Retaining Wall Calculator: How Much Material Do You Need?
Calculate retaining wall blocks, cap stones, and base gravel for any wall. Covers block counts by wall height, drainage requirements, geogrid reinforcement, and permit thresholds.
Related Calculators
Retaining Wall Block Types and Sizes
Several types of interlocking concrete retaining wall blocks are commonly available:
Each manufacturer provides wall design guides with specific height and geogrid requirements.
Block Count Calculation
Blocks per linear foot of wall (one course):
For a 12-inch wide block: 1 block per linear foot
For a 16-inch wide block: 0.75 blocks per linear foot
Courses per foot of wall height: 12 ÷ block height
For a 4-inch tall block: 3 courses per foot of height For a 6-inch tall block: 2 courses per foot
Total blocks = Wall length (ft) × Blocks per LF × Courses per foot
Example: 20 ft long × 3 ft tall wall using 4-inch × 12-inch blocks:
- Courses: 3 ft × 3 courses/ft = 9 courses
- Blocks per course: 20 ft × 1 block/ft = 20 blocks
- Total: 9 × 20 = 180 blocks + 5–10% waste = 190–200 blocks
Cap blocks: One cap per linear foot. 20 ft wall = 20 cap blocks.
Base Preparation
Every retaining wall requires a crushed stone base: - Minimum base depth: 6 inches of compacted crushed stone - First course buried: Bury the bottom course 1 inch per foot of wall height (minimum 4–6 inches for small walls)
Base volume (cu yd) = Wall length × (base width + 12 in backfill) × base depth ÷ 27
Drainage Requirements
All retaining walls require drainage behind the wall to prevent hydrostatic pressure buildup:
Drainage stone: 3/4-inch crushed stone backfill for the full height of the wall, 12 inches wide behind the blocks.
Drainage pipe: 4-inch perforated pipe at the base of the wall, wrapped in filter fabric, sloped to drain.
Volume of drainage stone: Wall length × 1 ft wide × wall height ÷ 27
For a 20 ft × 3 ft wall: 20 × 1 × 3 ÷ 27 = 2.2 cu yd of drainage stone
When Geogrid Reinforcement Is Required
Geogrid is a grid-like material buried in compacted backfill layers to tie the wall to the hillside. Most systems require geogrid: - When wall height exceeds 3–4 ft (manufacturer-specific) - At every 2–3 courses for walls 4–6 ft tall - At specified intervals for walls 6+ ft tall
Geogrid coverage: Typically 4–6 ft deep into the hillside per layer.
When You Need a Permit and Engineer
Wall height thresholds (varies by jurisdiction): - < 3 ft: Usually no permit required - 3–4 ft: Permit often required; DIY may be allowed - > 4 ft: Usually requires permit and in many jurisdictions, a licensed engineer's design - Walls with surcharge loads (driveway, structure above): Engineer required regardless of height
Check your local building department — requirements vary significantly by municipality and state. Failure on a tall retaining wall can be catastrophic.
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Frequently Asked Questions
- How many retaining wall blocks do I need?
- Multiply wall length (ft) × courses per foot × blocks per linear foot. For 4-inch tall blocks at 12-inch wide: (Wall Height ÷ 0.33) courses × wall length = total blocks. Add 5–10% waste. Example: 20 ft long, 3 ft tall = 9 courses × 20 blocks = 180 blocks + waste = ~195 blocks.
- How deep should a retaining wall base be?
- Minimum 6 inches of compacted crushed stone base. The first course should be buried 1 inch per foot of wall height (minimum 4–6 inches). For a 3-foot wall: bury at least 3 inches, more on slopes. Proper burial prevents frost heave and provides stability.
- Does a retaining wall need drainage?
- Yes — always. Without drainage, hydrostatic pressure from water-saturated soil builds behind the wall and can push it over. Install 4-inch perforated drain pipe at the wall base, sloped to an outlet, with 12 inches of 3/4-inch crushed stone backfill wrapped in filter fabric behind the entire wall.
- How tall can a retaining wall be without a permit?
- Most jurisdictions require a permit for walls over 3–4 feet. Many require an engineer's stamp for walls over 4 feet or any wall with a surcharge load (driveway or structure above). Check with your local building department — regulations vary significantly by location.
- What is geogrid and when do I need it?
- Geogrid is a polymer grid buried in compacted backfill layers to mechanically connect the wall to the hillside. Required when wall height exceeds manufacturer specifications (typically 3–4 ft for most interlocking block systems). Installed every 2–3 courses for taller walls, extending 4–6 ft into the hillside per layer.
Last updated 7/28/2026