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.
Exemple détaillé
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 (0.85 m³).
In metric: a wall 9 m long and 0.9 m high has a face of 8.1 m². A block whose face is 300 × 200 mm covers 0.06 m², so 8.1 ÷ 0.06 = 135 blocks, and 5% brings it to 142 blocks. Caps 300 mm long along 9 m is 30 caps. A 600 × 150 mm pad under 9 m of wall is 9 × 0.6 × 0.15 = 0.81 m³ of gravel (1.06 yd³).
Sur le chantier
The buried course is wall too. The detail buries the first course — say 6 in of an 8 in block — below finished grade. The wall you see is 3 ft; the wall you build is 3 ft 6 in. Type the height from the bottom of the buried course to the top of the last block: on a 30 ft wall that is 105 ft² instead of 90, and 166 blocks with 5% instead of 142. The depth of burial is the plan's number; the tool just needs the whole height.
Stone behind the wall. The drainage zone behind the blocks is a box: 30 ft long × 12 in wide × the 3.5 ft wall height = 105 ft³ = 3.9 yd³ of clear stone, plus the 1.1 yd³ base pad (in metric, 9 m × 300 mm × 1.05 m = 2.8 m³ plus the 0.81 m³ pad). Five yards of stone for a wall people think of as "a pallet of blocks". Run the zone through the gravel page at the width the detail gives.
Curves and corners. Measure a curved wall along the face at the middle of the block, and expect to cut every other cap on the inside of the curve — lift the waste to 10% on a wall with curves or corners. Steps in the wall (the grade rising along it) are a separate length at each height; add them.
What went wrong?
Quick referencefor a block wall
- Block face, ft²
- 12 × 8 = 0,667 · 18 × 6 = 0,75 · 12 × 6 = 0,5 · 16 × 8 = 0,889 · 18 × 8 = 1,0
- Blocs
- face area of the wall ÷ block face, × (1 + waste), rounded up
- Hauteur
- includes the buried course — the plan says how much
- Couronnements
- length in inches ÷ cap length, rounded up
- Stone
- base pad: L × W × D ÷ 27 · drainage zone: L × zone width × wall height ÷ 27 (feet)
- Block face, m²
- 300 × 200 mm = 0,06 · 450 × 150 = 0,0675 · 400 × 200 = 0,08
- Metric stone
- L × W × D in metres = m³ · 1 m³ = 1.31 yd³
Waste & overagewhat to add, and why
- Wall builders carry 5% on straight runs and 10% with corners, curves and steps — half blocks at a corner are a cut, and a cut block splits sometimes. The waste field takes it.
- Caps are matched to the block colour; buy the spares now, from the same pallet, because a cap broken by a mower in two summers will not match the next batch.
- Base gravel is ordered by the ton with a margin — the trench is never neat, and the pit does not take it back.
Mistakes to catchbefore you order
- Measuring wall height from grade instead of from the bottom of the first buried course — the buried course is block too.
- Typing the block's nominal face (18 × 6) as its actual face when the manufacturer's module differs; use the face size that lays up to the height, joint included.
- Counting caps off the wall length without adding for the corners where the cap turns.
What an inspector looks forthe building inspector, with the engineer of record on a designed wall
On this part of the job
- Whether the wall needed a permit and an engineer at all. Height is what usually triggers it, and the trigger height is in your own jurisdiction's rules — find it before you order block.
- The base: depth below grade, the thickness and compaction of the levelling pad, and that the first course is level and buried as the detail shows.
- Drainage — drain rock behind the wall, a perforated pipe with an outlet that actually daylights, and geotextile separating the rock from the soil. A wall without drainage is a wall with hydrostatic pressure behind it.
- Geogrid where the design calls for it: the right product, at the right courses, run back the full length shown, pulled tight and pinned.
- Backfill compacted in lifts behind the wall, using hand equipment close to the block so the courses are not pushed out.
- Surcharge — a driveway, a slope or a structure above the wall changes the design, and the inspector will look for what the engineer assumed.
Have this ready
- The engineered drawing and the geogrid schedule.
- The block manufacturer's installation guide.
- The drain outlet located and left visible.
Every figure an inspector measures against comes from the code your jurisdiction has adopted, the stamped drawing, or the manufacturer's printed instructions — so none of them are printed here. Look the number up in your own book, type it into the tool, and the arithmetic is yours.
La formule
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 (in millimetres, ÷ 1,000 for metres). 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. In metric, a 9 m wall 0.9 m tall is 8.1 m² of face; a 300 × 200 mm block face covers 0.06 m², so again about 135. 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 cubic yards and cubic 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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