Retaining wall calculator
This is a material take-off only. It counts blocks, caps and base gravel from the sizes you enter — nothing more. It does not design the wall, size its footing, address drainage or geogrid, or judge whether a wall of that height is stable or allowed. A retaining wall holding back real soil load is an engineered structure: consult a qualified engineer and your local authority before you build.
Enter the wall length and height and your block size, and the tool divides the face of the wall into blocks, counts a course of caps along the top, and works out the gravel volume for the levelling pad you describe. Every size is editable so it matches the product on your pallet.
Worked example
A wall 30 ft long and 3 ft high has a face of 90 square feet. A block whose face is 12 in × 8 in covers 96 square inches, which is 0.67 ft², so 90 ÷ 0.67 ≈ 135 blocks, and 5% waste brings it to about 142 blocks. Caps 12 in long along 30 ft is 30 caps. A 24 in × 6 in pad under 30 ft of wall is 30 × 2 × 0.5 = 30 ft³, or about 1.1 yd³ of gravel.
The formula
blocks = ⌈(length × height) ÷ block face area × (1 + waste)⌉
A wall face is a rectangle, and so is a block face, so the block count is just one area divided by the other. Keep the units square: a block quoted in inches becomes feet by dividing each side by 12 before you take its area. The base gravel is the wall length times the pad width times the pad depth — a simple box of stone.
Questions
- How many blocks do I need for a retaining wall?
- Work out the face area — wall length times wall height — and divide by the face area of one block. A 30 ft wall, 3 ft tall, is 90 square feet of face; a block whose face is 12 inches by 8 inches covers 0.67 square feet, so you need about 135 blocks before waste. Enter your block size and the tool does the division.
- How much base gravel goes under a retaining wall?
- That depends on the width and depth of the levelling pad you dig, which are yours to set here. The tool multiplies the wall length by the pad width and pad depth to give a gravel volume in yards or metres. It is a material estimate for ordering — how deep and wide the pad actually needs to be is a design question, not an arithmetic one.
- Do I need caps, and how many?
- Most segmental walls finish with a course of cap blocks glued on top. The count is the wall length divided by the length of one cap, rounded up. Set the cap length to your product and the tool counts them; leave the field alone if your wall is not capped.
- Will this tell me if my wall is strong enough?
- No, and it deliberately will not try. This is a material take-off — blocks, caps and base gravel. Whether a wall of a given height is stable, whether it needs geogrid, a wider footing or drainage behind it, and what your local rules allow before an engineer must sign off are all outside this tool. Ask an engineer and your authority for anything holding back real load.
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