Retaining Wall Calculator
Estimate the segmental blocks for a retaining wall from the wall length, height and block size: courses, blocks per course, cap blocks, buried course, base and drainage gravel, drain pipe, filter fabric, geogrid, cap adhesive, block weight and material cost.
How it works
blocks per course = ceil(length × 12 ÷ block length) · courses = ceil((height − cap height) ÷ block height) + 1 cap row · total = per course × wall courses + cap blocks · buried depth ≥ 6 in and ≥ 10% of total height · base gravel = length × base width × base depth × 1.15 ÷ 27 · drainage gravel = length × thickness × height ÷ 27 · drain pipe = length × 1.1 · geogrid = layers × length × (0.6 × height, at least 4 ft)
A segmental retaining wall is laid up in horizontal courses of interlocking block. Blocks per course is the wall length in inches divided by the block length, and the number of courses is the wall height divided by the block height, with the top row swapped for cap blocks if you use them. If you enter the height above ground instead of the total, the calculator adds a buried course: at least 6 in, or about a tenth of the wall height. Under the first course goes a compacted crushed-stone base, usually 6 in deep and as wide as the block plus 6 in each side, with 15% added for compaction. Behind the blocks a 12 in column of clean drainage gravel with a perforated pipe at the bottom lets water out, and filter fabric keeps soil from clogging it. Taller walls tie back into the hill with layers of geogrid; the estimate here assumes a layer every two courses reaching 60% of the wall height into the soil, which an engineer will confirm.
Worked example
A 30 ft long, 3 ft high wall in 12 × 8 in block: blocks per course = 30 × 12 ÷ 12 = 30. With a cap row the same height as the block, (36 − 8) ÷ 8 = 3.5, so 4 wall courses plus the cap row = 5 courses, 120 wall blocks and 30 caps, 150 in all (126 and 32 with 5% extra). The base is 30 ft × 24 in × 6 in = 1.11 cu yd, or 1.28 cu yd with compaction. The drainage gravel is 30 ft × 12 in × 40 in = 3.70 cu yd, with 33 ft of drain pipe. At $4.50 per block, $5 per cap, $50 per cu yd of gravel and $1 per ft of pipe, the materials come to about $1,009.
Sources: Inch Calculator — retaining wall blocks, caps, base and backfill gravel · Omni Calculator — retaining wall blocks, cap row and backfill volume · NCMA — segmental retaining wall design guidance
Frequently asked questions
How deep should the base course sit?
Bury the bottom course about one-tenth of the wall height, and at least 6 in, on a compacted gravel leveling pad about 6 in deep. Choose Height above ground and the calculator adds the buried course for you.
When does a retaining wall need engineering?
Most jurisdictions require an engineered design and a permit above about 3–4 ft, and sooner if there is a slope, driveway or structure surcharging the top. Taller walls also need geogrid soil reinforcement tied back into the hillside. Check local code before you build.
Do I need drainage behind the wall?
Yes — water pressure is what topples retaining walls. Backfill the first foot behind the block with free-draining gravel and run a perforated drain pipe along the base to daylight, wrapped in filter fabric so soil does not clog it.
How much gravel does a retaining wall need?
Two loads: a compacted base under the first course (wall length × base width × 6 in, plus about 15% for compaction) and drainage stone behind the wall (wall length × 12 in × wall height). For a 30 ft, 3 ft wall in 12 in deep block that is roughly 1.3 cu yd of base and 3.7 cu yd of drainage gravel.
Do cap blocks need glue?
Yes. Caps are set on a bead of masonry or landscape block adhesive so they cannot be knocked or pried off. One 10-oz tube does roughly 10 ft of cap.
Related calculators
Estimate for planning. Add a waste margin and confirm against your supplier specs and local building codes.