Retaining Wall Calculator
Enter your wall length, height, and block size to get the block count — plus cap blocks, base trench gravel, and the drainage backfill most DIYers forget.
Walls over 4 ft usually need an engineer and a permit.
One top course to finish the wall.
6″ deep × 18″ wide compacted trench.
12″ of clean stone behind the wall.
How it works
We multiply wall length by height to get the face area, divide by the face of one block, and add waste for cuts. The base assumes a compacted gravel trench 6 inches deep and 18 inches wide; drainage backfill assumes 12 inches of clean stone the full height behind the wall — the detail that keeps walls standing.
Formula: Blocks = ceil(Length × Height ÷ block face × (1 + waste)); Drainage = L × 1 ft × H ÷ 27
Good for
- Garden terraces and planting beds
- Slope retention up to 4 ft
- Raised patio edges
- Driveway side walls
Tips
- Bury the first course — about 10% of wall height (minimum one block) below grade.
- Always backfill with clean ¾″ stone and a perforated drain pipe, never soil alone.
- Step the wall back (setback lip on most blocks) as it rises.
- Anything over 4 ft of retained height is engineer territory in most jurisdictions.
References
Estimates use standard industry volume math. Cross-check your numbers against the pros:
Related calculators
Paver Calculator
Enter your patio or walkway dimensions and paver size to get the paver count — plus the base gravel, bedding sand, and joint sand for a full install.
Open calculator →Concrete Slab Calculator
Enter your slab dimensions to get cubic yards, the number of 80-lb bags, and an optional cost estimate — with a waste margin so you don’t run short.
Open calculator →Fence Post Concrete Calculator
Enter your post count, hole size, and depth to get the exact number of concrete bags — no more guessing a bag per post and running short.
Open calculator →Retaining Wall Calculator FAQ
How many blocks do I need for a 20 ft × 2 ft wall?
That's 40 sq ft of wall face. With standard 12×4″ blocks (0.33 sq ft each) and 10% waste, you need about 132 blocks plus 20 cap blocks.
Do I really need drainage gravel behind the wall?
Yes. Water pressure — not soil weight — topples most residential walls. Use 12 inches of clean stone with a perforated drain at the base, wrapped in filter fabric.
How deep should the base trench be?
A 6-inch compacted gravel base in a trench about 18 inches wide is standard for walls up to 4 ft. Bury the first course of blocks below grade for stability.
When do I need an engineer?
Most codes require engineering (and often a permit) for walls retaining more than 4 ft, walls with a slope or driveway above them, or terraced walls that act as one system.