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Retaining Wall Calculator

Plan a segmental block retaining wall before you order. Enter the wall length and height and the size of your block, and get the number of blocks, cap stones, drainage gravel and drain pipe for the job.

Quick answer: A retaining wall 20 ft long and 3 ft high built from 12 in by 4 in blocks needs about 210 blocks, 21 cap blocks and 2.96 cubic yards of gravel.

Your measurements

ft
ft
in
in
qty
in
in
in
in
%

Wall blocks needed

210blocks

Courses (rows) of block
10
Blocks per course
20
Blocks with 5% extra
210 blocks
Cap blocks
21 caps
Gravel (backfill and base)
2.96 cubic yards
Drain pipe along the wall
20 ft

Walls taller than about 3 to 4 feet, or walls holding back a slope or a driveway, often need a permit or an engineered plan. Check with your local building office.

How to use this calculator

  1. Measure the length of the wall and the visible height in feet.
  2. Enter the face width and the height of one block in inches.
  3. Set the number of courses buried below grade. One course is common for small walls.
  4. Enter the cap block length, the width of gravel behind the wall and the width and depth of the base trench.
  5. Read the block count, cap count and gravel volume, and add a little for cuts.

How the numbers are calculated

The number of visible courses is the wall height divided by the block height, rounded up. The buried courses are added on top. The blocks per course are the wall length divided by the block width, rounded up, and the total is courses times blocks per course plus your waste.

Gravel is worked out in two parts. The backfill behind the wall is length times height times the backfill width. The base is length times base width times base depth. Both are added and converted to cubic yards by dividing by 27.

Worked example

A wall 20 ft long and 3 ft high built from 12 in by 4 in blocks has 9 visible courses. With 1 buried course that is 10 courses of 20 blocks, or 200 blocks, and 210 with 5% extra. The caps need 21 blocks. Gravel for a 12 in backfill and a 24 in by 6 in base comes to 2.96 cubic yards.

Tips before you buy

  • Dig the base trench level and compact it well. A weak base is the most common reason a wall fails.
  • Use clean crushed stone behind the wall, not soil, so that water can drain out of it.
  • Lay a perforated drain pipe at the base of the backfill and run it out to daylight.
  • Taller walls and walls that hold back a slope, a driveway or a building often need an engineered design and a permit.
  • Add geogrid between courses if the manufacturer calls for it at your wall height.

Frequently asked questions

How many blocks do I need for a retaining wall?

Divide the wall length by the block width to get blocks per course, then multiply by the number of courses. Add 5% for cuts.

How much gravel do I need behind a retaining wall?

A common guide is at least 12 inches of clean gravel behind the wall for its full height, plus a gravel base under the first course.

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

It depends on your area. Many places set the limit at 3 or 4 feet. Check with your local building office before you start.

Why bury the first course?

Burying the first course locks the wall in place and helps stop it sliding forward. Your block maker will list the depth they recommend.

Related calculators

These results are estimates for planning purposes. Material sizes, densities and local requirements vary, so confirm quantities with your supplier or a qualified contractor before ordering.

Last updated October 11, 2026.