Retaining wall block calculator

Enter the wall's length and height and the face size of one block. You'll get the number of blocks and caps, and the gravel for drainage behind the wall.

Wall length (feet)

Wall height above ground (feet)

Block face length (inches)

Block face height (inches)

First course

–wall blocks

Rows of blocks: –
Blocks per row: –
Cap blocks: –
Drainage gravel (cu yd): –

Includes 5% extra blocks for cuts. Walls over 3 to 4 feet often need an engineer and a permit.

Common block sizes

Block type	Typical face size
Small garden wall block	12 × 4 in
Medium wall block	16 × 6 in
Large wall block	18 × 8 in

Measure the front face of the block you plan to buy, since sizes vary between brands.

How the math works

The wall height divided by the block height gives the rows, plus one buried row for the base. The wall length divided by the block length gives the blocks per row. A 20 foot wall, 2 feet high, built from 12 by 4 inch blocks needs 7 rows of 20 blocks. That's 140 blocks, or 147 with 5% extra, plus 20 caps.

Don't skip drainage

Water trapped behind a wall pushes on it and is the most common reason walls bulge or fall. Fill about 12 inches behind the wall with clean crushed stone, and add a perforated drain pipe at the bottom for walls over 2 feet. The calculator estimates that gravel for a 12 inch wide zone.

Common questions

How deep should the first row be buried?

A common rule is one inch for every eight inches of wall height, with a minimum of about six inches, or half a block for small walls. Follow the block maker's instructions, which are written for that block.

What goes under the first row?

A trench filled with about 6 inches of compacted crushed stone. The first row has to be perfectly level, since every row above follows it.