Retaining Wall Block Calculator

Enter your wall size and block dimensions to see how many blocks, caps and gravel to buy.

How the calculation works

blocks per course = wall length × 12 ÷ block length, rounded up

courses = wall height ÷ block height, rounded up, plus the buried courses

Multiply them together for total blocks, then add 5% for cuts and breakage. Caps run along the top in a single row. The gravel estimate assumes a 12" wide drainage zone behind the wall, running its full height, and a 6" deep by 24" wide leveling pad under the first course.

Worked example

A 20 foot wall, 24" tall, built from 12" × 4" blocks with one buried course: 20 blocks per course × 7 courses = 140 blocks, or 147 with 5% extra. You'd also need 20 caps (21 with extra) and about 2.5 cubic yards of gravel.

Tips

• The first course matters most. Set it level on a compacted gravel pad. Every course above follows it.
• Add a drain pipe behind walls over about 2 feet, so water pressure doesn't push the wall over.
• Glue the caps on with concrete adhesive made for masonry.

Frequently asked questions

How many retaining wall blocks per square foot?

A 12" × 4" block covers 1/3 square foot of wall face, so you need 3 per square foot. A 16" × 6" block covers 2/3 square foot, so 1.5 per square foot.

How tall can I build a retaining wall without a permit?

Many US building codes require a permit and engineering for walls over 4 feet, measured from the bottom of the footing. Some towns set a lower limit, so check with your local building department.