Retaining Wall Calculator
Calculate blocks, gravel, and drainage materials for your retaining wall.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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Request a ToolHow to Use the Retaining Wall Calculator
This calculator estimates the number of retaining wall blocks you need along with all supporting materials: cap blocks, base gravel, backfill gravel, perforated drain pipe, filter fabric, and cap adhesive. Enter your wall dimensions and get a complete material list instantly.
- Enter wall length and height. Measure the total length of wall you are building in feet. Height is the exposed wall face from grade to top, also in feet. For walls over 4 feet, consult a structural engineer before proceeding.
- Choose your block size. Standard 12" x 4" blocks are the most common retaining wall block in North America. Large 18" x 6" blocks are used for taller walls and give a more substantial look. The calculator uses each block's face dimensions to determine coverage per square foot.
- Set a waste factor. The default 5% accounts for cuts at corners and ends. Increase to 8-10% for walls with many corners or curves.
- Read your results. The primary result is total face blocks. The breakdown shows cap blocks for the top course, base gravel for the footing trench, backfill gravel behind the wall, drain pipe length, filter fabric area, and adhesive tubes for cap bonding.
All results update as you type. Use Share to send your inputs to a supplier or contractor, or Copy to paste the block count into a shopping list.
About the Retaining Wall Calculator
Retaining walls hold back soil on sloped lots and create usable flat areas in landscaping. A successful installation depends on more than just the blocks: proper drainage is critical to prevent hydrostatic pressure from toppling the wall over time.
The calculator uses industry-standard material quantities. Face blocks are calculated from wall area divided by each block's face coverage, with a waste factor for cuts. Cap blocks run the full length at one cap per block-length. Base gravel fills a 2-foot-wide by 6-inch-deep trench under the first course, providing a level, compactable footing. Backfill gravel behind the wall (1 foot deep across the full wall height) provides drainage while keeping soil pressure manageable. Perforated drain pipe runs the full wall length at the base to carry water away. Filter fabric wraps the gravel to prevent soil migration. Cap adhesive bonds the top course at one tube per 20 linear feet.
All calculations run in your browser. No data is stored or transmitted.
Frequently Asked Questions
How many blocks do I need for a 20-foot retaining wall that is 3 feet high?
A 20 x 3 foot wall has 60 square feet of face area. Using standard 12" x 4" blocks at 3 blocks per square foot, you need 180 face blocks before waste. With a 5% waste factor, order 189 blocks. You will also need 20 cap blocks, about 0.74 cubic yards of base gravel, 2.22 cubic yards of backfill gravel, 20 feet of perforated drain pipe, 100 square feet of filter fabric, and 1 tube of cap adhesive.
Do I need drainage behind a retaining wall?
Yes. Drainage is the most critical part of retaining wall construction. Without it, water saturates the soil behind the wall, multiplying the pressure the wall must hold. Over time, this hydrostatic pressure causes walls to lean, crack, or fail. Every retaining wall should have a layer of compactable gravel directly behind the blocks, a perforated drain pipe at the base to carry water to daylight, and filter fabric between the gravel and native soil to prevent clogging. Most block manufacturers include drainage requirements in their installation guides.
What is the difference between 12x4 and 18x6 retaining wall blocks?
Standard 12" x 4" blocks (12 inches long, 4 inches tall) are widely available at home improvement stores and suit most residential retaining walls up to 3-4 feet. They require 3 blocks per square foot of wall face. Large 18" x 6" blocks (18 inches long, 6 inches tall) cover more area per block at about 1.33 blocks per square foot. They are heavier, install faster for large projects, and can handle taller walls. Both types typically have a 8-inch depth for stability. Check the manufacturer's specifications for maximum wall height without engineering review.
How deep should the base gravel be for a retaining wall?
The standard base preparation is a 6-inch layer of compactable gravel (crushed stone or road base) in a trench that extends at least 2 feet wide and is buried so the first course of block sits below grade. For every foot of wall height, bury one additional inch of the base course below grade to help the wall resist overturning. For a 3-foot wall, the base should be buried at least 3 inches below finished grade. In frost-prone climates, dig deeper to get below the frost line and prevent frost heave from shifting your first course.