Holding Back the Earth
A retaining wall is a vital structural element in landscape architecture. Whether you are trying to flatten a sloped backyard to create a usable lawn, building a raised patio, or preventing a hillside from eroding into your driveway, a retaining wall physically holds back massive tons of soil.
Because segmental retaining wall (SRW) blocks are extremely heavy (often 50 to 80 pounds each) and usually delivered on wooden pallets by a forklift, you must calculate your exact requirements before ordering. Ordering too few stops your project cold. Ordering too many leaves you with thousands of pounds of concrete in your driveway that you cannot easily move or return.
The Science of Retaining Walls
Unlike a decorative brick wall, a retaining wall is an engineered structure. The soil behind the wall exerts immense horizontal pressure (called "hydrostatic pressure" when wet).
To prevent the wall from blowing out, standard interlocking retaining wall blocks feature a rear lip that forces each row to step backward slightly (the "batter"), leaning into the hill.
The 4-Foot Rule: In almost all jurisdictions, a homeowner can build a retaining wall up to 4 feet tall (including the buried base course) without a permit. If you need a wall taller than 4 feet, building codes mandate that you hire a licensed structural engineer, as the wall will require complex "geogrid" tie-backs buried deep into the hillside.
How to Calculate Retaining Wall Blocks
Calculating blocks is a simple two-dimensional area calculation. You must find the square footage of the face of your wall, and divide it by the square footage of the face of a single block.
The Formula
- Measure the Length and the Height of your planned wall in feet.
- Multiply Length × Height to find the Total Wall Area in square feet.
- Check the dimensions of your block (Length and Height) in inches.
- Convert the block dimensions into feet by dividing by 12.
- Multiply the block length × block height (in feet) to find the Block Face Area.
- Divide the Total Wall Area by the Block Face Area.
- Add a 5% to 10% Waste Factor to account for cuts at the ends of the wall or broken blocks.
Total Blocks = (Wall Length × Wall Height) ÷ ((Block Length ÷ 12) × (Block Height ÷ 12))
Example Calculation
You are building a wall that is 30 feet long and exactly 3 feet high. You are buying standard commercial blocks that are 16 inches long and 6 inches high.
- Total Wall Area:
30 ft × 3 ft = 90 sq ft - Convert Block to feet:
(16 ÷ 12 = 1.33 ft)and(6 ÷ 12 = 0.5 ft) - Block Face Area:
1.33 ft × 0.5 ft = 0.66 sq ft - Divide:
90 ÷ 0.66 = 136.3 blocks - Add 10% Waste:
136.3 × 1.10 = 150 blocks
You need to purchase exactly 150 retaining wall blocks.
The Hidden Base Course
Our calculator estimates the visible face of the wall. However, structural retaining walls cannot be built on top of grass or topsoil.
You must dig a trench, fill it with compacted crushed stone, and fully bury the entire first row of blocks (the "base course") below ground level to lock the bottom of the wall in place. When calculating the height of your wall, you must include the buried base course in your total height measurement.