About the Retaining Wall Calculator
This retaining wall calculator estimates materials for a segmental (interlocking block) wall: the number of wall blocks including the buried course, cap stones along the top, crushed stone for the leveling base and the drainage zone behind the wall, and a total material cost.
It suits garden walls, raised beds and terracing built from standard landscape blocks sold at home centers. Enter the wall length and exposed height, your block’s face width, height and depth, and local prices. The course count is rounded up so the wall reaches at least the height you enter.
The gravel estimate assumes a 6-inch compacted base trench 12 inches wider than the block depth and a 12-inch-wide drainage zone behind the full wall height, converted at about 1.4 tons per cubic yard. Walls taller than about 4 feet, or walls holding back slopes, driveways or buildings, usually need geogrid reinforcement, engineering and a permit — check local rules.
With the default inputs, the wall blocks is 166. Change any value above to recalculate instantly.
How to use the retaining wall calculator
- 1Measure the wall length along its face and the exposed height you want.
- 2Enter your block’s face width, height and depth from the product label.
- 3Set the buried courses (usually one) and whether you want caps.
- 4Enter local prices for blocks, caps and gravel.
- 5Read blocks, caps and gravel tonnage, then add adhesive for the caps.
Formula and method
The exposed height is divided by the block height and rounded up to whole courses, then the buried course(s) are added. Blocks per course are the wall length in inches divided by the face width, and the total is multiplied by the course count plus a breakage allowance. Caps run once along the top: wall length ÷ cap length plus the same allowance.
Gravel covers two zones: a leveling base trench as long as the wall, 12 inches wider than the block depth and as deep as the base you enter; and a 12-inch-wide drainage zone behind the blocks for the full built height including buried courses. Cubic feet are divided by 27 for cubic yards and multiplied by about 1.4 tons per cubic yard for crushed stone.
- H
- Exposed wall height (ft)
- L
- Wall length (ft)
- base
- Gravel base depth (ft)
- 1.4
- Approx. tons of crushed stone per cubic yard
Worked examples
30 ft long, 3 ft high garden wall
3 ft ÷ 6 in = 6 exposed courses plus 1 buried = 7. Each course needs 360 ÷ 16 = 22.5 blocks, so 157.5 × 1.05 → 166 blocks and 24 caps. Gravel is 30 cu ft of base plus 105 cu ft of drainage = 5 cu yd (7 tons). Total about $1,063.
50 ft raised-bed wall, 2 ft high, small 12 × 4 × 8 in blocks
24 in ÷ 4 in = 6 courses plus 1 buried = 7. At 50 blocks per course that is 350, or 368 with 5% extra, plus 53 caps. The narrower base and 28-inch drainage height need about 5.9 cu yd of gravel.
20 ft wall, 4 ft high with 18 × 8 in blocks, no caps
48 in ÷ 8 in = 6 exposed courses plus 1 buried = 7 courses of 13.33 blocks: 93.3 × 1.05 → 98 blocks. Gravel is 20 cu ft of base plus 93.3 cu ft of drainage = 4.2 cu yd.
Frequently asked questions
How many retaining wall blocks do I need?+
Divide the wall length in inches by the block face width to get blocks per course, then multiply by the number of courses — exposed height ÷ block height, plus one buried course. Add about 5% for cuts and breakage.
How deep should the first course be buried?+
Most block manufacturers recommend burying at least one full course, or roughly 10–12% of the wall height (often stated as 1 inch per 8 inches of height), whichever is greater. Taller walls and sloped ground need more embedment.
How much gravel do I need behind a retaining wall?+
Plan on a 6-inch compacted base about 24 inches wide under a 12-inch block, plus a 12-inch-wide column of clean crushed stone behind the wall for its full height. A 30 ft × 3 ft wall needs about 5 cubic yards.
How tall can a retaining wall be without an engineer?+
Many local codes allow walls up to about 4 ft (measured from the bottom of the footing) without engineering or a permit, unless they support a surcharge such as a driveway, slope or building. Always check with your building department.
Do retaining wall caps need glue?+
Yes. Caps are normally secured with a concrete or landscape block adhesive, since they have no interlocking lip. Plan on roughly one tube of adhesive per 10–15 linear feet of wall, depending on the product.