Calculate how many blocks you need to build a retaining wall.
Base & drainage
Estimated cost
Shopping list
Usage Tip
Bury the first course about one tenth of the wall height on compacted gravel; a level base course is what keeps the wall straight.
rows = round up( height × 12 ÷ block height )
total = rows × blocks per row
Counts round up to whole blocks.
How many retaining wall blocks do I need?
Work it out in two directions. The number of courses is the wall height divided by the block height; the blocks per course is the wall length divided by the block length. Multiply the two and add waste for cuts. A 20 ft long, 2 ft high wall built from 12 in long, 4 in high blocks needs 6 courses of 20 blocks — about 120 blocks, plus a row of caps on top. This calculator also sizes the base gravel, drainage stone, backfill, drain pipe, geogrid and the total cost.
Common block sizes
Retaining wall blocks vary widely. Typical face dimensions:
| Block | Face (L x H) | Blocks per face sq ft |
|---|---|---|
| Garden / mini | 12 x 4 in | 3.0 |
| Standard SRW | 12 x 6 in | 2.0 |
| Large | 18 x 6 in | 1.3 |
| Big block | 16 x 8 in | 1.1 |
Retaining wall base gravel calculator
A retaining wall stands on a compacted gravel base set in a trench. The trench is usually about 6 in deep and as wide as the block plus a foot, so the wall has a stable footing. Order roughly 20 percent extra gravel for compaction.
| Wall height | Base depth | Buried first course |
|---|---|---|
| Up to 2 ft | 6 in | 1 in per 8 in of height |
| 2 to 4 ft | 6 in | about 10% of wall height |
| Over 4 ft | engineered | engineered |
Retaining wall drainage guide
Water behind a wall is what topples it. Good drainage has three parts: a column of clean drainage gravel directly behind the blocks, a perforated drain pipe at the base that daylights to one side, and landscape fabric separating the gravel from the backfill soil so it does not clog. Run the pipe the full length of the wall and slope it to an outlet.
Common wall examples
| Wall | Blocks (12x4 in) |
|---|---|
| 10 ft x 1 ft garden bed edge | about 30 + caps |
| 20 ft x 2 ft wall | about 120 + caps |
| 30 ft x 3 ft wall | about 270 + caps |
Frequently asked questions
How do I calculate retaining wall blocks?
Courses equal wall height divided by block height; blocks per course equal wall length divided by block length. Multiply and add waste, then add a row of caps.
Do I need a permit for a retaining wall?
Usually for walls over about 4 ft, and often for any wall supporting a slope or structure. Check your local building department.
How much gravel goes behind a retaining wall?
A drainage column at least 12 in wide for the full height, plus a compacted base in the trench. The calculator estimates both.
Do I need geogrid?
Often for walls over 3 to 4 ft or where there is a slope or load above. Geogrid layers tie the wall back into the soil every couple of courses.
How many cap blocks do I need?
One row along the top, so roughly the wall length divided by the cap length. Caps are usually glued down with concrete adhesive.
How deep should the first course be buried?
About 1 in for every 8 in of wall height, or roughly a tenth of the wall height, so the base sits below grade.
Gravel Calculator · Project Cost Calculator · Material Estimator · Paver Calculator
Estimates are for planning. Retaining walls over about 4 ft, walls on slopes, and walls supporting structures typically require an engineer and a permit; reinforcement, drainage and base requirements vary with soil and load. Confirm block coverage and pallet counts with your supplier and follow local code.