How many blocks do I need for a retaining wall?
Blocks per course = wall length ÷ block face width. Courses = total wall height ÷ block height — and total height includes the buried first course, not just what you see.
A 30 ft wall 3 ft high, built from 12 in wide × 4 in tall block: 30 blocks per course, and the height is 3 ft plus a buried 4 in, which is 3.33 ft = 10 courses. That is 300 blocks, or 315 with 5% breakage.
The buried course is not optional and it is not padding. Bury one full block height, or 10% of the wall height, whichever is greater. It is what resists the toe of the wall being pushed out, and a wall built on the surface rotates forward within a few freeze cycles.
How the calculation works
Blocks = ceiling(length ÷ face width) × ceiling(total height ÷ block height), plus breakage. Cap blocks are one additional course, glued rather than stacked.
The base trench is 6 inches of compacted crusher run, dug about twice the block depth wide so the block sits on a level, fully supported bed with room behind it. That is roughly 0.5 ft × twice the block depth × the wall length, divided by 27 for yards.
Drainage stone is 12 inches of clean angular stone — #57 or similar with no fines — behind the full height of the wall, with filter fabric separating it from the retained soil and a perforated pipe at the bottom that daylights somewhere lower.
That drainage is the whole design. Water in the retained soil is what pushes walls over: saturated soil weighs half again as much as dry, and hydrostatic pressure behind a wall with no outlet is far beyond what stacked block resists. A wall built with perfect block work and no drain fails; one with mediocre block work and a proper drain usually does not.
Typical retaining wall cost, mid-2026
| Material | Unit | Typical range |
|---|---|---|
| Standard landscape block | each | $3.00 – $9.00 |
| Cap block | each | $5.00 – $14.00 |
| Crusher run base | per yd³ | $30.00 – $55.00 |
| Clean drainage stone (#57) | per yd³ | $40.00 – $90.00 |
| Non-woven filter fabric | per sq ft | $0.15 – $0.50 |
| Perforated pipe | per lf | $1.50 – $4.00 |
| GeogridRequired above ~3 ft | per sq ft | $0.40 – $1.20 |
| Masonry adhesive | per tube | $8.00 – $16.00 |
| Installation labour | per sq ft of face | $15.00 – $45.00 |
Walls over 4 ft — measured from the bottom of the footing, not from grade — generally need an engineered design and a permit. That is a legal threshold, not a suggestion.
What goes wrong on this job
- Spend the time on the base course. Every course above copies its errors and they compound — an eighth of an inch out of level at the bottom is two inches out by the tenth course.
- Never skip the drain. Hydrostatic pressure behind a wall is what actually knocks walls down, and it is the one failure you cannot fix afterwards without dismantling the whole thing.
- Use clean angular stone behind the wall, not soil and not pea gravel. It needs to drain freely and to interlock; rounded stone does neither.
- Step the base up in level runs on a slope rather than sloping the courses. Blocks are made to sit flat, and a sloped course leaves every joint slightly open.
- Check the block manufacturer geogrid table before you build over about 3 ft. It is specific to the block, the soil and the slope above the wall, and it is not something to eyeball.
Frequently asked questions
How many retaining wall blocks do I need?
Wall length ÷ block face width gives blocks per course; total height ÷ block height gives the number of courses. A 30 ft wall 3 ft high from 12 × 4 in block needs about 300 blocks including the buried course.
How deep should the first course be buried?
One full block height, or 10% of the wall height, whichever is greater. It resists the toe of the wall being pushed forward.
How high can I build a retaining wall without an engineer?
Commonly 4 ft, measured from the bottom of the footing rather than from finished grade — but the threshold varies by jurisdiction, so confirm locally before you order block.
Do I need drainage behind a retaining wall?
Yes, on any wall retaining real soil. Twelve inches of clean stone, filter fabric and a perforated pipe that daylights. Water pressure is the dominant cause of retaining wall failure.
What goes under a retaining wall?
Six inches of compacted crusher run in a trench roughly twice the block depth wide, levelled carefully. The base course sets up every course above it.