How to calculate retaining wall blocks
A retaining wall is built in courses — horizontal rows of blocks stacked on top of each other. To find the block count you work out how many blocks fit in one row, how many rows make up the height, and multiply. This calculator adds the cap row, a waste allowance, and an estimate of the compacted gravel base the wall sits on.
The formula, step by step
1. Blocks per course = wall length ÷ block length, rounded up.
2. Courses = wall height ÷ block height, rounded up.
3. Wall blocks = blocks per course × courses, × (1 + waste %).
4. Cap blocks = one extra row along the top (if selected).
Don't skip the base
A retaining wall is only as good as its base. Dig a level trench, then add about 6 inches of compacted gravel and bury the first course roughly one-tenth of the wall's height. This spreads the load and stops the wall from settling or leaning. The calculator estimates base gravel from your wall length and a trench about twice the block depth wide.
When to call a pro
Short garden walls (under about 3–4 ft) are a common DIY project. But taller walls — or any wall holding back a slope, driveway, or structure — face serious soil pressure and usually require drainage, geogrid reinforcement, and an engineer's design to meet code. Always check local requirements before building.
Common block sizes
| Block (L × H) | Per 20 ft course | Notes |
|---|---|---|
| 12 × 4 in | 20 blocks | Low garden walls |
| 12 × 6 in | 20 blocks | Very common |
| 18 × 8 in | 14 blocks | Larger walls |
Methodology & sources
How this calculator works: it divides the wall face area (length × height) by the face area of one block to get the block count, adds a waste allowance, and accounts for base and cap courses where relevant. Sources: block-count method follows segmental retaining wall (SRW) manufacturer guidance; for walls over about 4 ft or those retaining slopes/surcharges, consult an engineer and local code. Estimates are for planning only. Last reviewed: by the Toolchive Team.