Retaining Wall Block Calculator
Enter your wall's length and height and the block size you're using. You'll get the block count (with 5% extra), cap blocks, and the gravel for the base trench.
Results
How the retaining wall calculator works
A segmental retaining wall is a grid of blocks: the number of courses (rows) is your wall height divided by the block height, and the blocks per course is the wall length divided by the block length. Multiply, add 5% for cuts and chipped blocks, and that's your order.
Formula: ceil(height ÷ block height) × ceil(length ÷ block length) × 1.05 = blocks
| Wall height | Courses | Blocks (incl. 5%) |
|---|---|---|
| 1 ft | 3 | 32 |
| 2 ft | 6 | 63 |
| 3 ft | 9 | 95 |
The calculator also sizes the base trench: 6 inches of compacted crusher-run gravel in a trench 12 inches wide, which is the standard footing for garden-scale walls.
Building a wall that doesn't lean
- Bury the first course: dig the trench deep enough that the bottom block sits fully below grade — it locks the wall against sliding.
- Level the first course obsessively, block by block. Every error in course one is amplified at the top.
- Set each course back slightly (most blocks have a built-in lip) so the wall leans into the slope.
- Backfill with clean gravel directly behind the blocks — not soil — so water drains instead of pushing the wall over in winter.
- Walls over 3–4 ft usually need engineering and often a permit; check local rules before stacking higher.
Frequently asked questions
How many retaining wall blocks do I need per square foot?
With standard 12" × 4" blocks, you need 3 blocks per square foot of wall face. A 20 ft long, 2 ft high wall (40 sq ft) needs about 126 blocks including 5% extra.
What goes under a retaining wall?
A compacted gravel base: 6 inches of crusher run in a trench about twice as wide as the block, with the first course of blocks buried below grade on top of it.
How high can I build a retaining wall without an engineer?
Most block systems are rated for 3–4 ft as gravity walls. Above that (and in many towns, above 3 ft) you typically need geogrid reinforcement, engineering and a permit.
Do I need drainage behind a retaining wall?
Yes — at minimum a foot of clean gravel backfill directly behind the blocks. For walls over 2 ft, add a perforated drain pipe at the base leading to daylight. Trapped water is what destroys retaining walls.