Retaining Wall Calculator
Calculate blocks, courses, gravel backfill, and cost for your retaining wall project.
Quick Answer
For standard 16"×6" blocks, a 20 ft long × 3 ft tall wall needs about 90 blocks (99 with 10% waste), 15 cap blocks, and ~1.5 yd³ of gravel backfill — costing roughly $300–$900 in materials.
Wall Dimensions
Block Size
Enter wall dimensions and block size to see results.
💡 Walls over 4 feet tall usually require engineering or a permit — check local codes.
How to Calculate Retaining Wall Blocks
Formula
(Wall Length ÷ Block Width) × (Wall Height ÷ Block Height) = Blocks
Convert wall length and height to inches. Divide length by block face width for blocks per row. Divide height by block height for number of courses. Multiply to get total blocks. Add 10% for cuts and waste.
Example
For a 20 ft long × 3 ft tall wall using 16" × 6" blocks: 240" ÷ 16" = 15 blocks per row. 36" ÷ 6" = 6 courses. Total: 15 × 6 = 90 blocks (99 with 10% waste).
Tips & Best Practices
- •Dig a trench 6 inches deep and twice the block depth wide. Fill with compacted gravel.
- •Bury the first course at least halfway — this is critical for stability.
- •Backfill behind the wall with gravel (not soil) for drainage. Add landscape fabric to prevent soil migration.
- •Install a perforated drain pipe at the base behind the wall for proper water management.
- •Step back each course slightly (about ½ inch) for a natural batter angle.
Frequently Asked Questions
How much does a retaining wall cost?+
DIY retaining walls cost $5–$15 per sq ft of wall face (materials only). Professional installation runs $15–$40+ per sq ft. A 20 ft × 3 ft wall (60 sq ft) costs roughly $300–$900 in materials.
Do I need a permit for a retaining wall?+
Most jurisdictions require a permit for walls over 4 feet tall. Some require engineering for walls over 3 feet. Always check local building codes before starting.
How much gravel do I need behind a retaining wall?+
Plan for a 12-inch wide gravel backfill zone behind the wall, full height. Plus a 6-inch deep gravel base extending 6 inches beyond the block depth.
What type of block is best for retaining walls?+
Interlocking concrete blocks (like Allan Block or Versa-Lok) are the most popular for DIY. They stack without mortar and use a lip or pin system for setback. Natural stone is more attractive but harder to work with.
Common Retaining Wall Projects
Want a professional to handle this?
Compare quotes from local contractors to get a better sense of total project cost, timelines, and material options in your area.
Get Local Quotes →How Many Retaining Wall Blocks Do I Need?
Divide the wall length by the block length to get blocks per row, then multiply by the number of courses (rows of height). A 20 ft wall that is 3 ft high with standard blocks needs approximately 135 blocks.
Use the table below to quickly estimate block count for common wall sizes using standard 12"×8" retaining wall blocks.
| Wall Size (L × H) | Blocks | Gravel Backfill | Est. Block Cost |
|---|---|---|---|
| 10 ft × 2 ft | ~30 | ~0.5 yd³ | $45–$150 |
| 20 ft × 3 ft | ~135 | ~1.5 yd³ | $203–$675 |
| 30 ft × 3 ft | ~203 | ~2.2 yd³ | $305–$1,015 |
| 40 ft × 4 ft | ~360 | ~4 yd³ | $540–$1,800 |
Block counts include 5% waste. Costs based on $1.50–$5.00 per block depending on style and size.
How Much Does a Retaining Wall Cost?
A DIY retaining wall costs $5–$15 per square foot of wall face for materials. Professional installation runs $20–$45 per square foot.
Total cost depends on wall height, length, material choice, and site preparation. Taller walls cost more per foot due to engineering requirements.
Concrete Block Walls
Most popular DIY option. Interlocking blocks (no mortar) cost $1.50–$5 per block. Simple to build up to 3–4 feet tall. Best value for straight walls.
Natural Stone Walls
Premium look with fieldstone or flagstone. $8–$20 per sq ft of face. More labor-intensive but no two walls look alike.
Timber / Railroad Tie Walls
Budget-friendly at $5–$10 per sq ft. Easier to install on curves. Lifespan is shorter (10–15 years) than block or stone.
Do I Need Gravel Behind a Retaining Wall?
Yes. Backfill the area behind a retaining wall with 12 inches of crushed gravel for drainage. Without proper drainage, water pressure will push the wall over.
Proper drainage is the #1 factor in retaining wall longevity. Here are the critical steps:
Ordering in bulk?
Crushed stone base and crusher run are available for bulk delivery — typically more cost-effective than bags for a full wall project. Compare local pricing at AggregateMarkets.
Prices and availability vary by location. Use code MEADOWLARK for 5% off your first bulk order.