Retaining Wall Calculator
Calculate blocks, base material, backfill, and drainage for your retaining wall project. See also our Concrete Block Calculator, Gravel Calculator, and Fence Calculator.
How to Use the Retaining Wall Calculator
Enter the total wall length and desired height in feet. Select your block size — standard 6x16 inch blocks are most common for residential walls under 4 feet, while larger 8x18 inch blocks are used for taller or commercial walls. The setback per course determines how much each row steps back from the one below, creating the wall's batter (lean). The calculator provides a complete material list including wall blocks, cap blocks, base gravel, backfill material, drainage aggregate, and geogrid reinforcement for taller walls.
Formula
Courses = Wall Height (in) / Block Height (in)
Blocks per Course = Wall Length (in) / Block Width (in)
Total Blocks = Courses x Blocks per Course
Base Gravel = 0.5 ft deep x 2 ft wide x Length / 27 (cu yd) x 1.4 (tons)
Backfill = 1 ft deep x Height x Length / 27 x 1.4
Geogrid Layers = Courses / 2 (for walls over 3 ft)
Example Calculation
Wall: 20 ft long, 3 ft high, 6x16 blocks
Courses = 36 in / 6 in = 6 courses
Blocks per course = 240 in / 16 in = 15 blocks
Total blocks = 6 x 15 = 90 blocks
Cap blocks = 15
Base gravel = (0.5 x 2 x 20) / 27 x 1.4 = 1.04 tons
Backfill = (1 x 3 x 20) / 27 x 1.4 = 3.11 tons
Reference Table: Retaining Wall Block Sizes
| Block Type | Face Size | Depth | Weight | Coverage |
|---|---|---|---|---|
| Standard | 6" x 16" | 12" | 35-45 lbs | 0.67 sq ft/block |
| Large | 8" x 18" | 12" | 55-75 lbs | 1.0 sq ft/block |
| Small Garden | 4" x 12" | 8" | 20-25 lbs | 0.33 sq ft/block |
| Jumbo | 8" x 24" | 16" | 80-100 lbs | 1.33 sq ft/block |
| Cap Block | 3" x 16" | 12" | 20-30 lbs | Top course only |
| Corner Block | 6" x 16" | 12" | 40-50 lbs | Ends/corners |
Frequently Asked Questions
How tall can a retaining wall be without engineering?
Most building codes allow retaining walls up to 4 feet tall without a structural engineer's design. Walls over 4 feet typically require engineered plans, permits, and geogrid reinforcement. Some jurisdictions set the limit at 3 feet. Always check local codes before building.
What goes behind a retaining wall for drainage?
Proper drainage requires: a 4-inch perforated drain pipe at the base wrapped in filter fabric, 12 inches of clean drainage gravel (3/4 inch crushed stone) behind the wall, and filter fabric between the gravel and backfill soil. This prevents hydrostatic pressure buildup that can topple walls.
How deep should the base be for a retaining wall?
The base trench should be 6 inches deep for walls under 3 feet and 8-12 inches deep for taller walls. The base should be 24 inches wide (wider than the blocks) and filled with compacted crushed gravel. The first course of blocks should be buried below grade for stability.
What is geogrid and when do I need it?
Geogrid is a high-strength polymer mesh that extends from between block courses back into the retained soil, anchoring the wall. It's required for walls over 3-4 feet tall. Install geogrid every 2 courses (12-16 inches vertically), extending at least 3 feet behind the wall face or 60% of the wall height, whichever is greater.
How much setback should each course have?
Most retaining wall blocks have a built-in setback of 3/4 inch to 1 inch per course via a lip or pin system. This creates a batter (backward lean) of approximately 1 inch per foot of height. The setback helps the wall resist the lateral earth pressure pushing against it.
Can I build a retaining wall on a slope?
Yes, but you must step the base trench to follow the slope rather than angling it. Each step should equal one block height. The base must be level at each step. For slopes, consider building multiple shorter terraced walls (2-3 feet each) rather than one tall wall, which reduces earth pressure and eliminates the need for engineering.