Retaining Wall Calculator
Enter your wall length, height, and block size to estimate the number of blocks, the number of courses, and the base gravel needed for a segmental retaining wall.
Results
- Blocks needed
- 111
- Courses
- 5
- Blocks per course
- 20
- Base gravel (6 in)
- 0.37 cu yd
Estimates only. Round up and buy a little extra to account for waste, spills, and breakage.
This calculator turns wall length and height into a block count, plus the base gravel and drainage stone the wall needs behind it.
Segmental block walls above 4 feet usually require an engineered design and a permit, so check local code before you buy.
Also works as a retaining wall block calculator, block wall estimator, and wall base gravel calculator.
How to use the retaining wall calculator
- Enter the wall length in ft.
- Enter the wall height in ft.
- Enter the block length in in.
- Enter the block height in in.
- Enter the waste factor in %.
- Optionally add a price to estimate the total project cost.
- Read your quantities instantly — no sign-up, and you can print the estimate to take to the supplier.
Worked example
Inputs
- Wall length
- 20 ft
- Wall height
- 3 ft
- Block length
- 12 in
- Block height
- 8 in
- Waste factor
- 10 %
Results
- Blocks needed
- 111
- Courses
- 5
- Blocks per course
- 20
- Base gravel (6 in)
- 0.37 cu yd
Using the example values above, the retaining wall calculator returns 111 (blocks needed). Change any field to match your own project and the numbers update instantly.
Base and drainage are most of the job
Dig a trench deep enough to bury the first course fully, plus 6 inches of compacted crushed stone below it. Getting the first course dead level decides whether the whole wall looks right.
Backfill 12 inches of clean 3/4 inch drainage stone behind the blocks and run a perforated pipe at the base. Water pressure, not weight, is what topples most failed walls.
Block counts and setback
Standard 12 by 8 inch face blocks give 0.67 square feet each, so a 20 ft long, 3 ft high wall needs about 90 blocks plus cap units.
Most block systems set back about 1 inch per course, which leans the wall into the slope and adds stability.
How much extra should I order?
Always order a little extra. A 5% waste factor suits simple, square projects; 10% is right for most jobs; bump it to 15–20% for irregular shapes, diagonal patterns, or uneven ground. Running short almost always costs more than the leftover material.
Formula
Blocks per course = wall length ÷ block length. Courses = wall height ÷ block height. Blocks = courses × blocks per course, × (1 + waste%).
Frequently asked questions
- How many retaining wall blocks do I need?
- Divide the wall length by the block length for blocks per course, then multiply by the number of courses (wall height ÷ block height).
- How deep should the base of a retaining wall be?
- Set the first course on 4–6 inches of compacted gravel, with the bottom course buried about one tenth of the wall height.
- How many blocks do I need for a retaining wall?
- Divide the wall face area in square feet by the face area of one block. A 12 by 8 inch block covers 0.67 sq ft, so a 60 sq ft wall needs about 90 blocks.
- How much gravel behind a retaining wall?
- Allow a 12 inch wide column of clean stone the full height of the wall, plus 6 inches of compacted base below the first course.
- How tall can a retaining wall be without engineering?
- Commonly 4 feet in most jurisdictions, but confirm locally since some limit unengineered walls to 3 feet.
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