Retaining Wall Calculator
A 20 ft × 3 ft retaining wall (90 face sq ft) needs about 80 blocks + 0.4 cu yd base gravel + 2.2 cu yd drainage backfill. Enter your dimensions for an exact estimate.
Inputs
Estimate
- Wall face area60 sq ft
- Blocks needed80
- Base gravel (12 in wide × 6 in deep)0.37cu yd
- Drainage backfill (12 in wide × wall height)Crushed gravel placed behind wall for drainage.2.22cu yd
- Cost — low$240
- Cost — high$640
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Try Remodel AI freeHow to calculate retaining wall materials
A retaining wall has three main components: blocks (the visible wall face), base gravel (a level foundation), and drainage backfill (crushed gravel behind the wall to relieve water pressure). Each wall needs all three.
Block count = (wall length in inches / block length) × (wall height in inches / block height). For an 18-inch-long × 6-inch-tall block at a 20 ft × 3 ft wall: 240/18 × 36/6 = 14 × 6 = 80 blocks.
Base gravel goes under the wall, typically 12 inches wide × 6 inches deep. Drainage backfill goes behind the wall, 12 inches wide × the full wall height. Drainage prevents hydrostatic pressure from blowing out the wall.
Formulas used
- Blocks
(wall_length × 12 / block_length) × (wall_height × 12 / block_height)Convert dimensions to inches and divide by block dimensions to get rows × courses. - Drainage backfill
wall_length × 1 ft × wall_height / 27Crushed gravel behind the wall, 12 inches wide × full wall height. Result in cubic yards.
Frequently asked questions
How tall can a retaining wall be without engineering?+
Most jurisdictions allow DIY retaining walls up to 4 feet. Above 4 feet, you need a permit and often engineered drawings stamped by a licensed engineer.
How many blocks do I need for a 20×3 ft wall?+
With 18-inch × 6-inch blocks: 14 blocks per row × 6 rows = 84 blocks. Order 5% extra for cuts and damaged pieces = 88 blocks.
Do I need drainage behind a retaining wall?+
Yes — any wall over 2 ft tall needs drainage. Use 12 inches of crushed gravel backfill with a perforated drain pipe at the base, sloped to daylight.
How much does a retaining wall cost?+
DIY concrete block: $15–$40 per face square foot. Interlocking SRW block: $20–$50. Natural stone: $30–$80. Timber: $15–$30. Add 50–100% for professional installation.
How deep should the base be for a retaining wall?+
6 inches of compacted base gravel under the first course, plus burying the first course 1 inch per foot of wall height. A 3 ft wall = 3 in buried below grade + 6 in gravel base = 9 in excavation.