Retaining Wall Calculator
Calculate blocks, courses, gravel backfill, and cost for your retaining wall project.
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.