Retaining Wall Calculator
Enter the wall's length and exposed height and pick your block size. The calculator stacks the wall course by course and returns the number of wall blocks to buy, the cap blocks for the top row, and the compacted gravel base that goes underneath. Open Advanced options for setbacks, waste and pricing.
Your measurements
How far the wall runs, end to end.
Finished wall height above the ground. Walls over about 4 ft usually need an engineer and a permit.
The face dimensions of one block — width across the wall, height per course. Pick "Custom size" to type your own under Advanced options.
Cap blocks are the finished top row that hides the hollow cores and gives a clean edge.
Wall blocks to buy
126 blocks
6 courses · 20/course · incl. 5% waste
What to buy
- Wall block — 12" W × 8" H (standard)
- 126 blocks
- Cap block
- 20 blocks
- Gravel base
- 1.1 tons
6 courses (rows) of 20 blocks, including one buried base course below grade. A "course" is one horizontal row of block.
The finished top row that hides the cores and gives the wall a clean edge.
Compacted crushed stone under the first course (a ~2 ft wide, 6 in deep leveling pad) so the wall doesn't settle.
Estimates only. Verify against your supplier's coverage figures before ordering.
Footprint
Wall elevation to scale
6 courses · 20 blocks/course · cap row
How the wall stacks up
- Face area
- 60 ft²
- Courses (rows)
- 6 courses(incl. 1 buried)
- Blocks per course
- 20 blocks
- Blocks before waste
- 120 blocks
- With waste (+5%)
- 126 blocks
Length × exposed height — the visible face of the wall.
Exposed height ÷ 8 in block height, rounded up, plus one base course set below grade.
Wall length ÷ 12 in block width, rounded up.
Courses × blocks per course.
A little extra for breakage and the blocks you split at the ends.
The formula
Courses = ⌈height ÷ block height⌉ (+1 buried base) · Blocks/course = ⌈length ÷ block width⌉ · Blocks = courses × blocks/course
Example: A 20 ft × 3 ft wall in 12" × 8" block is 5 exposed courses plus a buried base course — 6 rows of 20 blocks = 120 blocks (126 with 5% waste), 20 cap blocks, and about 1.1 tons of gravel base.
How it works
- 1Find the courses (rows): exposed wall height ÷ the block's face height, rounded up. Add one buried base course below grade for stability.
- 2Find the blocks per course: wall length ÷ the block's face width, rounded up.
- 3Multiply courses × blocks per course for the total, then add ~5% for breakage and end cuts.
- 4Add a cap row — one block per length — to finish the top and hide the hollow cores.
- 5Lay it all on a compacted crushed-stone base (a ~2 ft wide, 6 in deep leveling pad) and backfill behind with drainage gravel.
Frequently asked questions
How many blocks do I need for a retaining wall?
Divide the wall height by the block height for the number of courses, and the wall length by the block width for blocks per course, then multiply. A 20 ft long, 3 ft tall wall in 12×8 inch block needs 5 courses of 20 blocks — 100 blocks — plus about 20 caps and one buried base course.
How tall can a retaining wall be without engineering?
Most codes let you build a segmental block wall up to about 3–4 feet of exposed height as a DIY project. Taller walls — or walls holding back a slope, driveway or structure — usually require an engineered design, geogrid reinforcement and a permit. Always check locally.
Do I need a gravel base under a retaining wall?
Yes. Set the first course on 4–6 inches of compacted crushed stone in a level trench so the wall doesn't settle or heave, and bury that bottom course below grade. Backfill behind the wall with drainage gravel so water escapes instead of pushing the wall over.
How much does a retaining wall cost?
Segmental block walls run roughly $20–$60 per face square foot installed, driven by block choice, wall height, drainage and site access. A 20 ft × 3 ft wall (60 ft²) is about $1,200–$3,600 installed; DIY material-only is far less, but block walls are heavy, slow work.