Retaining Wall Block Calculator
Calculate how many retaining wall blocks, courses and cap blocks you need from wall length and height, plus gravel for the base and the drainage zone behind it.
By CalcBuilt Editorial TeamUpdated September 11, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Wall blocks to buy
- 189 blocks
- 180 blocks in the wall plus 5% waste. Warning: 4 in tall, 8 in deep garden block is rated by most makers for walls up to about 2 ft; over that use a 6 in to 8 in tall block 10 in to 12 in deep
- Courses
- 9 courses
- 20 blocks per course at 12 in long, 4 in tall; bury the first course
- Cap blocks
- 20 caps
- One per 12 in of wall, set with masonry adhesive
- Base gravel
- 0.74 cu yd
- 24 in wide x 6 in deep of 3/4 minus crushed stone under the first course (block depth plus drainage zone), compacted in two lifts; order about 20% more loose
- Drainage gravel
- 2.22 cu yd
- 12 in of clean 3/4 stone behind the full height of the wall, over a 4 in perforated pipe at the base that daylights past the wall end, with fabric between stone and soil
Retaining Wall Block formula
Courses = Wall height (in) / Block height (in), rounded up
Blocks = Courses x Wall length (in) / Block length (in) x (1 + waste %)
Cap blocks = Wall length (in) / Block length (in), rounded up
Base gravel (cu ft) = Length x 2 ft x 0.5 ft, Drainage gravel (cu ft) = Length x Height x 1 ftWorked example
A 35 ft wall 2.5 ft tall in 16 x 6 in blocks: 30 / 6 = 5 courses. Each course takes 420 / 16 = 26.25 blocks, so 5 x 26.25 = 131.25, or 132 blocks, and 138 with 5% waste, plus 27 caps.
Base gravel: 35 x 2 x 0.5 = 35 cubic feet, or 1.3 cubic yards. Drainage gravel: 35 x 2.5 x 1 = 87.5 cubic feet, or 3.24 cubic yards. Order about 1.5 yards of 3/4 in minus for the base and 3.5 yards of 3/4 in clean stone for drainage, 4.5 to 5 yards in total.
Quick reference
| Wall height | 12 x 4 in blocks | 16 x 6 in blocks | Base gravel | Drainage gravel |
|---|---|---|---|---|
| 1 ft | 30 | 15 | 0.37 cu yd | 0.37 cu yd |
| 2 ft | 60 | 30 | 0.37 cu yd | 0.74 cu yd |
| 3 ft | 90 | 45 | 0.37 cu yd | 1.11 cu yd |
| 4 ft | 120 | 60 | 0.37 cu yd | 1.48 cu yd |
How to calculate retaining wall blocks
Divide the finished wall height, including the buried first course, by the block height and round up for the number of courses. Divide the wall length in inches by the block face length for blocks per course. Multiply the two, then add 5 percent for a straight wall or 10 percent for curves, corners and steps, where blocks are split to fit. Cap blocks are one per block length along the top.
Height counts from the bottom of the buried course, not from grade. A wall that shows 3 feet above ground with one 4 inch course buried is 3 feet 4 inches tall for the count, or 10 courses of 4 inch block. IRC R105.2 exempts walls up to 4 feet measured from the bottom of the base with no surcharge; a slope, driveway, pool or fence above the wall is a surcharge. Over that line you need a permit and an engineer's stamped design, typically $500 to $1,500 for a residential wall in 2026, and the design will call for geogrid. Two terraced walls closer together than twice the height of the lower one count as a single wall. Block makers also cap each product. Most 4 inch garden blocks are rated to about 2 feet; taller walls take 6 to 8 inch blocks 10 to 12 inches deep.
Base, drainage and backfill
Dig a trench about 24 inches wide (block depth plus 12 inches of drainage stone) and deep enough for 6 inches of base plus the buried course. Bury one full course, or 1 inch per 8 inches of wall height. Compact the subgrade, then place 3/4 inch minus crushed stone in two 3 inch lifts and compact each. Set the first course block by block with a torpedo level both ways and a rubber mallet; 1/8 inch out here is an inch out at the top. Stack the rest with the joints offset by half a block; the rear lip or pins set the batter on their own.
