How many blocks do I need for a retaining wall?

Length x height of the wall face in feet, divided by the face of one block (16 x 6 in is 0.667 sq ft), plus 5% for cuts, plus one buried course. A 20 ft wall showing 2 ft is 79 blocks of 16 x 6; a 30 ft wall showing 3 ft is 166. The retaining wall guide has three block sizes; the calculator does any block with the joint set to zero.

Does a retaining wall need a concrete footing?

Not a segmental block wall under about 3 to 4 ft: it sits on 6 inches of compacted crushed stone in a trench, with the first course buried. A concrete footing makes the base rigid, and a rigid base under a wall meant to flex slightly cracks blocks. Mortared block and poured walls are a different system and do need a footing.

How deep should the base be for a retaining wall?

6 inches of compacted crushed stone (3/4 inch minus) in a trench twice as wide as the block is deep, so 20 inches for a 10 inch block, plus the height of one buried course: dig about 12 inches. On soft clay go to 8 inches of stone and a geotextile under it.

How much gravel goes behind a retaining wall?

12 inches of clean, open-graded stone (3/4 inch clean, no fines) the full height of the wall, with a 4 inch perforated pipe at the bottom. For 20 ft at 2 ft that is 1.97 tons; for 30 ft at 3 ft, 4.42 tons. The pipe daylights at the low end or into a drain; water that cannot get out is what pushes walls over.

How high can a DIY retaining wall be?

Most block systems are rated to 3 or 4 ft as a gravity wall on level ground with drainage; above that, or with a slope, a driveway or a building above, the wall needs geogrid reinforcement and an engineer, and many towns require a permit past 3 or 4 ft. If you need more height, two lower walls with a planted terrace between are cheaper and safer.

Do retaining wall blocks need glue?

Only the caps: a bead of concrete adhesive on the top course holds them on. The wall blocks themselves interlock with a rear lip or pins and are held by their own weight and the setback. Glueing every course does nothing for strength and makes the wall impossible to adjust.

How to build a block retaining wall under 3 ft (with quantities)

Published 2026-09-13 · 7 min read

A retaining wall under about 3 ft is a weekend job with segmental blocks: no mortar, no footing, just a trench, a compacted base, one course set dead level and the rest stacked on it with the drainage that keeps it standing. This is the job in order, with the quantities for a 16 x 6 in block, so a 20 ft wall showing 2 ft is known in advance to be 79 blocks (5 courses, one of them buried), 16 caps, 0.8 tons of base stone and 2 tons of drainage stone.

Brick and block calculator Gravel calculator

Before you start

Three questions. Is there anything above the wall (a slope, a shed, a drive) that will load it? Is it over about 3 ft anywhere along its length? Does the ground stay wet? A yes to the first two means an engineered design, not this article; a yes to the third means the drainage matters even more than usual. Call the locating service before you dig, and check the local permit threshold.

How much you need

16 x 6 in blocks, dry-stacked, 5% extra for cuts, one course buried below grade. Base: 6 in of crushed stone, 20 in wide. Drainage: 12 in of clean stone behind the wall, full exposed height. Caps are one per block length plus a spare.

16 x 6 in block, 5% extra, one buried course
Wall (length x exposed height)	Courses	Blocks	Caps	Base stone (tons)	Drainage stone (tons)
10 ft x 1 ft	3	24	9	0.41	0.49
20 ft x 1 ft	3	48	16	0.82	0.98
30 ft x 1 ft	3	71	24	1.23	1.47
50 ft x 1 ft	3	119	39	2.05	2.46
10 ft x 2 ft	5	40	9	0.41	0.98
20 ft x 2 ft	5	79	16	0.82	1.97
30 ft x 2 ft	5	119	24	1.23	2.95
50 ft x 2 ft	5	197	39	2.05	4.92
10 ft x 3 ft	7	56	9	0.41	1.47
20 ft x 3 ft	7	111	16	0.82	2.95
30 ft x 3 ft	7	166	24	1.23	4.42
50 ft x 3 ft	7	276	39	2.05	7.37

Other block sizes, the bags for small walls and metric blocks are in the retaining wall guide; the block calculator and the gravel calculator take any size. Order the base stone and the drainage stone as two products: stone with fines for the base, clean stone with none for the drainage.

