Retaining Wall Calculator: How Many Blocks Do I Need?

The Formula

Wall Length (ft) x Wall Height (ft) / Block Face Area (sq ft) = Number of Blocks

Segmental retaining wall blocks vary by brand. This example uses a block with an 18” x 6” face: 0.75 sq ft.

A 40-foot wall, 3 feet tall:

40 x 3 / 0.75 = 160 blocks

Add 5-10% waste for cuts at corners and curves: 176 blocks.

Block Sizes Vary

Not all retaining wall blocks are the same size. Common face dimensions:

• Large format blocks: faces around 46” x 18”, roughly 6 sq ft each.
• Standard blocks: faces around 16” to 18” wide and 6” to 8” tall, 0.67 to 1 sq ft.
• Small garden wall blocks: around 12” x 4”, 0.33 sq ft.

The block you pick changes the count dramatically. That same 40 x 3 wall: 160 of the 18” x 6” blocks, 20 large-format blocks, or 360 small garden blocks. Use your block’s actual face size from the spec sheet.

Cap Pieces

Every finished retaining wall needs a cap course. Caps are sold separately and typically match the block width.

Cap count: Wall Length / Cap Width

For a 40-foot wall with 18” caps: 40 / 1.5 = 27 cap pieces. Buy 28.

Caps glue down with construction adhesive. Two tubes of landscape block adhesive per 40 feet of wall is typical.

Base Course

The base course sits in a trench below grade. This is the most important part of the wall. A bad base creates a crooked wall within two years.

Trench width: block depth + 12 inches behind for gravel. Trench depth: 6 inches of compacted gravel base, with the first course of block buried.

A standard block is 12 inches deep. So the trench is 24 inches wide (12” block + 12” gravel behind) and about 12 inches deep (6” gravel + 6” block height, with the block top near grade level).

Base gravel for our 40-foot wall: 40 ft long x 2 ft wide x 0.5 ft deep = 40 cubic feet = 1.48 cubic yards

Use the base material the block maker specifies, compacted. Most call for a well-graded crushed stone.

Backfill Gravel

Behind the wall and between the block and the retained soil, you need drainage gravel. This is 3/4” clean stone (no fines) that lets water pass through instead of building hydrostatic pressure against the wall.

The gravel zone should extend at least 12 inches behind the block, from base to within 6 inches of the top (where you cap it with soil to grow grass).

Volume for our 40-foot x 3-foot wall: 40 ft x 1 ft wide x 2.5 ft tall (3 ft wall minus 6” of topsoil cap) = 100 cubic feet = 3.7 cubic yards

The gravel calculator handles the volume-to-tonnage math for different stone types if your supplier quotes by weight instead of by the yard.

Drainage Pipe

A perforated 4” drain pipe runs along the base of the wall, behind the first course, sitting in the gravel base. This carries water to a daylight outlet at the end of the wall or to a catch basin.

Length: wall length plus whatever extra run you need to reach a daylight point. A 40-foot wall might need 55 feet of pipe if the outlet is 15 feet past the end.

Use perforated pipe behind the wall (holes down, on top of the gravel base) and transition to solid pipe where it exits beyond the wall end. Add a filter fabric sock over the perforated section to keep fines out — or wrap the entire gravel zone in filter fabric. Either works.

When You Need Geogrid

Taller walls need geogrid reinforcement, and where it starts depends on the block, the soil, and what sits above the wall. Taller walls may also require a concrete footing beneath the base course instead of just compacted gravel. Geogrid is a plastic mesh that ties the wall face to the soil mass behind it, turning the whole thing into a gravity structure.

The block manufacturer’s design charts say when grid is needed and where it goes. Two code limits sit on top of that. IRC R404.4 requires an engineered design for any retaining wall not braced at the top that holds back more than 48 inches of soil, and for walls over 24 inches that also carry a surcharge such as a driveway, slope, or building. Many jurisdictions also require a permit at a lower height.

Grid commonly extends back about 60% of the wall height or more, per the design. On a 4-foot wall that’s roughly 2.5 feet behind the block face. That means excavating a wider terrace behind the wall to lay the grid flat before backfilling over it.

The grid itself isn’t the expensive part. The extra excavation and compaction are.

Curves and Corners

Inside curves use the same block count (the gaps between blocks at the back are just wider). Outside curves require cutting the block face at an angle so the fronts meet tight. Budget 15% waste for walls with curves, versus 5-10% for straight walls.

90-degree corners need corner blocks (if the manufacturer makes them) or blocks cut at 45 degrees on a wet saw. Not all block systems handle corners gracefully. Versa-Lok and Allan Block have corner units. Others require creative cutting. For masonry walls using standard bricks instead of segmental blocks, the brick calculator covers per-square-foot counts and mortar estimates.

Running the Numbers

Trussly has a retaining wall calculator that takes wall length, height, and block size, then gives you block count with 5% waste, caps, base gravel, and drainage aggregate. It shows the formula behind each quantity and flags walls tall enough to need engineering.

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