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 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To find retaining wall blocks, divide the wall height by the block height for courses and the wall length by the block length for blocks per course, then multiply. A 20 ft wall 3 ft tall with 12 x 4 in blocks needs 9 courses of 20 blocks, 180 blocks or 189 with 5% waste, plus 20 cap blocks, 0.74 cubic yards of base gravel and 2.22 cubic yards of drainage gravel.

Results

Wall blocks to buy: 189 blocks; 180 blocks in the wall plus 5% waste
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 compacted crushed stone under the first course (block depth plus drainage zone)
Drainage gravel: 2.22 cu yd; 12 in of clean gravel behind the full height of the wall, over a perforated drain pipe at the base

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 ft

Worked 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 4.5 to 5 yards of 3/4 in clean stone in total.

Quick reference

Blocks and gravel per 10 ft of wall (no waste)
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. Keep gravity walls under 4 feet; above that, or with a slope or driveway above the wall, get an engineered design with geogrid.

Base, drainage and backfill

Dig a trench about 24 inches wide and deep enough for 6 inches of compacted 3/4 inch minus crushed stone plus the buried course. Level the base carefully; every error in the first course shows in all the ones above. Set the first course, check it for level in both directions, then stack the rest with the blocks offset by half a block.

Backfill directly behind the blocks with at least 12 inches of clean 3/4 inch drainage stone, with a perforated drain pipe at the base that daylights beyond the wall end. Water pressure, not soil weight, pushes over most failed walls. Compact native soil behind the drainage zone in 6 to 8 inch lifts, and glue the cap course with masonry adhesive.

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

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

For a 3 ft wall in 12 x 4 in blocks, 9 courses of 20 blocks is 180 blocks plus 20 caps, or about 189 blocks with 5% waste.

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)

Next steps for this project

The usual order for a concrete & masonry project. See all concrete & masonry calculators.

Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.