Retaining Wall Block Calculator (Blocks, Caps, Base Gravel & Drainage Rock)
How many retaining wall blocks you need for a straight wall or a circle (fire pit, tree ring), including the buried course, cap units, base gravel for the leveling pad and drainage rock behind the wall. Block sizes are editable because every product line differs.
Formula
Buried depth = max(exposed height ÷ bury ratio, minimum). Courses = ⌈(exposed height + buried depth − cap height) ÷ block height⌉. Blocks per course = ⌈run ÷ block face length⌉, where run = wall length, or π × outside diameter for a circle. Wall blocks = ⌈blocks per course × courses × (1 + waste%)⌉; caps = ⌈run ÷ cap length × (1 + waste%)⌉. Base gravel cu yd = run(ft) × trench width(in) × base depth(in) ÷ 144 ÷ 27; drainage rock cu yd = run(ft) × zone depth(in) × stacked block height(in) ÷ 144 ÷ 27 (straight walls only).
Worked example
Inputs: Wall shape = straight; Wall length (straight) or outside diameter (circle) = 20 ft; Exposed wall height above grade (including cap) = 24 in; Block face length (from the block label) = 12 in; Block height (from the block label) = 4 in; Cap height (0 = no caps) = 0 in; Cap face length = 12 in; Bury 1 in of block for every __ in of exposed height (from your block maker) = 8 in; Minimum buried depth (from your block maker) = 6 in; Compacted base depth = 6 in; Base trench width = 24 in; Drainage rock zone behind the block = 12 in; Block waste/cut allowance = 5 %.
| Wall run ft | 20 |
|---|---|
| Face area sq ft | 40 |
| Buried depth in | 6 |
| Blocks per course | 20 |
| Courses | 8 |
| Buried courses | 2 |
| Wall blocks | 168 |
| Cap units | 0 |
| Trench depth in | 12 |
| Base gravel cubic yards | 0.74 |
| Drainage rock cubic yards | 1.98 |
Add the hollow-core fill from your block's spec sheet if your blocks have cores.
FAQ
How many retaining wall blocks do I need for a 20 ft wall 2 ft high?
With 12 × 4 in blocks, the 1-in-8 bury rule and a 6 in minimum buried depth: 24 in exposed + 6 in buried = 30 in, so 8 courses (32 in) of 20 blocks, 160 blocks, 168 with 5% waste. That is 40 sq ft of exposed face, about 0.74 cu yd of base gravel and 1.98 cu yd of drainage rock.
How many blocks for a fire pit?
Multiply the outside diameter by π and divide by the block face length for each course. A 4 ft outside diameter ring with 12 in blocks needs 13 blocks per course (150.8 in ÷ 12). Tapered blocks change the count, so dry-lay the first ring.
How deep do you bury the first course?
It depends on the block maker. Allan Block adds 1 in of buried block per foot of wall height; Rockwood and others use 1 in per 8 in of exposed height with at least 6 in buried. Both are editable here; follow the guide for the block you buy.
How much gravel goes under a retaining wall?
Allan Block and Rockwood both call for at least 6 in of compacted base in a trench about 24 in wide, so roughly 1 cu yd covers 27 ft of wall. Rockwood suggests adding 10% for trench and compaction variation.
Do I need a permit or engineer?
Allan Block requires a drain pipe for gravity walls over 4 ft, and taller walls often need geogrid and an engineered design. Ask your local building department; this tool only counts materials.
Sources
- Allan Block, Basic Installation guide: 24 in wide base trench, 6 in of base plus 1 in of buried block per 1 ft of wall height, wall rock in the cores and 12 in behind the block, drain pipe for gravity walls over 4 ft
- Rockwood Retaining Walls, Classic installation manual (PDF): bury 1 in of block per 8 to 10 in of wall height with at least 6 in of block embedment, minimum 6 in compacted base, 12 in of drainage aggregate behind the blocks, add 10% to base material
- Geometry (volume/area formulas); no external constant needed
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Last reviewed 2026-10-16.