Free landscaping calculator
Retaining Wall Calculator — blocks & gravel
Blocks and caps for the wall you see, plus the base trench and drainage gravel that keep it standing.
Base is a 6″-deep, 12″-wide compacted gravel trench; backfill is a 12″ gravel column behind the wall.
How many retaining wall blocks do I need?
Multiply wall length by wall height to get the face area in square feet, then divide by the face area of one block. The calculator adds 5% for cut blocks and breakage, counts cap blocks for the top course, and — the part most first-timers miss — sizes the gravel: a compacted base trench under the wall and a drainage column behind it. Walls fail from water pressure far more often than from weak blocks.
| Block face size | Blocks per 100 sq ft |
|---|---|
| 12 × 4 in | 300 |
| 16 × 6 in | 150 |
| 18 × 8 in | 100 |
The base and drainage matter more than the blocks
A lasting wall starts below grade: dig a trench about 12 inches wide and deep enough for 6 inches of compacted gravel plus one buried course of block — a common rule is to bury one inch of wall for every eight inches of height. Behind the wall, a 12-inch-wide column of clean drainage gravel (ideally with a perforated pipe at the base on taller walls) lets water escape instead of pushing the wall over.
How tall can I build?
Most block manufacturers rate standard gravity walls to 3–4 feet. Above that — or on slopes, or holding back a driveway — most municipalities require an engineered design and a permit. Check local code before building anything you can't comfortably step over.
Frequently asked questions
How many blocks do I need for a 20 foot wall 2 feet high?
That wall has 40 square feet of face. Using 16 by 6 inch blocks (0.67 sq ft each), you'd need about 63 blocks including 5 percent extra, plus roughly 15 cap blocks for the top course.
Do I need gravel behind a retaining wall?
Yes. A 12-inch-wide column of clean crushed gravel behind the wall relieves water pressure, which is the main reason retaining walls lean and fail. Taller walls should add a perforated drain pipe at the base.
How deep should the base be for a retaining wall?
Dig a trench for 6 inches of compacted gravel, then bury the first course of block about 1 inch for every 8 inches of wall height. A 3-foot wall should have its first course buried roughly 4 to 5 inches.
How tall can a retaining wall be without an engineer?
Most standard blocks are rated for gravity walls up to 3 or 4 feet. Many cities require an engineered design and permit above that height, and sooner if the wall supports a slope or driveway. Always check local code.