How many blocks do I need for a retaining wall?
Enter the wall length and height, choose your block size, and set a waste allowance — you'll get the number of blocks to buy, the courses and blocks per course, the capping units and the base gravel, instantly, in feet. For example, a 20 ft long, 3 ft high wall in 12 × 6 in blocks is 6 courses of 20 blocks — about 126 blocks with a 5% waste allowance.
Includes a 5% waste allowance for cuts and breakage. Stack whole courses from a level, compacted gravel base; the caps and base gravel are estimates — confirm the block and cap sizes your supplier stocks.
Blocks = courses (height ÷ block height, rounded up) × blocks per course (length ÷ block width, rounded up), plus a waste allowance. Base gravel = length × base width × base depth. These are material-planning figures, not a structural design or a code value — tall walls (often over about 1.2 m / 4 ft) usually need an engineered design and a permit; check with your local building authority.
Common questions
- How many blocks do I need for a retaining wall?
- Count the courses and the blocks in each course, then multiply. Courses = wall height ÷ block height, rounded up; blocks per course = wall length ÷ block width, rounded up. Multiply the two and add 5–10% for waste. For example, a 20 ft long, 3 ft high wall built from 12 × 6 in blocks is 6 courses of 20 blocks = 120, or about 126 with a 5% allowance. The calculator above does it for you in feet or metres.
- How do I calculate blocks for a retaining wall?
- It is a whole-block grid. First find the number of rows: divide the wall height by the block height and round up, because you cannot leave a course unfinished. Then find the blocks in one row: divide the wall length by the block width and round up. Multiply rows × blocks-per-row for the bare count, then add a waste allowance. A capping row along the top is one extra block per block-width of length.
- How much gravel do I need for the base of a retaining wall?
- The levelling base is a trench of compacted gravel under the bottom course. Its volume is simply the wall length × the base width × the base depth. As a common planning starting point a base around 12 in (300 mm) wide and 6 in (150 mm) deep is typical for a low garden wall, which on a 20 ft wall is roughly 0.37 cubic yards. Confirm the base size against your block manufacturer's instructions, because taller or load-bearing walls need a wider, deeper base.
- What size are retaining wall blocks?
- Segmental retaining-wall (SRW) blocks vary by maker, but common faces are about 12 in wide and 4–6 in high for garden-wall blocks, and 16–18 in wide by 6–8 in high for larger structural units; in metric, roughly 300–450 mm wide by 100–200 mm high. A bigger block means fewer pieces and faster laying. Pick the size in the calculator that matches the block you will actually buy, since the count changes a lot with block size.
- How tall can I build a retaining wall myself?
- Material count is the easy part; structure is not. Many areas allow a low landscape wall — often up to about 1.2 m (4 ft) — to be built without an engineer, but above that, or where the wall holds back a slope, a driveway or a structure, an engineered design, geogrid reinforcement and a permit are commonly required. This calculator sizes the blocks and base only; check your local building authority for the height limit and reinforcement rules before you build.
- How much extra should I add for waste?
- About 5–10% is normal. The allowance covers blocks cut to length at the ends and around curves or steps, plus the occasional cracked or off-colour unit. Use the higher end for a curved or stepped wall with lots of cuts, and keep a few spare blocks and caps from the same batch, because concrete colour can shift between production runs.
Want the full walk-through? Read the retaining wall guide → Pouring a concrete footing first? Try the concrete calculator →
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-06.