How Many Retaining Wall Blocks Do I Need?
Retaining wall blocks are sold by the piece, but the wall is measured by its length and height. Here is how to turn those into a block count - by the course, with the caps and a wastage allowance - for any block size.
By the Calculate My Reno Team / Published
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Open the Retaining Wall Calculator
Retaining wall blocks are priced per piece, but you measure the wall by its length and height - so the job is turning those two numbers into a block count, by the course, with the cap row and a little extra for cuts.
The formula
A dry-stacked wall is built one course (row) at a time, so the count is blocks-per-course times courses - not a “blocks per square foot” figure:
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 )
The retaining wall calculator does this for any block size and pools several wall sections into one order. The two numbers that drive everything are the wall length and the block size.
Blocks per course, by block size
How many blocks one course needs per length of wall - multiply by the number of courses for the wall total:
| Block face width | Blocks per m of wall | Blocks per ft of wall |
|---|---|---|
| 300 mm (12 in) | 3.3 | 1.0 |
| 400 mm (16 in) | 2.5 | 0.75 |
| 450 mm (18 in) | 2.2 | 0.67 |
| 600 mm (24 in) | 1.7 | 0.5 |
Wider blocks cover faster with fewer pieces; smaller blocks curve more easily and waste less on a tight radius.
Count the courses from the height
The number of courses is the wall height divided by the block height, rounded to the nearest whole course:
- A 150 mm (6 in) high block gives about 6.7 courses per metre of height, or 2 courses per foot.
- A 200 mm (8 in) high block gives 5 courses per metre, or 1.5 per foot.
- The bottom course is usually buried below grade (about a tenth of the wall height), so order for the full stack even though the finished wall above ground is a little lower.
Worked example
A 6 m long, 0.9 m high wall in 300 × 150 mm blocks, 5% wastage:
- Blocks per course: 6 ÷ 0.3 = 20.
- Courses: 0.9 ÷ 0.15 = 6.
- Wall blocks: 20 × 6 = 120, × 1.05 = 126 blocks.
- Cap blocks: 6 ÷ 0.3 = 20, × 1.05 = 21 caps.
In imperial, a 20 ft long, 3 ft high wall in 12 × 6 in blocks works out the same way: 20 per course × 6 courses = 120, plus the caps and waste.
A few tips
- Buy all the blocks in one batch so the colour matches across the whole wall, and keep a few spares.
- Dry-lay the first course to check the wall length lands without a thin sliver at the end, then adjust your start point.
- Order the base gravel and the drainage backfill at the same time, and price the whole job with the cost to build a retaining wall.
- For a wall over about 1 to 1.2 m (3 to 4 ft), or one holding back a driveway or slope, get an engineered design with geogrid before you order.
Frequently asked questions
How do I calculate how many retaining wall blocks I need?
Divide the wall length by the block face width and round up for the blocks in one course, then multiply by the number of courses, which is the wall height divided by the block height. Add a cap row and a wastage allowance. A 6 m long, 0.9 m high wall in 300 × 150 mm blocks is 20 per course × 6 courses = 120 blocks, plus 20 caps and about 5% waste. The retaining wall calculator does it for any block size.
How many retaining wall blocks are in a course?
One course is the wall length divided by the block face width, rounded up to a whole block. A 20 ft wall in 12 in blocks is 20 blocks per course; the same wall in 16 in blocks is ceil(20 ÷ 1.33) = 15. You round up because a part block at the end still has to be cut from a whole one.
Do I count blocks by the square foot?
No - retaining wall blocks are counted by the course, not by face area, because the block height and width matter separately. The number of courses comes from the wall height, and the blocks per course from the wall length, so a "blocks per square foot" figure does not capture it. Use blocks per course × courses instead.
How many extra blocks should I order?
Allow about 5% for a straight wall and 10% where there are curves, corners and steps with blocks cut to fit. Split-face and tumbled blocks vary in colour between batches, so order a little long and keep a few spares for future repairs - colours get discontinued.