Retaining Wall Calculator

Enter the wall length and height and the block dimensions, and get the number of courses, blocks per course, total blocks, capstones, base gravel and drainage stone.

Retaining walls hold back real loads — read the note on height limits before you start.

How to use the retaining wall calculator

Enter the wall length and finished height.

Enter your block dimensions.

Read courses, blocks, base and drainage stone.

Why use the Build Tally retaining wall calculator

Course breakdown

See courses high and blocks per course.

Base and drainage

Gravel volumes for both included.

Capstone count

Top course quantity given.

Safety guidance

Clear notes on when engineering is required.

Courses and block counts

The number of courses is the wall height divided by the block height, rounded up, and blocks per course is the wall length divided by the block length. Most segmental wall blocks are 12 inches long by 4 or 6 inches high, so a 20 ft wall 3 ft high is 9 courses of 20 blocks — about 180 blocks before waste. The first course is usually buried, so add its height to your total when measuring.

The base course decides everything

A retaining wall is only as straight and stable as its first course. Standard practice is to excavate below grade, compact the subgrade, then lay 6 inches of compacted crushed stone and level the first course meticulously — checking every block front to back and side to side. Errors in the base course multiply upward and cannot be corrected higher up.

Drainage and height limits

Water pressure behind a wall is what pushes it over, so free-draining gravel behind the blocks and a perforated drain pipe at the base are essential, not optional. Height limits matter too: most jurisdictions require engineered design and a permit above about 4 feet, and lower where the wall supports a driveway, structure or slope above it. Check your local rules before building.

Read the guide: How Much Gravel Do I Need? Tons, Cubic Yards and the Right Depth

Frequently asked questions

How many blocks for a retaining wall?

Courses (height ÷ block height) × blocks per course (length ÷ block length). Enter your figures above for the exact count.

How high can I build without engineering?

Commonly up to about 4 feet, but limits vary and are lower where the wall supports a load. Check your local code — this is a genuine safety matter.

What goes behind a retaining wall?

Free-draining gravel and a perforated drain pipe at the base. Trapped water is the main cause of wall failure.

How deep should the base be?

Typically 6 inches of compacted crushed stone, with the first course buried below finished grade.

Do I need geogrid?

Taller walls and walls with a slope or load above usually require geogrid reinforcement layers. An engineer specifies this.

How many capstones do I need?

One per block length along the top course, which the calculator shows.