Retaining Wall Calculator
Calculate how many wall blocks, cap stones, gravel backfill, and landscape fabric you need for your retaining wall.
How to Calculate Retaining Wall Materials
To estimate retaining wall blocks, calculate the wall face area (length × height) and divide by the face area of one block. Add a waste factor for cuts and curves. Behind the wall, you'll need a gravel backfill zone (typically 12 inches wide) for drainage, plus landscape fabric to prevent soil from clogging the gravel. A compacted gravel base trench is essential for the first course.
Common Block Sizes
| Block Size (W × H) | Face Area | Blocks per Sq Ft | Approx. Weight |
|---|---|---|---|
| 12×4 in | 0.33 sq ft | 3.0 | ~30 lbs |
| 16×6 in | 0.67 sq ft | 1.5 | ~50 lbs |
| 18×6 in | 0.75 sq ft | 1.3 | ~60 lbs |
| 8×4 in | 0.22 sq ft | 4.5 | ~15 lbs |
Retaining Wall Height Limits
Most building codes allow homeowners to build retaining walls up to 4 feet tall without a permit or engineer. Walls over 4 feet typically require a structural engineer's design, a building permit, and geogrid reinforcement. Always check your local building codes before starting. Even for walls under 4 feet, proper drainage and a compacted base are critical for long-term stability.
Drainage Is Critical
The number one reason retaining walls fail is poor drainage. Water pressure behind the wall (called hydrostatic pressure) can push the wall over. To prevent this, backfill behind the wall with 12 inches of clean crushed gravel, install landscape fabric to keep soil out of the gravel, and place a perforated drain pipe at the base that directs water away from the wall.
Tips for Your Retaining Wall Project
- Dig a trench 6 inches deep and twice the block width for the base — compact the gravel base thoroughly.
- Bury the first course halfway (2–3 inches) below grade for extra stability.
- Stagger block joints by half a block length on each course (like brickwork) for structural strength.
- Use construction adhesive between the last two courses and on cap stones to prevent shifting.
- Backfill with gravel as you build each course — don't wait until the wall is finished.
- Set blocks with a slight backward lean (about 1 inch per foot of height) called "batter" for added stability.
Frequently Asked Questions
Divide the wall face area (length times height in feet) by the face area of one block. A standard 12x4-inch block covers 0.33 sq ft, so a 20-foot-long by 3-foot-high wall (60 sq ft) needs about 180 blocks before waste. Add 5-15% extra for cuts, curves, and breakage. Use our calculator above for an exact count including cap stones and all materials.
Most building codes allow retaining walls up to 4 feet tall without a permit or engineer. Walls over 4 feet typically require a building permit, a structural engineer's design, and may need geogrid reinforcement. However, requirements vary by location, so always check with your local building department. Even for walls under 4 feet, proper drainage and a compacted base are essential.
DIY retaining walls using concrete blocks cost $5-$15 per square foot of wall face for materials, including blocks, gravel, fabric, and drainage. A 20-foot-long by 3-foot-high wall (60 sq ft) costs roughly $300-$900 in materials. Professional installation costs $20-$50 per square foot including labor and materials. Natural stone walls cost significantly more.
Yes, drainage is critical and the number one reason retaining walls fail is poor drainage. Water pressure behind the wall (hydrostatic pressure) can push the wall over. Every retaining wall needs 12 inches of gravel backfill behind it, landscape fabric to keep soil out of the gravel, and a perforated drain pipe at the base that directs water away from the wall.
A properly built concrete block retaining wall lasts 50-100 years. Natural stone walls can last even longer. The key factors for longevity are proper drainage, a compacted gravel base, and correct construction techniques (staggering joints, using adhesive on top courses, and building with a slight backward lean). Walls that fail prematurely almost always have drainage issues.