Step 1: the trench

Mark the line, then dig a trench 20 in wide and about 12 in deep, so there is room for 6 in of stone plus the buried first course. Keep the bottom level along the wall; on a slope, step the trench in whole block heights rather than following the grade. Compact the soil at the bottom.

Step 2: the base

Fill with crushed stone in two lifts, compacting each with a plate compactor until it rings. Screed the top flat with a straight board; this surface decides whether the wall is level, so spend the time here. On wet or soft ground lay a woven geotextile in the trench first.

Step 3: the first course, dead level

Set the first block, tap it down, and check it level front to back and along the wall. Set the next one to it, and so on; check every block, both ways, and check back along the run every few blocks with a long level. This course is buried and nobody will see it, and it is the only one that matters. If the ground drops, start the next step down with a full course below and bring it up to meet the first.

Step 4: the drain pipe

Behind the first course, lay 4 in perforated pipe on a couple of inches of clean stone with the holes down, falling to the low end where it daylights or joins a drain. Wrap it in a sock or cover it with fabric so the fines cannot clog it.

Step 5: stack, set back, backfill as you go

Sweep each course clean and stack the next with the joints staggered by half a block; the rear lip on most blocks sets the wall back into the slope automatically. After every course, backfill behind the wall with the clean drainage stone to 12 in, then soil beyond that, and compact the soil in thin layers with a hand tamper (a plate compactor within 3 ft of the wall pushes it over). Cut blocks with a chisel and hammer or a saw for the ends and any corner.

Step 6: caps and the top

Finish the drainage stone about 6 in below the top, lay fabric over it and top with soil so the lawn or bed does not wash into the stone. Glue the caps on with concrete adhesive, overhanging the face by an inch, and grade the soil above the wall so surface water runs away from it, not over it.

Building walls for other people?

The calculators give you the block count and the stone. The Pro quote builder turns that into a quote with your labour, the pipe, the caps and adhesive, delivery and margin, printed with your logo.

Frequently asked questions

How many blocks do I need for a retaining wall?: Length x height of the wall face in feet, divided by the face of one block (16 x 6 in is 0.667 sq ft), plus 5% for cuts, plus one buried course. A 20 ft wall showing 2 ft is 79 blocks of 16 x 6; a 30 ft wall showing 3 ft is 166. The retaining wall guide has three block sizes; the calculator does any block with the joint set to zero.
Does a retaining wall need a concrete footing?: Not a segmental block wall under about 3 to 4 ft: it sits on 6 inches of compacted crushed stone in a trench, with the first course buried. A concrete footing makes the base rigid, and a rigid base under a wall meant to flex slightly cracks blocks. Mortared block and poured walls are a different system and do need a footing.
How deep should the base be for a retaining wall?: 6 inches of compacted crushed stone (3/4 inch minus) in a trench twice as wide as the block is deep, so 20 inches for a 10 inch block, plus the height of one buried course: dig about 12 inches. On soft clay go to 8 inches of stone and a geotextile under it.
How much gravel goes behind a retaining wall?: 12 inches of clean, open-graded stone (3/4 inch clean, no fines) the full height of the wall, with a 4 inch perforated pipe at the bottom. For 20 ft at 2 ft that is 1.97 tons; for 30 ft at 3 ft, 4.42 tons. The pipe daylights at the low end or into a drain; water that cannot get out is what pushes walls over.
How high can a DIY retaining wall be?: Most block systems are rated to 3 or 4 ft as a gravity wall on level ground with drainage; above that, or with a slope, a driveway or a building above, the wall needs geogrid reinforcement and an engineer, and many towns require a permit past 3 or 4 ft. If you need more height, two lower walls with a planted terrace between are cheaper and safer.
Do retaining wall blocks need glue?: Only the caps: a bead of concrete adhesive on the top course holds them on. The wall blocks themselves interlock with a rear lip or pins and are held by their own weight and the setback. Glueing every course does nothing for strength and makes the wall impossible to adjust.

Figures are planning estimates from the calculators on this site and typical product data. Check the yield, size or coverage printed on what you buy, round up, and follow your local building code for structural work.