How many retaining wall blocks per square foot?

3 of the 12 x 4 in block, 1.5 of the 16 x 6 and 1 of the 18 x 8, dry-stacked with no joint. Multiply by the wall length x height in feet, add 5% for the cuts at corners and curves. Check the face size on your block; makers round the labels.

How many blocks for a 2 ft high retaining wall?

63 blocks of 16 x 6 in for 20 ft (4 courses), 95 for 30 ft and 158 for 50 ft. In the 12 x 4 in block: 126, 189 and 315. Bury the first course, so a wall that shows 2 ft is really about 2 ft 6 in of blocks: use the next row down in the tables if your block is not a multiple.

How much gravel for a retaining wall base?

A trench 6 inches deep and twice the block depth wide (20 inches for a 16 x 6 block): 0.82 tons or 35 bags of 0.5 cu ft for 20 ft, 2.05 tons for 50 ft, compacted in two lifts. Use 3/4 inch minus crushed stone that locks together, not pea gravel.

How much drainage gravel behind a retaining wall?

12 inches of clean, open-graded stone the full height of the wall, with a perforated pipe at the bottom: 1.97 tons for 20 ft at 2 ft high, 7.37 tons for 50 ft at 3 ft. Water pressure, not soil, is what pushes walls over; the drainage stone is not optional.

How high can I build a retaining wall without an engineer?

Most block makers rate their systems for 3 to 4 ft (about 1 m) as a gravity wall on a proper base with drainage; above that, or with a slope, a driveway or a structure above the wall, you need geogrid and an engineered design, and many towns require a permit above 3 or 4 ft. A 50 ft wall at 3 ft is already 237 blocks of 16 x 6 in and 7.4 tons of drainage stone; two 2 ft walls with a terrace between them are usually the cheaper answer.

How many cap blocks do I need?

Caps are sold by the linear foot of wall: a 20 ft wall takes about 15 caps of 16 in plus one or two spares for cuts. They are not in the block counts above; add them separately with the adhesive (roughly one tube per 15 to 20 ft of cap).

How many blocks for a retaining wall? (12x4, 16x6 and 18x8 in)

A 20 ft long retaining wall 2 ft high is 40 sq ft of face, which takes 63 blocks of 16 x 6 in (4 courses of 15) with 5% extra for cuts, 126 of the small 12 x 4 in garden block, or 42 of the big 18 x 8. Under it goes about 0.8 tons of crushed stone for the base and 2 tons of drainage stone behind it. At 3 ft the same wall is 95 blocks of 16 x 6, and 3 ft is where most makers say to bring in an engineer. The tables cover 10 to 50 ft walls, three block sizes, the stone, and metric blocks.

Open the brick and block calculator

Blocks for walls 1, 2 and 3 ft high

Dry-stacked, 5% extra for cuts; courses and blocks per course rounded up. Sizes are the face (length x height); depth only affects the base width.

1 ft high (3 courses of 4 in, 2 of 6 in), 5% extra
Wall length	Face (sq ft)	12 x 4 in	16 x 6 in	18 x 8 in
10 ft	10	32	16	11
20 ft	20	63	32	21
30 ft	30	95	48	32
40 ft	40	126	63	42
50 ft	50	158	79	53

2 ft high, 5% extra
Wall length	Face (sq ft)	12 x 4 in	16 x 6 in	18 x 8 in
10 ft	20	63	32	21
20 ft	40	126	63	42
30 ft	60	189	95	63
40 ft	80	252	126	84
50 ft	100	315	158	105

3 ft high, 5% extra
Wall length	Face (sq ft)	12 x 4 in	16 x 6 in	18 x 8 in
10 ft	30	95	48	32
20 ft	60	189	95	63
30 ft	90	284	142	95
40 ft	120	378	189	126
50 ft	150	473	237	158

Base and drainage stone

Base trench 6 in deep x 20 in wide (for a 16 x 6 block), compacted crushed stone; drainage 12 in of clean stone behind the wall for its full height. 5% extra on both.

Crushed stone base and drainage stone
Wall	Base (tons)	Base (bags)	Drainage (tons)	Drainage (cu yd)
10 ft x 1 ft	0.41	18	0.49	0.39
20 ft x 1 ft	0.82	35	0.98	0.78
30 ft x 1 ft	1.23	53	1.47	1.17
40 ft x 1 ft	1.64	70	1.97	1.56
50 ft x 1 ft	2.05	88	2.46	1.94
10 ft x 2 ft	0.41	18	0.98	0.78
20 ft x 2 ft	0.82	35	1.97	1.56
30 ft x 2 ft	1.23	53	2.95	2.33
40 ft x 2 ft	1.64	70	3.93	3.11
50 ft x 2 ft	2.05	88	4.92	3.89
10 ft x 3 ft	0.41	18	1.47	1.17
20 ft x 3 ft	0.82	35	2.95	2.33
30 ft x 3 ft	1.23	53	4.42	3.5
40 ft x 3 ft	1.64	70	5.9	4.67
50 ft x 3 ft	2.05	88	7.37	5.83

How the numbers are worked out

Face area = length x height. A 16 x 6 in block covers 0.667 sq ft, so 1.5 blocks per square foot; the brick and block calculator does the same sum for any face size when you set the joint to zero. The stone is length x width x depth through the gravel calculator, and the concrete block wall guide covers mortared CMU walls, which are a different job.

