall, the area rises to 100 sq ft and the need to 125 blocks with waste, since 112.5 blocks exact rounds up to 113 before waste is added. With 12 in long blocks, the area stays 80 sq ft but the need rises to 132 blocks with waste. With 0% waste, the area stays 80 sq ft and the need falls to 90 blocks. Wall size, block size and waste each move the count on their own, so a wrong block size is as costly as a wrong measurement.

The reverses, worked backwards

The length behind 99 blocks is 20 ft: at 15 ft the need is 75 blocks (60 sq ft wall, 68 blocks exact, 16 in × 8 in blocks, 10% waste); at 30 ft it is 149 blocks (120 sq ft wall, 135 blocks exact, 16 in × 8 in blocks, 10% waste). The height behind 99 blocks is 4 ft: at 3 ft the need is 75 blocks (60 sq ft wall, 68 blocks exact, 16 in × 8 in blocks, 10% waste); at 5 ft it is 125 blocks (100 sq ft wall, 113 blocks exact, 16 in × 8 in blocks, 10% waste). The waste behind 99 blocks is 10%: at 0% the need is 90 blocks (80 sq ft wall, 90 blocks exact, 16 in × 8 in blocks, 0% waste); at 20% it is 108 blocks (80 sq ft wall, 90 blocks exact, 16 in × 8 in blocks, 20% waste). Each reverse moves one input and holds the rest, so the step is exactly one dimension or one waste change.

What the defaults assume

The defaults assume a 20 ft by 4 ft rectangular wall face, 16 in by 8 in blocks, and 10% cutting and breakage waste. They do not assume cap blocks, base gravel, drainage pipe or adhesive — a wall with a finished top needs its caps counted separately, and the base is bought by the ton, not by the block.

Who uses this

• A homeowner building a garden wall: the 99-block order settles the supplier call before the first block is laid.
• A DIY builder terracing a slope: the same maths on the wall alone gives the blocks, not a whole garden.
• A contractor quoting a job: wall area, block size, waste — the quote follows the same three steps, and the waste line shows the client why.
• A landlord budgeting a boundary wall: the wall size off the plans, divided by block area, gives the block line on the wall budget.

Accuracy & limitations

The arithmetic is exact for the inputs entered; the judgement is the block size and the waste allowance. Real walls step, curve and need caps, and the block on the pallet overrides any planning figure; a wall measured with its footing over-orders, and a block size read off a different product under-orders. Treat 99 blocks as the planning order for a 20 ft by 4 ft wall at 16 in by 8 in blocks, and raise the waste to 15% for a stepped or curved wall.

Cite this calculator

Bright Side Kit, “Retaining Wall Calculator: 99 Blocks for a 20×4 ft Wall”, https://brightoolbox.com/retaining-wall-calculator/, accessed 3 November 2026. Students and researchers: cite the calculator name, the page URL and the date you used it, and quote the defaults (20 ft × 4 ft wall, 16 in × 8 in blocks, 10% waste) so the 99-block result can be checked.

Frequently asked questions