Retaining Wall

Estimating Blocks and Gravel for a Retaining Wall

The number of blocks a retaining wall needs depends on stacking enough rows to reach the wall’s height, with enough blocks in each row to span its length. Enter your wall’s length and height, the block’s own length and height, a backfill depth, and a waste allowance, and this calculator estimates how many blocks to order and how much drainage gravel you’ll need behind the wall.

The Formula

$$\text{Rows} = \left\lceil \frac{\text{Wall Height}}{\text{Block Height}} \right\rceil$$ $$\text{Blocks Per Row} = \left\lceil \frac{\text{Wall Length}}{\text{Block Length}} \right\rceil$$ $$\text{Blocks Needed} = \text{Rows} \times \text{Blocks Per Row}$$ $$\text{Gravel Volume} = \text{Wall Length} \times \text{Wall Height} \times \text{Backfill Depth}$$

Worked Example

A 6 m × 0.9 m retaining wall, using 400 mm × 200 mm blocks with a 30 cm gravel backfill and a 10% waste margin:

1. Rows: $\lceil 0.9 \div 0.2 \rceil = 5$.
2. Blocks per row: $\lceil 6 \div 0.4 \rceil = 15$.
3. Raw block count: $5 \times 15 = 75$ blocks.
4. With a 10% waste margin: $75 \times 1.10 \approx 83$ blocks.
5. Gravel volume: $6 \times 0.9 \times 0.3 = 1.62 \text{ m}^3$.

Key Factors to Consider

• A wall’s first (base) row is the foundation everything above it depends on being level and properly compacted. A well-prepared, compacted gravel base beneath the first course of blocks is widely considered the single most important step for a retaining wall’s long-term stability — an uneven or poorly-compacted base can cause the whole wall to shift or lean over time regardless of how well the rest is built.
• Taller walls generally need to be set back (battered) slightly rather than built perfectly vertical. Many block systems are designed with a slight built-in setback per course specifically to lean the wall back into the soil it’s retaining, which improves stability against the soil pressure pushing outward — check the specific block system’s own design for its intended batter.
• Soil type behind the wall affects how much pressure it actually needs to resist. Heavier, denser, or more water-saturated soil exerts more lateral pressure than lighter, well-draining soil — this is exactly why the drainage gravel layer matters so much, since it keeps water from accumulating and adding hydrostatic pressure on top of the soil’s own weight.
• Geogrid reinforcement is commonly required for taller retaining walls, beyond the blocks and gravel alone. Horizontal geogrid layers embedded into the soil behind the wall at intervals add significant additional resistance against the wall tipping or sliding — many jurisdictions’ permit requirements for taller walls specifically call for engineered reinforcement like this.

Common Mistakes

• Skipping the drainage gravel. A retaining wall holds back soil that traps rainwater — without a gravel backfill layer to let water drain away, pressure can build up behind the wall and cause it to lean or fail over time.
• Ignoring permit requirements for taller walls. Many local jurisdictions require a permit, and sometimes an engineered design, once a wall exceeds roughly 1.22 m / 4 ft — check with your local building department before starting.
• Confusing this with a flat hardscaping project. This calculator is for a stacked, vertical wall — for a flat patio or walkway instead, see the Paver Calculator or Concrete Calculator.

Useful to Know

• Need to figure out how much drainage gravel by itself, for a project beyond just this wall’s backfill? Gravel Calculator estimates volume and weight from length, width, and depth.
• Building a wood deck instead of a block wall? Decking Calculator estimates board count and fasteners from your deck’s dimensions.
• Planning a flooring project on the level ground this wall creates? Flooring Calculator estimates material and cost from room dimensions.