Retaining wall calculator

How many retaining wall blocks you need

Count the courses, then the blocks in each course. The courses fill the exposed height plus the buried part, less the cap, divided by the block height and rounded up. The blocks per course are the wall length divided by the block face, rounded up.

A 20 ft wall showing 24 in, with 6 in buried and a 2 in cap, in Pavestone RockWall Large blocks (17.44 in face, 6 in high) is 5 courses of 14: 70 blocks, or 74 with 5% for cuts. In the RockWall Small block (11.63 in face, 4 in high) the same wall is 7 courses of 21, 155 blocks with cuts.

Cap blocks and adhesive

Caps run along the top, one face length at a time. At 12 in each, the example needs 20 caps. They are glued down: a 10 fl oz tube of LOCTITE PL 500 Landscape gives about 13.6 ft of the 3/8 in bead its data sheet recommends outdoors, and with two beads along the wall that is 3 tubes for 20 ft.

Corners and curves

The count is for a straight wall. Corners and curves add cuts, so keep the allowance and round up when you order from a yard that takes returns.

Base and drain gravel

The first course sits on a compacted leveling pad of 3/4 in minus base, at least 6 in wider than the block on each side and 6 in deep after compaction. For the 7 in deep RockWall Large that is a 19.0 in wide pad, 0.6 cu yd for the example. Behind the blocks goes at least 12 in of clean drainage aggregate for the full height, 2.0 cu yd here, with 2 lengths of 4 in perforated pipe at the base.

Why drainage matters

Water that sits behind a wall pushes on it, and wet soil pushes harder. The stone lets water fall to the pipe, and the pipe carries it out to daylight or a drain. Belgard spaces outlets no more than 50 ft apart and at the low points. Keep regular soil out of the drain stone, or it clogs.

Tons or cubic yards

Gravel sells by the ton, the cubic yard or the bag. We use 1.4 tons per cubic yard and add 10% for compaction, and the shopping list counts 0.5 cu ft bags. Your yard knows the weight of what it sells, so ask before you order in bulk.

Retaining wall cost

The calculator prices the list at the Home Depot Hamilton Mill store, one-piece shelf prices read Nov 6, 2026, before tax and delivery: the RockWall Large block at $5.97, the RockWall Small at $2.97, the 12 in cap at $2.87, a tube of PL 500 at $6.48 and a 0.5 cu ft bag of paver base at $6.47, with Quikrete drainage rock at $6.18 a 0.5 cu ft bag from Lowe's near Chamblee, GA. Blocks bought by the pallet cost less each.

For the 20 ft wall showing 24 in, that is $1,454.81 in materials priced so far. Every line has a price. We have not read a named installed range for retaining walls, so we print none.

How tall can you build without an engineer

The calculator stops at 48 in. Under the model code, IRC R404.4, a wall not supported at the top that retains more than 48 in of unbalanced fill, or a wall over 24 in that holds a load besides the soil, is designed by engineering. A driveway, a parked car, a shed or a slope rising behind the wall all count as that load, so tick the box and the limit drops to 24 in.

The permit test is different. IRC R105.2 exempts a wall up to 4 ft measured from the bottom of the footing to the top, unless it supports a surcharge, and the calculator adds the pad, the buried course and the exposed height to show where yours lands. Local codes change both numbers.

Walls beside a patio

A low wall often holds the edge of a patio on a slope. Count the pavers, base and sand with the paver calculator, or the pour with the concrete slab calculator, and price the whole job in the patio cost calculator. The paver patio cost guide sets the sizes side by side. Call 811 before any digging.