Retaining Wall Calculator – Blocks & Base Gravel

Measure your wall's total length and finished height in feet, then enter your block's face length and face height in inches (both are printed on the product page or the pallet tag). The calculator multiplies rows by columns to give the exact number of retaining wall blocks, then sizes the base gravel trench that goes underneath. Need cap blocks, metric units, a waste allowance or a cost estimate? Open More options. Everything works from just the four main boxes, so you can get an answer in seconds and fine-tune afterward.

A dry-stack retaining wall is really a grid: whole blocks stacked in rows. To find how many retaining wall blocks you need, you don't guess from square footage — you count rows and columns. The number of rows equals your wall height divided by one block's face height, rounded up. The number of blocks per row equals wall length divided by one block's face length, rounded up. Multiply the two and you have the total. A 20-foot-long, 2-foot-high wall built from 12 in × 4 in blocks needs 6 rows of 20 blocks — 120 blocks. Because partial blocks aren't sold, both counts round up, which is why a slightly longer wall can jump a whole column. This row-by-column method is what the pros use, and it's far more accurate than dividing wall area by block area.

Rows = ⌈wall height ÷ block face height⌉

Blocks per row = ⌈wall length ÷ block face length⌉

Total blocks = rows × blocks per row

Base gravel is the leveling pad under the first course:

Base gravel (cu ft) = wall length × trench width × trench depth

Divide by 27 for cubic yards. The default trench is 12 in wide and 6 in deep — a typical footing for a low garden wall. Under More options you can add a cap row (one solid block per column) and a 5–10% waste allowance for cuts and breakage.

How many blocks for a 10-foot wall? A 10 ft long, 2 ft high wall using 12 in × 4 in blocks = 6 rows × 10 = 60 blocks, plus about 0.19 cu yd of base gravel.

A 40-foot, 3-foot-high wall: with 16 in × 6 in blocks that's 6 rows × 30 = 180 blocks.

Adding a cap: on the 20-foot wall above, turning on the cap row adds 20 solid cap blocks along the top for a finished edge.

- Use the face size, not the box size. Retaining wall blocks are counted by face length × face height; the buried depth of the block doesn't change the count.
- Add 5–10% for waste on any wall with curves, corners or cuts — enter it under More options.
- Don't skip the base. A 6-inch compacted gravel base is what keeps the wall from heaving; this calculator sizes it for you.
- Bury the first course. Set the bottom row roughly 10% of the wall height below grade for stability, and still count it as a full row.
- Walls over 3–4 ft often need engineering, geogrid reinforcement or a permit — check local code before you buy.