How This Calculator Works
The calculator divides your wall length by block length to get blocks per row, then divides wall height by block height to get total rows. It adds 5% waste for cuts at corners and ends. Backfill gravel is estimated at 12 inches deep behind the full wall height. Drainage fabric covers the back of the wall plus a 3-foot base wrap to prevent soil from clogging the gravel.
Retaining Wall Block Reference
| Block Type | Dimensions (L x H x D) | Weight | Cost Each | Best For |
|---|---|---|---|---|
| Standard Landscape | 12" x 4" x 8" | ~30 lbs | $3-5 | Garden walls, raised beds, borders |
| Large Retaining | 18" x 6" x 12" | ~60 lbs | $5-8 | Structural walls, slopes, terracing |
| Mini Wall | 12" x 4" x 6" | ~24 lbs | $2-4 | Garden edging, small borders, tree rings |
Backfill Requirements
Proper backfill is critical for retaining wall performance. Behind every retaining wall, you need 12 inches of compacted 3/4-inch crushed stone wrapped in drainage fabric. This gravel zone lets water flow down and away instead of building pressure against the blocks. At the base, a 6-inch deep compacted gravel pad extends 12 inches beyond the block depth for stability.
| Component | Material | Dimensions | Purpose |
|---|---|---|---|
| Base pad | 3/4" crushed stone | 6" deep x (block depth + 12") wide | Level foundation, drainage |
| Backfill | 3/4" crushed stone | 12" wide x full wall height | Relieve hydrostatic pressure |
| Drainage fabric | Non-woven geotextile | Full height + 3 ft base wrap | Prevent soil from clogging gravel |
| Drain pipe (optional) | 4" perforated PVC | Full wall length at base | Channel water away for walls over 3 ft |
Height Limits and Engineering
Most municipalities allow gravity retaining walls (no reinforcement) up to 3-4 feet tall without a building permit or engineer. Walls over 4 feet typically require a licensed engineer's stamp, a building permit, and may need geogrid reinforcement or tiebacks. The general rule: a gravity wall can be built as high as the block depth — an 8-inch deep block supports roughly 24 inches of wall height without geogrid.
FAQ
How many blocks per square foot of retaining wall?
With standard 12 x 4-inch blocks, you need 3 blocks per linear foot per course (12-inch blocks cover 1 ft each). Each row is 4 inches tall, so a 1-foot-tall wall needs 3 rows. That works out to about 3 blocks per square foot of wall face. Large 18 x 6-inch blocks need about 1.3 per square foot.
Do I need a permit for a retaining wall?
In most areas, walls under 3-4 feet (measured from bottom of footing to top of wall) do not require a permit. Walls over 4 feet almost always do, and many jurisdictions require an engineer's design above that height. Check your local building department — setback requirements and surcharge loads (like a driveway at the top) can also trigger permit requirements at any height.
How much does a retaining wall cost per linear foot?
DIY materials for a 3-foot-tall wall run $15-25 per linear foot (blocks, cap, gravel, fabric). Professional installation adds $25-50 per linear foot for labor. A 20-foot wall at 3 feet tall costs roughly $300-500 for DIY materials or $800-1,500 professionally installed.
Can I build a retaining wall without drainage?
Not recommended. Water pressure behind a retaining wall is the number one cause of failure. Even a 2-foot garden wall benefits from gravel backfill. For walls over 2 feet, drainage is critical — without it, water saturates the soil, increases pressure, and can push the wall over during heavy rain.
Related Calculators
- Gravel Calculator — calculate backfill and base gravel amounts
- Landscape Rock Calculator — for decorative rock around your wall
- Brick Calculator — for brick retaining walls and veneers
- Soil Volume Calculator — for filling raised areas behind the wall
⚠ Important Notice
Retaining walls over 4 feet tall typically require engineering design and a building permit in most jurisdictions. This calculator provides material estimates for planning — it does not account for soil type, drainage, surcharge loads, or seismic requirements. Consult a licensed structural engineer for walls that retain significant earth loads or are near structures.
Prices shown are approximate 2026 U.S. averages and may vary significantly by region, retailer, and market conditions. Always get local quotes for accurate pricing.