How the block count works
A block wall is a grid: the wall length decides how many blocks fit in each row, and the wall height decides how many rows you stack. The first row sits below ground, so it's added on top of the visible height.
Blocks per row = wall length (in) ÷ block length, rounded up
Rows = visible height (in) ÷ block height, rounded up, + buried rows
Blocks = blocks per row × rows × (1 + extra %)
Example: a 30 ft wall standing 2 ft above ground with 12 × 4 inch blocks needs 30 blocks per row and 6 visible rows plus 1 buried row, so 7 rows. That's 210 blocks, or 221 with 5% extra, plus 30 caps.
Gravel for the base and drainage
The calculator also estimates crushed stone for two jobs:
- Base: a trench about 24 inches wide and 6 inches deep of compacted stone under the first row.
- Drainage zone: about 12 inches of clean stone behind the wall, from the base to near the top.
Weight is based on about 1.35 tons per cubic yard. Add a perforated drain pipe the length of the wall at the base, and landscape fabric between the stone and the soil. For more detail on stone, see the gravel calculator.
Common block sizes
| Block type | Typical face size | Blocks per sq ft of wall face |
|---|---|---|
| Small garden wall block | 12 × 4 in | 3 |
| Medium wall block | 16 × 6 in | 1.5 |
| Large segmental block | 18 × 8 in | 1 |
Sizes vary by brand, so measure the block you plan to buy or check its spec sheet.
Safety and permits
Short garden walls are a reasonable DIY job. Walls taller than about 3 to 4 feet, walls holding back a slope, a driveway or a building, and walls near property lines often need geogrid reinforcement, an engineered design and a permit. Check with your local building department before you dig.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block length to get blocks per row, and the wall height by the block height to get the number of rows, then multiply. Add one buried row below ground and about 5 percent extra for cuts.
How much of a retaining wall should be buried?
A common rule is to bury the first course of block, or about 10 percent of the wall's total height, whichever is more. Check the block manufacturer's instructions; taller walls need more.
How tall can a retaining wall be without an engineer?
Many areas allow homeowners to build segmental block walls up to about 3 or 4 feet tall without engineering. Taller walls, walls holding back a slope, a driveway or a structure, usually need geogrid reinforcement, an engineered design and a permit.
What goes behind a retaining wall?
Most block walls have about 12 inches of clean crushed drainage stone directly behind the blocks, a perforated drain pipe at the base, and landscape fabric between the stone and the soil. Good drainage is the main thing that keeps a wall from failing.
How much base gravel does a retaining wall need?
A typical base trench is about 24 inches wide and 6 inches deep, filled with compacted crushed stone, plus however deep you bury the first course. The calculator includes this base in its gravel estimate.