Retaining wall calculator

Enter the wall size and block dimensions to see how many blocks, caps and cubic yards of gravel to buy.

By the Calcist team · Checked against known answers · Updated October 27, 2026

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How retaining wall blocks are calculated

Courses = wall height (in) ÷ block height, rounded up, + 1 buried course
Blocks per course = wall length (in) ÷ block face length, rounded up
Caps = wall length (in) ÷ cap length, rounded up

Worked example

A wall 20 ft long and 2 ft high, with 12 × 4 in blocks: 24 ÷ 4 = 6 courses, plus 1 buried = 7. Each course is 240 ÷ 12 = 20 blocks, so 140 blocks; with 5% extra, buy 147, plus 20 caps. A 6 in base pad 24 in wide is 0.74 cu yd of gravel, and 12 in of drainage stone behind the 2.33 ft of block is 1.73 cu yd: about 2.5 cu yd in total.

Gravel assumptions

The base pad here is 24 inches wide, and the drainage stone fills the given width behind the wall for its full height, including the buried course. Most block makers call for a compacted gravel leveling pad and free-draining stone behind the wall; follow your block maker's installation guide for exact sizes.

Limitations

Height limits: walls taller than about 3 to 4 ft, or holding back a slope, driveway or building, usually need an engineered design, geogrid and a permit.
Setback: many blocks step back as they rise, which slightly changes the length of upper courses.
Curves and corners need extra blocks; add more than 5% for them.

Sources

Unilock: How deep should a paver base be?

How we checked this

Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:

20 × 2 ft wall, 12 × 4 in blocks, buried course, 5%: 147 blocks ✓
Same: 2.469 cu yd of gravel ✓
Without a buried course: 6 courses, 120 blocks ✓
A 5 ft wall warns about engineering ✓

Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.

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Questions people ask

How many retaining wall blocks do I need?

Multiply the courses (height ÷ block height, plus a buried course) by the blocks per course (length ÷ block length). A 20 × 2 ft wall of 12 × 4 in blocks takes 140.

How much gravel do I need behind a retaining wall?

With 12 inches of drainage stone behind a 20 ft wall 2⅓ ft tall, about 1.7 cubic yards, plus about 0.7 cu yd for a 6 in base pad.

Should the first course of a retaining wall be buried?

Usually yes. Block makers commonly recommend burying the first course, or about 10% of the wall height, for stability.

How tall can a retaining wall be without an engineer?

It depends on local code, but many areas require an engineered design for walls over about 4 feet.

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