Retaining Wall Calculator
Enter the wall size and the block you are using. This counts the blocks and the caps, and the gravel that goes under and behind them.
How the math works
Courses = wall height in inches, divided by the height of one block, rounded up. Blocks per course = wall length in inches, divided by the width of one block, rounded up. Blocks = courses x blocks per course. A 12 x 4 inch block gives 3 courses per foot of height and 1 block per foot of length.
Common questions
- How many blocks do I need for a 20 foot wall 3 feet high?
- About 180 of the common 12 by 4 inch block: 9 courses of 20. The 4 inch face height means three courses per foot, which is why that block adds up fast.
- How deep should the base of a retaining wall be?
- Six inches of compacted gravel for a wall up to 3 feet, eight inches up to 4. The first course is also buried, usually one full block or a tenth of the wall height, whichever is more.
- Do I need drainage behind a retaining wall?
- Yes. A foot of clean stone behind the wall and a perforated pipe at the base, sloped to daylight. Walls fail from water pressure far more often than from weight.
- How high can I build without a permit?
- Usually 4 feet, but it varies by town and a wall carrying a slope or a driveway above it can trip the rule lower. Check before you buy, because an engineered wall needs geogrid and a wider base.
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