What this estimate includes
This estimate shows wall blocks, cap blocks, base gravel, and drainage gravel for a basic retaining wall layout.
Before you order
Retaining walls can be structural. Wall height, drainage, soil pressure, slope, and local code can change requirements. Walls over 4 ft may require engineering or permits.
Frequently Asked Questions
How many retaining wall blocks do I need?
Divide the wall face area (length x height) by the face area of one block (block length x block height). For a 20 ft wall that is 2 ft high using 16 x 6 inch blocks, you need about 60 blocks plus 10% waste -- about 66 blocks total.
How much gravel goes under a retaining wall?
A standard base for a garden-scale retaining wall is 4-6 inches of compacted gravel at least 12 inches wide. Taller walls need a wider, deeper base. Use the gravel calculator to estimate base material volume.
Do retaining walls need drainage gravel?
Yes -- drainage behind the wall is critical. Without it, hydrostatic pressure from saturated soil can topple even well-built walls. Place 6-12 inches of crushed stone directly behind the wall, with drainage fabric between the gravel and soil.
How many cap blocks do I need?
Cap blocks run along the top course of the wall. You need one cap block for every block length along the wall. For a 20 ft wall with 16-inch cap blocks, that is about 15 cap blocks. Use the cap block option in the calculator above.
How much waste should I add for retaining wall blocks?
For a straight wall, 5-10% is standard. For curved walls, corners, or complex patterns, add 10-15%. Blocks at corners and ends often require cuts.
When does a retaining wall need engineering?
Most municipalities require a permit and engineering review for retaining walls over 4 feet in height. Walls near property lines, driveways, slopes, or buildings may need review at lower heights. Always check local code before building.
Can I build a retaining wall myself?
For walls under 2 feet high on level ground away from structures, DIY is generally feasible with segmental concrete blocks. Above that height, consult a contractor or engineer. This calculator is for material estimating only and does not provide structural guidance.
Disclaimer: This calculator provides material estimates only. Retaining walls can be structural elements subject to soil pressure, drainage, and load conditions not captured by this tool. Walls over 4 feet, near buildings, or on slopes require professional engineering review. Verify all quantities with your block supplier and consult your local building department before construction. See all
outdoor calculators.