Enter your wall length and height and pick your block size, and this retaining wall calculator gives the blocks per course, the number of courses, the total blocks to buy, the cap blocks and the compacted gravel base. Works for concrete, cinder block and SRW walls, in feet and inches.
Total wall run: 20.0 ft
Base pad runs about 150 mm (6 in) wider than the block; the backfill is the free-draining gravel zone behind the wall.
Cap blocks
21
Courses high
9
Base gravel
0.43 yd³
Gravel to order (two different materials)
Under ~4 ft (1.2 m), no geogrid is usually needed - bury the bottom course and build on a compacted leveling pad. Check your local code before you start.
A segmental retaining wall is dry-stacked, not mortared, so it is counted by the course rather than by area. Each course is a row of whole blocks spanning the wall length, and the wall is as many courses high as the block height divides into the wall height:
blocks per course = ceil( wall length ÷ block width )
courses = round( wall height ÷ block height )
wall blocks = ceil( blocks/course × courses × wastage )
cap blocks = ceil( wall length ÷ cap width × wastage )
base gravel = wall length × (block depth + 150 mm) × pad depth
backfill gravel = wall length × wall height × backfill zone
curved length = π × diameter × arc ÷ 360 (developed run of a curve)
Building an L-shape, a stepped wall, a radiused bed or a tree ring? Add a section for each run - set it to Straight, Curved (diameter + arc) or Round (a full circle) - and the block counts pool into one order. The calculator sizes two lots of gravel: the compacted leveling pad the wall sits on, in a trench a little wider than the block, and the free-draining backfill zone behind the blocks that carries water down to the drain pipe - order them as two different materials. All math runs in exact metric internally, so switching between feet and metres never changes the answer.
The quick answer is blocks per course × number of courses, plus a cap row and a little waste. You do not need the wall area or a “blocks per square foot” figure - the block face width sets how many fit in a course, and the block height sets how many courses tall the wall is. Enter your wall length, your wall height and the block size above and the calculator returns the blocks per course, the courses, the total blocks to buy, the caps and the base gravel in one go - in square feet or metres, for a concrete, cinder, SRW or system block.
This example follows the unit system you pick in the calculator above.
A 20 ft long, 3 ft high wall in 12 × 4 in blocks (8 in deep), with a 12 in cap, on a 6 in gravel pad and a 12 in backfill zone, 5% wastage:
Common segmental block sizes and how many fit in one course - multiply by the number of courses for the wall total:
| Block | Face (W × H) | Depth | Blocks per m of course | Blocks per ft of course |
|---|---|---|---|---|
| Garden wall block | 300 × 150 mm (12 × 6 in) | 200 mm (8 in) | 3.3 | 1.0 |
| Standard SRW | 400 × 150 mm (16 × 6 in) | 300 mm (12 in) | 2.5 | 0.75 |
| Large SRW | 450 × 200 mm (18 × 8 in) | 300 mm (12 in) | 2.2 | 0.67 |
| Leveling pad | 100-150 mm (4-6 in) deep | block depth + ~150 mm wide | - | - |
So a 20 ft wall, 6 courses high, in 12 in blocks is 20 × 6 = 120 blocks before waste - the calculator does the rounding and adds the caps and base gravel.
In the United States, this is a segmental retaining wall (SRW). Under the International Building Code, a permit is needed for any wall over 4 ft (1.2 m) in total height, and at any height if it carries a surcharge such as a driveway or a slope above.
For the recognised design method and the approval rules that apply to your wall, see the CMHA (formerly NCMA) Design Manual for Segmental Retaining Walls. A tall or surcharged wall is an engineered structure - the calculator sizes the blocks and base, not the geogrid reinforcement, which depends on the soil you are holding back.
Go deeper on any part of the job:
Work out one course first - the wall length divided by the block face width, rounded up to whole blocks - then multiply by the number of courses, which is the wall height divided by the block height. A 20 ft long, 3 ft high wall in 12 × 4 in blocks is 20 blocks per course × 9 courses = 180 blocks, plus a cap row and a waste allowance. Enter your wall and block sizes above for the exact count.
Divide the wall height by the height of one block and round to the nearest whole course, because you build to a course line and cannot lay a part course. A 3 ft wall in 4 in blocks is 9 courses; a 3 ft wall in 6 in blocks is 6 courses. Remember the bottom course is usually buried, so the finished wall above ground is a little lower than the full stack.
Yes. The same block, course and base maths applies to any dry-stacked unit - concrete retaining wall block, cinder block, split-face SRW block, or a system block like Allan Block, Versa-Lok, Keystone, Pavestone, Belgard or Oldcastle. Pick a preset or type your block size in, and the calculator counts the blocks, caps and base gravel. It works just as well with blocks from Home Depot, Lowe's or Menards - the size is what matters, not the brand.
You can. Enter the timber or tie size as the block - for a 6×6 timber that is a 6 in (about 140 mm) face height and your chosen length, with as many courses as the wall is high. The block count then becomes the number of timbers or ties per course times the courses. Timber walls do not take cap blocks, so ignore the cap line, and remember they are usually held with deadmen and spikes rather than stacked dry.
Set the wall section to Curved (enter the diameter and the arc angle) or Round for a full circle, and the calculator works out the developed length - π × diameter × arc ÷ 360 - then counts the blocks, caps and base by course as normal, with a diagram of the curve. Curve-rated retaining blocks are tapered to fit a radius; standard straight blocks leave wedge gaps on a tight curve, so bump the waste. For a mortared round fire pit lined with fire brick, the [brick & block calculator](/brick-calculator/) has a matching round mode.
It depends where you are, but as a rough guide many places draw the line around 3 to 4 ft (1 to 1.2 m) of exposed height - above that you usually need a building permit and a wall designed by an engineer, with geogrid reinforcement built into the soil behind the blocks. A wall holding back a driveway, pool or slope (a surcharge) can need engineering at any height. The calculator flags when your wall is over the usual threshold.
Build the material list first - blocks, caps, base gravel, drainage backfill and adhesive from this calculator - price each line locally, then add labor, which is often half or more of an installed wall price. A taller wall costs much more per foot because it needs an engineered design and geogrid. See the cost guide below for the full breakdown rather than a single per-foot rate.
Allow about 5% for a straight wall with few cuts, and 10% where there are curves, corners and steps that need blocks cut to fit. Split-face and tumbled blocks also vary in colour between batches, so order a little long and keep some spares. The calculator adds your chosen waste allowance to the wall and cap counts before rounding up.
Turn a wall length, height and block size into a block count, by the course.
Read guide→Size the base leveling pad and the drainage backfill gravel for a retaining wall.
Read guide→Estimate retaining wall cost from the material list plus labor.
Read guide→Pavers, gravel sub-base, bedding sand, jointing sand and cost for a patio, path or driveway.
Open calculator→Volume and bags of gravel for driveways, paths and drainage at any depth.
Open calculator→Concrete blocks (cinder block / CMU), plus mortar and sand, from your wall sizes.
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