How Many Retaining Wall Blocks Do I Need? Courses, Caps & a Worked Example

Planning estimate, not a structural design. Retaining walls are counted in courses, not by area — and the buried course, the caps, and the drainage are where estimates go wrong. This guide gives you the method, then run your wall through our retaining wall calculator.

The course method

Count in two steps. Courses = total wall height in inches ÷ block face height, rounded up. Blocks per course = wall length in inches ÷ block face length, rounded up. Multiply: a 20 ft long, 2 ft high wall in 16 × 8 in blocks is 3 courses of 15 blocks = 45 blocks. Area math (wall area ÷ block face area) gives the same 45 here only by luck — on a wall with a half-block remainder at the end of every course, area math undercounts by a full block per course.

The buried course

The first course sits below finished grade: bury at least one full block height, or 10% of total wall height, whichever is more. For a typical 8-inch unit that is at least 8 inches below grade. The wall height in your count must be the total height including that buried course, not just the height you see. Bury deeper if the ground slopes away in front of the wall, since the soil at the toe is what resists the wall sliding forward.

Caps

Caps are flat finishing units for the top course: one cap per block on the top course, usually glued in place, and usually priced separately from wall blocks. A 20 ft wall in 16-inch blocks needs 15 caps. Caps shed water off the top of the wall and cover the hollow cores and connection pins of the blocks below — do not skip them, and do not count them as structural courses.

Drainage: the part that keeps the wall standing

Water pressure behind a wall is the leading cause of retaining wall failure. Standard practice: a 12-inch column of free-draining crushed stone directly behind the blocks, a drainage pipe at the base for walls over about 3 feet, and a gravel base trench 4–6 inches deep and a few inches wider than the block on each side. Budget the stone separately — roughly a ton of drainage gravel per 8–10 feet of 3-foot wall, depending on block depth — and never backfill with clay soil, which holds water against the wall.

Worked example: 30-foot wall, 3 feet high

Courses. 36 in ÷ 8 in = 5 courses (including the buried course).
Blocks per course. 360 in ÷ 16 in = 22.5 → 23 blocks.
Wall blocks. 5 × 23 = 115 blocks.
With waste. 115 × 1.10 = 126.5 → 127 blocks.
Caps. 23 cap blocks for the top course.

The 4-foot line

Most jurisdictions require an engineered design and a permit above 3–4 feet of exposed height, and surcharge loads — a driveway, a slope, or a building above the wall — can lower that threshold. Segmental dry-stacked blocks are engineered for gravity walls out of the box, but a mortared CMU wall holding back soil is a structural element needing a reinforced footing, vertical rebar, and grouted cores. When in doubt, check with your building department before you dig.

Frequently asked questions

How many retaining wall blocks do I need for a 20-foot wall?

A 20 ft long, 2 ft high wall with 16 × 8 in blocks is 3 courses of 15 blocks = 45 blocks — order 50 with 10% waste, plus 15 cap blocks.

How many blocks per square foot of retaining wall?

Roughly 1 block per 0.89 sq ft for a 16 × 8 in face, but course rounding at the ends of the wall is why you should count whole blocks per row instead of dividing area.

How deep should the base course be buried?

At least one full block height, or 10% of the total wall height, whichever is more. For a typical 8-inch unit that is at least 8 inches below finished grade.

Do retaining walls need drainage gravel?

Yes — always. A 12-inch column of free-draining crushed stone behind the blocks, plus a drainage pipe at the base for walls over about 3 feet, relieves the water pressure that causes walls to lean and fail.

How tall can a retaining wall be without an engineer?

Most jurisdictions set the trigger at 3–4 feet of exposed height, and surcharge loads above the wall can lower it. Check with your building department before you dig.

The rest of the wall job

Blocks are only part of the order — figure gravel for the base and drainage, and for a mortared block wall instead, the concrete block calculator covers blocks and mortar.