Retaining Wall Size Calculator

Multiply courses by blocks per course. Blocks per course = wall length divided by block length, rounded up; courses = wall height divided by block height, rounded up (plus one buried course). The cap row replaces the top course of wall blocks. Example: a 20 ft long, 3 ft high wall with 6x16-inch blocks needs 15 blocks per course x 7 courses (6 exposed + 1 buried) = 90 wall blocks + 15 cap blocks; order 99 + 17 caps with a 10% waste allowance.

Dig for a 6-inch compacted crushed-stone leveling pad plus buried block: bury about 1 inch of block per foot of wall height (minimum 6 inches buried). For a 3 ft wall with 12-inch-deep blocks that is a 12-inch-deep, 24-inch-wide trench (block depth plus 6 inches of working room on each side). Manufacturer specifications require the compacted base pad to be at least 6 inches thick.

You need gravel twice: the 6-inch compacted base pad under the wall and the 12-inch drainage zone behind it. A 20 ft long, 3 ft high wall needs about 0.74 cubic yards (roughly 1 ton) for the base pad plus 2.22 cubic yards (roughly 3 tons) for the drainage backfill — about 4 tons total. One cubic yard of crushed stone weighs about 1.35 tons.

Yes — trapped water is the most common reason retaining walls fail. Install a 4-inch perforated drain pipe directly behind the first course, sloped to daylight or a storm drain, and backfill a 12-inch-wide zone of clean crushed stone behind the full height of the wall. A 20 ft wall needs 20 linear feet of pipe. Never use rounded pea gravel or soil as backfill against the wall.

When the total wall height — including the buried course — exceeds 4 feet, plan on geogrid soil reinforcement and have the wall designed by a licensed engineer (most jurisdictions also require a permit above 4 ft). A common layout is one grid layer every 16 inches of height, each layer extending back about 0.7 times the wall height. Example: a 30 ft long, 5 ft high wall needs 3 layers, each 4 ft long — 90 linear feet of 3-ft-wide grid (30 square yards).

Batter is the slight backward lean built into segmental block walls — each course steps back about 3/4 inch. Total top setback = (number of courses - 1) x 3/4 inch. A 7-course wall sets back 4.5 inches at the top; an 11-course wall sets back 7.5 inches. Keep this in mind when placing the wall near property lines or structures.

DIY material cost for a 20 ft x 3 ft segmental block wall is roughly $704.50: 99 blocks ($420.75) + 17 caps ($63.75) + 4 tons of gravel ($200) + 20 ft of drain pipe ($20), before geogrid or tools. Professionally installed segmental walls typically run $15 to $50 per square foot of wall face — a 60 sq ft wall would be roughly $900 to $3,000 installed.

For garden walls up to 3 ft, pressure-treated 6x6 timber is cheapest and simplest (an 8 ft x 3 ft timber wall takes 6 timbers plus 2 deadman anchors). For DIY walls up to 4 ft, dry-stacked segmental block with proper base and drainage is the most forgiving. Above 4 ft, or where vehicles or structures surcharge the wall, use an engineered segmental wall with geogrid or a poured concrete wall with rebar (a 10 ft x 3 ft poured wall needs about 1.36 cubic yards of concrete plus 78 linear feet of rebar).

In most US jurisdictions, any retaining wall over 4 feet tall measured from the bottom of the footing — including the buried portion — requires engineered plans and a building permit; some areas require permits at any height or near property lines. This calculator is a planning estimate, not engineering advice. Always check your local building code and call 811 before you dig.