Introduction
A retaining wall fails for two reasons: bad drainage and bad material counts. You can’t fix the first after the fact. You can fix the second before you order. This tool gives you the numbers before you leave the house.
Take a 24 ft wall at 3.5 ft exposed height with 0.5 ft buried. Using standard 18 in × 6 in blocks, you’d need 6 rows (including the buried course) and about 17 blocks per row. That’s roughly 102 blocks before waste. Add 5% and you’re at 108. The base trench takes 6 in of gravel across a 16 in width. The backfill behind the wall runs the full height plus burial, 12 in wide. Total gravel lands near 90 cubic feet, or about 3.3 cubic yards.
Those are the numbers a supplier needs. This calculator gives them to you in one pass, with the waste factor and cost math built in. It’s an estimating tool, not a structural design tool. For walls over 4 feet, you’ll still need an engineer’s stamp and a geogrid plan.
How to Use the Retaining Wall Calculator
Pick your currency from the dropdown if you want the cost estimate in something other than US dollars.
Enter the wall length and choose the unit: feet, inches, meters, centimeters, or millimeters.
Enter the exposed wall height — the part you’ll see above grade.
Enter the burial depth. Most block walls bury 6 to 12 inches of the first course. Half a foot is a common default.
Input the block length. Standard landscape blocks run 12 to 18 inches.
Input the block height. Four to 8 inches covers most segmental blocks.
Input the block depth. This is the front-to-back measurement, usually 8 to 12 inches.
Set the base gravel depth and width. Six inches deep by 16 inches wide is typical for a residential wall.
Set the backfill gravel width — the distance behind the wall that gets drained. Twelve inches is standard.
Enter the cost per block, cost per cap, and gravel cost. Choose the gravel pricing unit from the dropdown.
Set the waste factor. Five percent is a safe default for a straight wall. Bump it to 10% for curves or step-downs.
Add any additional costs for drain pipe, filter fabric, or adhesive. Hit Calculate.
How the Retaining Wall Formula Works
The calculator converts everything to feet, then counts blocks by rows and columns.
Formula: Total Height (ft) = Exposed Height + Burial Depth
Formula: Number of Rows = ceil(Total Height ÷ Block Height (ft))
Formula: Blocks per Row = ceil(Wall Length (ft) ÷ Block Length (ft)) + 1
Formula: Total Blocks = Rows × Blocks per Row
Formula: Blocks with Waste = ceil(Total Blocks × (1 + Waste % ÷ 100))
Formula: Cap Blocks with Waste = ceil(Blocks per Row × (1 + Waste % ÷ 100))
Formula: Base Gravel (ft³) = Wall Length × Base Width × Base Depth
Formula: Backfill Gravel (ft³) = Wall Length × Backfill Width × Total Height
Formula: Total Gravel (ft³) = Base Gravel + Backfill Gravel
Formula: Block Cost = Blocks with Waste × Cost per Block
Formula: Total Cost = Block Cost + Cap Cost + Gravel Cost + Additional Costs
The ceil function rounds up. A wall that needs 16.2 blocks per row becomes 17. The +1 on blocks per row accounts for the half-block you almost always need at the end of a run. The tool enforces a minimum of 2 blocks per row.
Burial depth matters for two reasons. First, it adds to the total height the wall must cover, which adds rows. Second, the buried portion still needs backfill gravel behind it. A 0.5 ft burial on a 3 ft wall gives you 3.5 ft of total height. On a 24 ft wall with 6 in blocks, that’s ceil(3.5 ÷ 0.5) = 7 rows instead of 6.
Gravel cost can be entered three ways. A per-cubic-foot price multiplies straight through. A per-cubic-yard price gets divided by 27 first. A per-cubic-meter price gets divided by 35.3147. The tool handles that conversion so you can use whatever unit your supplier quotes.
Gravel weight is estimated at 100 lb/ft³. That’s a reasonable average for crushed stone. Actual weight varies with moisture and aggregate type.
Worked Example
You’re building a 28 ft wall with 4 ft of exposed height and 0.75 ft buried. Your blocks are 16 in long, 6 in high, and 10 in deep. Base gravel runs 6 in deep by 18 in wide. Backfill gravel is 12 in wide. Blocks cost $3.75 each. Caps cost $4.50 each. Gravel is $32 per cubic yard. You’re adding $60 for drain pipe and fabric.
Step 1: Total height.
4 + 0.75 = 4.75 ft.
Step 2: Convert block dimensions to feet.
Length: 16 ÷ 12 = 1.333 ft. Height: 6 ÷ 12 = 0.5 ft. Depth: 10 ÷ 12 = 0.833 ft.
Step 3: Number of rows.
ceil(4.75 ÷ 0.5) = ceil(9.5) = 10 rows.
Step 4: Blocks per row.
ceil(28 ÷ 1.333) + 1 = ceil(21.0) + 1 = 21 + 1 = 22 blocks.
Step 5: Total blocks.
10 × 22 = 220 blocks.
Step 6: Blocks with 5% waste.
ceil(220 × 1.05) = ceil(231) = 231 blocks.
Step 7: Cap blocks with waste.
ceil(22 × 1.05) = ceil(23.1) = 24 caps.
Step 8: Base gravel.
28 × 1.5 × 0.5 = 21 ft³.
Step 9: Backfill gravel.
28 × 1.0 × 4.75 = 133 ft³.
Step 10: Total gravel.
21 + 133 = 154 ft³. In cubic yards: 154 ÷ 27 = 5.70 yd³.
Step 11: Gravel cost.
5.70 × $32 = $182.40.
Step 12: Block and cap cost.
Blocks: 231 × $3.75 = $866.25. Caps: 24 × $4.50 = $108.00.
Step 13: Total cost.
$866.25 + $108.00 + $182.40 + $60.00 = $1,216.65.
The summary tab shows 231 blocks, 24 caps, 154 ft³ of gravel (5.70 yd³), and $1,216.65 total cost. The breakdown tab lists every converted dimension and intermediate value.
