Retaining Wall Block Calculator – How Many Blocks? | CalcLoft
Find out how many blocks you need for a retaining wall — rows (courses), blocks per row and the total to order, including waste for cuts and breakage.
How many retaining wall blocks do I need?
Retaining wall blocks are laid in rows, so count them in two steps. First find the number of rows (courses): divide the wall height in inches by the block height and round up. Then find how many blocks fit along the wall in one row: divide the wall length in inches by the block length and round up. Multiply the two together:
Blocks = rows × blocks per row
For example, a 20 ft long, 2 ft high wall built with 8 in × 4 in blocks (8 in long, 4 in high) has 24 ÷ 4 = 6 rows and 240 ÷ 8 = 30 blocks per row, for a total of 6 × 30 = 180 blocks.
Always use the block's actual face dimensions — the real length and height you see on the front of the unit — not its nominal or trade size. If the first course sits below grade or the wall backs into a slope, add the buried height to the wall height before calculating.
Block counts for common walls (8 in × 4 in blocks)
The table below shows totals for an 8 in long, 4 in high block, plus the quantity to buy with the usual 10% waste allowance.
| Wall size (L × H) | Rows | Blocks per row | Total blocks | Buy with 10% waste |
|---|---|---|---|---|
| 10 × 1 ft | 3 | 15 | 45 | 50 |
| 10 × 2 ft | 6 | 15 | 90 | 99 |
| 20 × 2 ft | 6 | 30 | 180 | 198 |
| 20 × 3 ft | 9 | 30 | 270 | 297 |
| 30 × 2 ft | 6 | 45 | 270 | 297 |
For a 10 ft long, 1 ft high wall: 12 in ÷ 4 in = 3 rows and 120 in ÷ 8 in = 15 blocks per row, so 3 × 15 = 45 blocks, and 45 × 1.1 rounds up to 50 with waste. Block size changes the math quickly — the same 20 × 2 ft wall is 6 rows of 15 (90 blocks) with 16 in blocks, and 4 rows of 15 (60 blocks) with 16 in long, 6 in tall units.
Base gravel, drainage and tall-wall rules
The block count above covers the wall face only. Every retaining wall needs a compacted crushed-stone base at least 4–6 in deep and several inches wider than the block on each side — a 20 ft wall with a 6 in deep, 1.5 ft wide base uses roughly 0.6 cu yd of gravel, ordered separately. Behind the wall, backfill the bottom courses with clean gravel and run a perforated drain pipe so water can escape instead of pushing on the wall. Cap units, adhesive and any cut blocks are extra.
In most areas, walls up to about 3–4 ft tall can be built as a self-supporting gravity wall without engineering. Anything taller, or a wall holding back a steep slope or heavy load, needs a structural engineer's design and usually a permit — check your local building code before you start.
Related guides
Retaining wall block calculator FAQ
How many blocks for a 20 × 2 ft retaining wall?
With 8 in × 4 in blocks you need about 180 blocks: 6 rows (courses) times 30 blocks per row. Order roughly 198 to include a 10% waste allowance for cuts and breakage. With longer or taller blocks the count drops fast — try the calculator above with your block size.
How tall can a retaining wall be before I need an engineer?
Most residential codes let you build segmental retaining walls up to about 3–4 ft without an engineer's review. Above that height you should have a structural engineer design the wall and check whether a permit is required — local rules vary, so confirm with your building department.
What size retaining wall block should I choose?
For a low garden wall (2 ft or less), lighter 8 in × 4 in units are easy to handle and give the blockiest look. Taller walls are faster with bigger units: 6 in tall blocks mean fewer rows to lay, and 12–16 in long blocks mean fewer pieces and joints per row. Match the block size to the wall's height and to how heavy a unit you can lift.
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