Retaining Wall Calculator
A retaining wall holds back soil on a sloped lot, preventing erosion and creating usable level space. Before purchasing materials, you need to know how many blocks the project requires. This calculator takes your wall length, wall height, and the dimensions of your chosen block to determine the number of courses (rows) and the total block count. Standard concrete retaining wall blocks are commonly 6 inches or 8 inches tall and 16 inches long, but decorative blocks come in other sizes. The formula is straightforward: divide the wall height by the block height to get the course count, divide the wall length by the block length to get blocks per course, then multiply. The result is the minimum number of blocks. In practice, always add 5 to 10 percent for waste from cuts, broken blocks, and adjustments at corners. Taller walls (generally over 3 feet) typically require batter (a slight backward lean), deadman anchors, and proper drainage. Consult a landscape contractor or structural engineer for walls over 4 feet tall, and check with your local building department regarding permit requirements.
How to calculate retaining wall blocks
The calculation uses the dimensions of your wall and your chosen block:
Courses = Wall height (in) / Block height (in)
Blocks per course = Wall length (in) / Block length (in)
Total blocks = Courses x Blocks per course x (1 + Waste factor)
Worked example
Wall: 20 ft long, 3 ft high. Block: 6 in tall, 16 in long. Waste: 10%.
- Wall height in inches = 3 x 12 = 36 in
- Courses = 36 / 6 = 6 courses
- Wall length in inches = 20 x 12 = 240 in
- Blocks per course = 240 / 16 = 15 blocks
- Base total = 6 x 15 = 90 blocks
- With 10% waste = 90 x 1.10 = 99 blocks (round up to 100)
Frequently asked questions
How do I calculate the number of blocks for a retaining wall?
Divide the wall height by the block height to get the number of courses. Divide the wall length by the block length to get blocks per course. Multiply courses by blocks per course for the total block count. Always add 5 to 10 percent for cuts and waste.
What is the most common block size for retaining walls?
Standard concrete retaining wall blocks are typically 6 inches tall and 16 inches long, or 8 inches tall and 16 inches long. Some decorative blocks are 4 inches tall. Always verify the actual dimensions with your supplier before purchasing.
Do I need a permit for a retaining wall?
Permit requirements vary by municipality. Most jurisdictions require a permit for retaining walls over 3 to 4 feet tall. Taller walls often require engineered plans. Contact your local building department before starting construction.
How much does a retaining wall cost?
Block retaining wall costs vary widely depending on block type, site access, and labor rates. Concrete block walls typically range from $15 to $30 per square foot installed. DIY material costs are much lower. Get local quotes for accurate pricing.
Should I add a waste factor to my block count?
Yes. Plan for at least 5 to 10 percent extra blocks to cover cuts at corners and wall ends, damaged blocks, and irregular terrain. For complex layouts with many corners, add 10 to 15 percent.
Sources
- USDA Natural Resources Conservation Service: Soil Erosion.
- International Residential Code (IRC): local permit thresholds for retaining walls.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. General information only. Consult a licensed contractor for structural advice.