Retaining Wall Block Calculator
Estimate the number of blocks for a segmental retaining wall by its face area. Assumes a standard block covering about 0.67 sq ft of face.
Retaining Wall Block calculator
How it's calculated: Wall length ft × height ft = face area in square feet; add 5% waste, multiply by about 1.5 blocks per square foot, and round up.
How to estimate retaining wall block
- Walls over 3-4 feet usually need engineering and a permit. Check local code before building.
- Bury the first course at least one block height below grade and set it on compacted gravel.
- Add cap blocks for the top course and geogrid for taller walls - both are separate line items.
- Backfill behind the wall with clean gravel and run a perforated drain pipe along the base - trapped water is what pushes walls over.
- On a slope, step the base trench so every course stays dead level; never tilt a course to follow the grade.
Frequently asked questions
How many retaining wall blocks do I need?
Multiply wall length by height for face area, then about 1.5 standard blocks per square foot. Add 5% for breakage and buried courses.
Do I need drainage behind a retaining wall?
Yes. Saturated soil pushes far harder than dry soil. Put about 12 inches of clean gravel behind the blocks with a perforated pipe at the base draining to daylight - skipping this is the top cause of leaning walls.
How does the block math work out in practice?
Take a 20 ft wall 3 ft tall: that is 60 sq ft of face, and at about 1.5 blocks per square foot plus 5% waste you land near 95 blocks, plus cap blocks for the top course.
How deep do I bury the first course?
A common rule is to bury about 10% of the wall height, with a minimum of one full block height, set on at least 6 inches of compacted gravel.
When does a retaining wall need an engineer?
Most codes draw the line at 3-4 feet of retained height - above that you typically need engineering and a permit. Anything loaded from above by a driveway, structure or steep slope should be engineered regardless of height.