Estimate retaining wall blocks, caps, base gravel and drainage gravel from wall length and height, using typical base and backfill sizes you can change.
Retaining Wall Formula
For a straight rectangular wall, round blocks per course and the number of courses up separately, then apply the waste allowance. Enter total block height including buried courses and excluding caps:
B = ceil(ceil(L / bl) * ceil(H / bh) * (1 + W))
A single cap row, when used, is counted along the length only:
C = ceil(ceil(L / cl) * (1 + W))
Compacted base gravel is a volume, converted to cubic yards. Multiply this by (1 + A/100) for the entered base ordering allowance A (%):
Vbase = (L * Wb * Db) / 27
Drainage backfill behind the wall is the wall face area times the backfill thickness:
Vback = (L * H * T) / 27
- B = number of wall blocks
- C = number of cap blocks
- L = wall length
- H = entered total block-wall height, including buried courses and excluding caps; drainage mode uses the entered gravel-zone height
- bl = block length (face), bh = block height (course height)
- cl = cap block length
- W = waste/breakage fraction (for example 0.05 for 5%)
- Wb = base width, Db = base (gravel) depth
- T = drainage backfill thickness
- Vbase, Vback = gravel volume in cubic yards (dividing by 27 converts cubic feet to cubic yards)
Dividing the wall length by the block length gives the number of blocks in one course, and dividing the wall height by the block height gives the number of courses. Round each up before multiplying for the raw block count, then round up again after adding waste. Caps are separate and receive the same waste allowance; blank cap length uses the selected block face length. The base gravel and drainage gravel are handled as separate volumes because they use different dimensions: the base runs along the footing under the wall, while the backfill sits against the full height of the wall face. Add product-specific block-core fill when required; blank adds none. The illustrative bulk density starts at 1.4 US short tons per ordered cubic yard and must be checked with the supplier. Cost subtotals include only supplied unit prices and exclude labor, excavation, pipe, fabric, reinforcement, delivery and tax.
Typical Block Sizes and Gravel Depths
These dimensions are illustrative references, not structural design limits. Confirm product face and cap sizes, embedment, base dimensions and drainage requirements from the actual product and project design. The calculator requires base width and depth rather than assuming width is twice block depth.
| Block face (length x height) | Blocks per square foot of wall | Typical use |
|---|---|---|
| 12 in x 4 in | 3.0 | Garden and small walls |
| 16 in x 6 in | 1.5 | Mid-size walls |
| 18 in x 6 in | 1.33 | Product and design specific |
| Layer | Typical depth | Purpose |
|---|---|---|
| Base gravel (leveling pad) | 6 in to 8 in | Stable, compacted footing |
| Base width | Use the product / project design | Spreads the wall load |
| Drainage backfill | 12 in | Drains water away from the wall |
Example Problems
Example 1. A wall is 40 ft long and 3 ft high using 16 in by 6 in blocks (block length 1.333 ft, block height 0.5 ft) with a 5% waste allowance.
Blocks per course = ceil(40 / (16/12)) = 30. Courses = ceil(3 / 0.5) = 6. Raw blocks = 30 × 6 = 180. With 5% waste: ceil(180 × 1.05) = 189 blocks. With 16-inch caps and the same waste, order ceil(30 × 1.05) = 32 caps separately.
Example 2. The same 40 ft wall sits on a base 1.5 ft wide and 0.5 ft deep, with 1 ft of drainage gravel behind the 3 ft face.
Base gravel = (40 x 1.5 x 0.5) / 27 = 1.11 cubic yards. Drainage gravel = (40 x 3 x 1) / 27 = 4.44 cubic yards. Total gravel = about 5.6 cubic yards before compaction allowance.
FAQ
How much extra should I order for waste?
The calculator starts at a 10% planning allowance for blocks and caps. Choose the actual allowance with the supplier; curves, corners, stepped sections and cutting plans can change the quantity.
Do I count the cap row in the block total?
No. Cap blocks are usually a different unit and are counted separately as a single row along the wall length. The calculator keeps them apart so you can price them on their own.
Why is gravel given in cubic yards?
Bulk gravel is sold by the cubic yard. The volume is figured in cubic feet from your measurements, then divided by 27 to convert to cubic yards. The base order applies an editable 20% planning allowance to compacted geometry; this is not a measured universal compaction factor. Drainage gravel uses its entered geometry plus any added core fill. Confirm both order quantities and bulk density with the supplier.