Building it

Dig the trench, compact the stone base in two lifts, and spend the time on the first course: level along and across, every block. The rest of the wall copies whatever the first course does.
Set the wall back into the slope (most blocks have a lip or pin that does this), and bury at least one course, more on a slope.
Lay the perforated drain pipe on the stone base behind the first course, daylight it at the low end, and backfill with the drainage stone as you go up; soil only in the last 6 to 8 inches.
Over about 3 ft, on a slope, or under a drive: geogrid every two or three courses and an engineer's drawing. The French drain guide covers the pipe and stone behind the wall in more detail.

Metric: 300 x 100 and 400 x 150 mm blocks

Blocks with 5% extra for walls 3 to 15 m long and 0.3 to 0.9 m high, dry-stacked.

Metric blocks, 5% extra
Wall	300 x 100 mm	400 x 150 mm
3 m x 0.3 m	32	16
6 m x 0.3 m	63	32
10 m x 0.3 m	105	53
15 m x 0.3 m	158	79
3 m x 0.6 m	63	32
6 m x 0.6 m	126	63
10 m x 0.6 m	210	105
15 m x 0.6 m	315	158
3 m x 0.9 m	95	48
6 m x 0.9 m	189	95
10 m x 0.9 m	315	158
15 m x 0.9 m	473	237

Frequently asked questions

How many retaining wall blocks per square foot?: 3 of the 12 x 4 in block, 1.5 of the 16 x 6 and 1 of the 18 x 8, dry-stacked with no joint. Multiply by the wall length x height in feet, add 5% for the cuts at corners and curves. Check the face size on your block; makers round the labels.
How many blocks for a 2 ft high retaining wall?: 63 blocks of 16 x 6 in for 20 ft (4 courses), 95 for 30 ft and 158 for 50 ft. In the 12 x 4 in block: 126, 189 and 315. Bury the first course, so a wall that shows 2 ft is really about 2 ft 6 in of blocks: use the next row down in the tables if your block is not a multiple.
How much gravel for a retaining wall base?: A trench 6 inches deep and twice the block depth wide (20 inches for a 16 x 6 block): 0.82 tons or 35 bags of 0.5 cu ft for 20 ft, 2.05 tons for 50 ft, compacted in two lifts. Use 3/4 inch minus crushed stone that locks together, not pea gravel.
How much drainage gravel behind a retaining wall?: 12 inches of clean, open-graded stone the full height of the wall, with a perforated pipe at the bottom: 1.97 tons for 20 ft at 2 ft high, 7.37 tons for 50 ft at 3 ft. Water pressure, not soil, is what pushes walls over; the drainage stone is not optional.
How high can I build a retaining wall without an engineer?: Most block makers rate their systems for 3 to 4 ft (about 1 m) as a gravity wall on a proper base with drainage; above that, or with a slope, a driveway or a structure above the wall, you need geogrid and an engineered design, and many towns require a permit above 3 or 4 ft. A 50 ft wall at 3 ft is already 237 blocks of 16 x 6 in and 7.4 tons of drainage stone; two 2 ft walls with a terrace between them are usually the cheaper answer.
How many cap blocks do I need?: Caps are sold by the linear foot of wall: a 20 ft wall takes about 15 caps of 16 in plus one or two spares for cuts. They are not in the block counts above; add them separately with the adhesive (roughly one tube per 15 to 20 ft of cap).

How many bricks per square metre (and per square foot)?
Bricks and blocks per square metre and per square foot for the common formats, how the mortar joint changes the count, and what to order for a 10 m² wall.
How many bricks per square foot? Walls, patios and floors
Modular bricks per square foot of wall (about 7) and laid flat on a patio (4.5 to 5), bricks for 100 to 500 sq ft with 5% waste, mortar, and the UK brick per sq ft.
How many bricks do I need for a fire pit? 30, 36 and 48 inch
Bricks per course and in total for round fire pits of 30 to 48 in inside diameter at 3 to 5 courses, in modular, UK and fire brick sizes, with 5% waste, and square pits.
How many bags of mortar per 100 bricks (and per 100 blocks)?
80 lb and 60 lb mortar bags per 100, 500 and 1,000 bricks or 8x8x16 blocks from the label yields, mortar volume per square metre of wall, and 25 kg bags for UK bricks.

Last updated 2026-09-13. Figures are planning estimates from typical product data: check the yield, size or coverage printed on what you buy and round up.