YardHow to use the retaining wall calculator
Calculate retaining wall blocks
The first course of a block retaining wall should be buried below ground on a compacted gravel pad about 6 inches deep. Walls over about 4 ft high usually need an engineered design - check local rules. Estimates only.
- Enter the wall length in feet, not counting any returns or corners.
- Enter the height above ground in feet. The calculator adds one buried course below ground when the buried-course option is on, which is how block walls should be built.
- Enter your block length and height in inches (12 × 4 in is a common size) and the cap block length.
- Read off the total blocks, cap blocks and gravel. Run the calculator again for each separate wall section and add the results.
How the block count works
Blocks per course is simply the wall length divided by the block length. Courses is the wall height divided by the block height, rounded up. Multiply the two and you have the blocks for the visible wall. This calculator then adds one buried course, because the first course of a retaining wall should sit below ground on the gravel pad, and a waste allowance (10 percent by default) for cuts, corners and the odd broken block.
Worked example: a 20 ft wall with 12 inch blocks needs 20 blocks per course. A 3 ft wall with 4 inch blocks needs 9 courses above ground, plus 1 buried course makes 10, so 200 blocks before waste, and 220 with 10 percent extra. A 12 × 4 inch block face covers a third of a square foot, which is why a quick check is 3 blocks per square foot of wall face.
Do not forget the base and the backfill
More retaining walls fail from a poor base than from the blocks themselves. The calculator allows for a gravel base pad about 2 ft wide and 6 inches deep along the whole wall, levelled and compacted, with the first course buried on it. Behind the wall it allows a drainage layer of clean gravel about 12 inches wide for the full height, which lets water escape instead of pushing the wall over. Many builders also lay a perforated drainage pipe in that gravel at the base. Keep topsoil and mulch out of the drainage layer.
Common block sizes
| Block (length × height) | Face area | Blocks per sq ft of wall |
|---|---|---|
| 12 in × 4 in | 0.33 sq ft | 3.0 |
| 16 in × 6 in | 0.67 sq ft | 1.5 |
| 18 in × 6 in | 0.75 sq ft | 1.33 |
| 8 in × 4 in (small garden block) | 0.22 sq ft | 4.5 |
Block depth (front to back) is usually 8 to 12 inches and does not change the count, but it does change the width of the trench and base pad you dig. Check the depth of the block you are buying before you dig.
Tips before you order
- Walls over about 4 ft high, and walls holding back a slope, driveway or building, usually need an engineer's design and often a permit. Check local rules first.
- Order blocks and caps from the same batch so colours match, and keep a few spares for future repairs.
- Step the base up in sections on a slope instead of following the slope, keeping every course level.
- Compact the base and each backfill layer as you go. Most leaning walls were never compacted.