Retaining Wall Calculator
Calculate how many landscaping blocks and tons of drainage gravel you need to build a structural retaining wall.
On this page
Retaining Wall Cross Section
A retaining wall is more than just stacked blocks. To prevent the wall from collapsing, you must have a compacted gravel base pad, and a 12-inch column of drainage gravel backfilled directly behind the blocks.
How to Use the Retaining Wall Calculator
A retaining wall isn't just a row of bricks—it's an engineering system. Our calculator tells you how many decorative blocks to buy, and calculates the exact amount of hidden structural gravel required to keep the wall standing.
- Measure the wall: Enter the total length and desired height of the wall. Remember that your wall height must include the first row of blocks, which will be buried underground.
- Enter block dimensions: Hardware stores sell blocks in various sizes (e.g., 12" wide x 4" tall, or 16" wide x 6" tall). Enter the face dimensions of the specific block you are buying.
Retaining Wall Material Formula
We calculate the total surface area of the wall and divide it by the face area of one block. Then we calculate the volume of a 12-inch wide trench behind the wall for the backfill gravel.
Step 1: Calculate Area
Wall Area = Length (ft) × Height (ft)
Step 2: Calculate Blocks
Block Area (sq ft) = (Block Length in. × Block Height in.) ÷ 144
Total Blocks = (Wall Area ÷ Block Area) × 1.05 Waste Factor
Step 3: Calculate Drainage Gravel
Backfill Volume = Length × Height × 1 foot (depth)
Tons of Gravel = (Volume × 105 lbs) ÷ 2000
Retaining Wall Example Calculation
You want to build a garden wall that is 30 feet long and 3 feet tall using standard 12" x 4" blocks.
- Wall Area: 30 × 3 = 90 sq ft.
- Block Area: (12 × 4) ÷ 144 = 0.333 sq ft per block.
- Blocks Needed: (90 ÷ 0.333) × 1.05 = 283.5 (Buy 284 blocks).
- Base Gravel: 30 ft × 1 ft × 0.33 ft = 10 cu ft (0.5 Tons).
- Backfill Gravel: 30 ft × 3 ft × 1 ft = 90 cu ft.
- Backfill Tons: (90 × 105) ÷ 2000 = 4.72 Tons.
Tips for Building a Retaining Wall
- Leveling the base is everything: The most difficult and important part of the job is making the very first row of blocks perfectly level. Use a heavy hand tamper to compact your base gravel, and use a 4-foot carpenter's level across multiple blocks. If the first row is off by 1/8 inch, the top row will be visibly crooked.
- Glue the top caps: The top row of blocks (the 'capstones') must be secured using specialized exterior masonry construction adhesive so they aren't knocked off by kids or lawnmowers.
- Set back the blocks: Most retaining wall blocks have a lip on the back bottom edge. This forces each row to automatically "step back" about 3/4 of an inch into the hillside. This set-back creates a stronger center of gravity, preventing the wall from tipping forward.
Frequently Asked Questions
Why do I need so much gravel for a retaining wall?
Water is the number one cause of retaining wall failure. When it rains, soil absorbs water and becomes incredibly heavy. If you don't have a 12-inch column of drainage gravel behind the wall, the hydrostatic pressure of the wet soil will push your wall completely over.
Do I need to bury the first row of blocks?
Yes! The entire first course (row) of retaining wall blocks must be buried completely underground. This 'embedment' prevents the bottom of the wall from kicking out. Our calculator assumes you will be burying the first row, so include that buried row in your total 'Wall Height' measurement.
How tall can I build my wall without an engineer?
In most local municipalities, you can build a gravity retaining wall up to 3 or 4 feet tall (including the buried block) without requiring an engineer or a building permit. Anything taller than 4 feet typically requires a structural engineer and geogrid reinforcement.
Do I need a drainage pipe?
Yes, you should install a 4-inch perforated corrugated plastic pipe at the very bottom of your gravel backfill trench. This catches water filtering down through the gravel and carries it safely away from the ends of the wall.