Backfill directly behind the blocks with at least 12 inches of clean 3/4 inch drainage stone. At the base put a 4 inch perforated pipe (with a sock in silty soil) that daylights beyond the wall end or runs to a drywell. Put non-woven fabric between the stone and the native soil so fines do not clog it. Water pressure, not soil weight, pushes over most failed walls. Compact the native backfill in 6 to 8 inch lifts, hand tamper or walk-behind only within 3 feet of the wall. Where the wall is over about 3 feet, or anything sits above it, geogrid goes on every second course. Cut it to the length in the maker's chart, about 60 percent of the wall height at minimum, and pull it taut before backfilling. Glue the cap course with masonry adhesive, about 15 caps per 10 oz tube.
What this calculator leaves out
The perforated pipe, fabric and geogrid, cap adhesive, corner and half blocks, and steps. The 20 percent compaction allowance on the base stone, since the result is compacted volume. Excavation and haul-off of the trench spoil, plate compactor rental, the permit and engineer's fee over 4 feet, and delivery. A pallet of 12 x 4 in garden blocks holds about 100 and weighs about 2,500 lb; 189 blocks is two pallets and a boom truck.
Mistakes that cost money
- The 4 ft permit limit (IRC R105.2) runs from the bottom of the base, not grade; a wall showing 3-1/2 ft is over it.
- No perforated pipe at the base: water, not soil, tips most walls; 4 in perforated pipe in the stone, daylighted past the wall end.
- A first course 1/8 in out of level is 1 in out by course 8; level every base block both ways with a torpedo level.
- No geogrid under a slope or driveway: the wall bellies out within a few years; lay grid every second course to the maker's chart.
- Running the plate compactor against the back of the blocks pushes the top courses out; hand tamp within 3 ft of the wall.
- Native clay backfilled against the blocks holds water and heaves; 12 in of clean 3/4 in stone behind the wall, fabric between stone and soil.
Key facts
- Most building departments require an engineered design and a permit for retaining walls over 4 ft tall, including the buried course.
- The first course should be buried about 1 in for every 8 in of wall height, with a minimum of one full course below grade.
- A 12 x 4 x 8 in garden wall block weighs about 25 lb and covers 1/3 sq ft of wall face; 100 sq ft of wall takes 300 blocks.
- Segmental blocks are made with a rear lip or pins that set back each course 3/4 to 1 in, so the wall leans into the slope.
- One cubic yard of 3/4 in crushed stone weighs about 1.4 tons and covers 54 sq ft at 6 in deep.
Frequently asked questions
Why is my retaining wall leaning after a few years?
Almost always water. Clay backfilled straight against the blocks, no perforated pipe at the base, or a pipe with no outlet lets water build up behind the wall and push. Frost does the rest. A wall over about 3 ft with a slope or driveway above it and no geogrid bellies out the same way. The fix is to pull it down to the base and rebuild with 12 in of clean stone, a drained pipe, fabric and grid.
How much gravel do I need behind a retaining wall?
Plan on a 12 in wide zone of clean 3/4 in stone the full height of the wall: length x height x 1 ft, divided by 27 for cubic yards. A 20 x 3 ft wall needs about 2.2 cubic yards.
How deep should the base be for a retaining wall?
6 in of compacted crushed stone under the first course, with the first course buried at least 4 in or one full block height.
How tall can I build a retaining wall without an engineer?
Most codes allow up to 4 ft measured from the bottom of the footing or base without an engineer, provided nothing but level ground sits above the wall. Check with your local building department.
Sources and references
- SRW-MAN-003 Segmental Retaining Wall Installation Guide (Concrete Masonry & Hardscapes Association)
- Chapter 4 Foundations (Section R404.4 Retaining Walls), 2021 IRC (ICC)
- Chapter 1 Scope and Administration (Section R105.2 Work Exempt From Permit), 2021 IRC (ICC)
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.