Retaining Wall Calculator
Count the blocks, caps and base stone needed to build a retaining wall.
What you'll need to order
- retaining wall block
- cap block
- crushed stone base
- drainage gravel
- filter fabric
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
blocks = (length × 12 ÷ block length) × (height ÷ block height) × 1.05
The first course is buried, so a wall that stands 24 inches above grade needs a fourth course below it. Add one course to your height before calculating if you want a specific exposed height.
Worked example
With wall length 20 ft, wall height 24 in, block length 12 in, block height 8 in, this calculator returns 63 .
Waste allowance
The headline figure includes a 5% allowance for waste and over-ordering. The exact requirement is shown alongside it.
Frequently asked questions
How many blocks do I need for a retaining wall?
Divide the wall length in inches by the block length for blocks per course, divide wall height by block height for the number of courses, then multiply. A 20 ft wall 24 inches high using 12 by 8 inch blocks needs about 63 blocks.
How deep should the base of a retaining wall be?
Bury the first course entirely — a minimum of 6 inches, or one tenth of the wall height for taller walls. Below that goes 6 inches of compacted crushed stone in a trench at least twice the block width.
How tall can I build a retaining wall without an engineer?
Most jurisdictions cap unengineered walls at 3 to 4 ft. Above that you need a permit and a design, because the soil pressure grows with the square of the height. Check your local code before you buy.
Do I need drainage behind a retaining wall?
Yes, on any wall holding back soil. Backfill the first 12 inches behind the blocks with clean stone and run a perforated drain at the base. Water pressure behind a wall is what pushes it over, not the soil weight.