Retaining Wall Calculator
Block count and base material for a segmental retaining wall.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Courses = ⌈ exposed height ÷ block height ⌉ + buried courses
Blocks per course = ⌈ wall length ÷ block face length ⌉
Blocks = courses × blocks per course; caps = blocks per course
Base gravel = length × 24 in × 6 in; drainage stone = length × 12 in × total wall height
Run the calculation to see each step with your own numbers.
A 40 ft wall, 3 ft exposed, 6 in × 12 in blocks, 1 buried course, 5% waste.
Courses = ⌈36 ÷ 6⌉ + 1 = 7; per course = ⌈480 ÷ 12⌉ = 40
Blocks = 7 × 40 = 280 → order 294; caps = 40
Base gravel ≈ 40 × 2 × 0.5 ÷ 27 = 1.5 yd³; drainage stone ≈ 40 × 1 × 3.5 ÷ 27 = 5.2 yd³
- A straight gravity SRW. Curves, corners and steps add cut blocks. Walls over ~3–4 ft exposed, or with a slope or surcharge above, need geogrid reinforcement and an engineer.
- One buried course per ~8 in of exposed height (bury the base for stability); adjust the buried-courses input to your manufacturer's spec.
- Base pad 24 in wide × 6 in deep of compacted crushed stone; drainage column 12 in of ¾-in clean stone behind the wall with a perforated drain at the base.
- Geogrid, drain pipe, filter fabric and the retained soil are separate.
- Do I need an engineer?
- For a straight gravity wall under about 3–4 ft of exposed height with level ground behind it, usually not. Taller walls, tiered walls, walls with a slope or driveway above, or walls near a foundation need geogrid and an engineer's design — and often a permit.
- How many courses go below grade?
- About one buried course per 8 in of exposed wall height, minimum one, so the base can't kick out. Set the buried-courses input to match your block manufacturer's spec.
- What is the drainage stone for?
- A 12-in-wide column of clean 3/4-in stone behind the blocks, with a perforated pipe at the base, lets water out instead of building pressure against the wall. It's the most-skipped step and a common cause of failure.
Layout
Standard SRW geometry; base-pad and drainage-column dimensions from SRW manufacturer installation guides (Allan Block, Versa-Lok, Keystone) and NCMA SRW design manual.
About the Retaining Wall Calculator
The Retaining Wall Calculator estimates the materials for a segmental (interlocking block) retaining wall — wall blocks, cap units, the compacted base pad and the drainage stone behind the wall — from the wall length, exposed height and block size. It assumes a straight gravity wall; curves and corners add cut blocks. Walls over about 3–4 ft exposed, tiered walls, or any wall with a slope or driveway above need geogrid reinforcement and an engineer's design.
What you enter
- Wall length (face)
- Exposed wall height
- Block height — Common SRW units are 4, 6 or 8 in high.
- Block length (face) — Common face length 12 or 18 in.
- Buried courses — Bury about 1 course per 8 in of exposed wall height for stability (minimum 1).
- Waste allowance — Default 5%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Wall blocks — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
- Cap units — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
- Base gravel — A 6-in compacted leveling pad, 24 in wide.
- Drainage backfill stone — A 12-in-wide column of ¾-in clean stone behind the wall, full height.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
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- Gravel Calculator — Gravel by volume and by delivered weight for driveways, paths and fill.
- Crushed Stone Calculator — Crushed stone by volume and weight. Shares the density question with the Gravel Calculator.
- French Drain Calculator — Gravel, pipe and fabric for a French drain of a given length and trench size.
Part of the Masonry calculators.