Retaining Wall Block Calculator
Enter the wall length (or the diameter of a round wall), how tall it will stand above the ground, and the block you are using. This calculator adds the buried course, counts blocks and caps, and sizes the two amounts of stone every block wall needs: the crushed-stone leveling pad and the clean drainage gravel behind it.
What you'll need
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How many blocks, and why the buried course matters
Segmental retaining wall blocks are dry-stacked, so the count is simple: blocks per course is the wall length divided by the block length, and the number of courses is the total height divided by the block height. The part people forget is the buried course. The first course sits below grade — at least 6 inches, or roughly 1 inch for every 8 inches of exposed wall — so a wall that stands 24 inches above the lawn is really 30 inches of block.
For a round wall, such as a raised bed or a fire pit ring, the wall length is the circumference: π × diameter. Tapered blocks make tight circles possible; with square-ended blocks, the inside radius needs extra cutting, so add another 5%.
The two kinds of stone every block wall needs
| Layer | Material | Size |
|---|---|---|
| Leveling pad | Crusher run / road base, compacted | 6 in deep, block depth + 12 in wide |
| Drainage zone | Clean angular stone (#57) | 12 in wide, pad to 6 in below the top |
| Drain | 4 in perforated pipe, daylighted | Full length, at the back of the pad |
| Separation | Non-woven filter fabric | Between the drain stone and the soil |
The leveling pad is what keeps the wall straight: it is compacted, then the first course is set level on it block by block. The drainage stone is what keeps it standing. Water pressure behind a wall is the most common reason walls bulge or fail, and a 12 inch column of clean stone with a pipe at the bottom relieves it. The calculator sizes both, with a compaction allowance on the pad.
When a wall needs an engineer
Most residential codes, including IRC section R404.4, require an engineered design once a wall retains more than 4 feet of unbalanced soil. Slopes, driveways or buildings above the wall change the loads and can require engineering at lower heights, and many towns require a permit sooner. Taller walls typically need geogrid reinforcement layered back into the soil — the block count stays similar, but the excavation and backfill grow a lot.
Worked example
A 30 ft straight wall, 24 inches exposed, in 16 × 6 inch blocks: 23 blocks per course, 5 courses (24 in + 6 in buried), 121 blocks with 5% extra, 32 caps, about 1.8 tons of crusher run for the pad and 3.4 tons of #57 drainage stone.
Buying the stone? The crushed stone calculator compares #57, #8 and crusher run, and the delivery calculator works out truck loads. Pairing the wall with a patio? The paver calculator handles the paving.
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Frequently asked questions
- How many retaining wall blocks do I need?
- Divide the wall length by the block length for blocks per course, then multiply by the number of courses — including the buried first course. A 30 ft wall 24 inches tall in 16 x 6 inch block is 23 blocks per course × 5 courses = 115, or about 121 with 5% extra.
- How deep should the first course be buried?
- At least 6 inches, or about one inch for every eight inches of exposed height, whichever is more. The buried course keeps the base of the wall from kicking out.
- How much gravel goes behind a retaining wall?
- A 12 inch wide zone of clean angular stone such as #57, from the leveling pad to about 6 inches below the top, with a perforated drain pipe at the bottom. For a 30 ft wall standing 30 inches tall (36 inches of block with the buried course) that is about 3 cubic yards, or roughly 4.3 tons.
- How tall can a block wall be without an engineer?
- Most residential codes require an engineered design once a wall retains more than 4 feet of soil, and sooner if there is a slope, a driveway or a structure above it. Many towns also require a permit below that height — check locally.
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