How the retaining wall calculator works
Dry-stacked segmental blocks are counted course by course. The number of courses is the exposed height divided by the block height, rounded up, plus one course set below ground: burying the first row keeps the toe of the wall from sliding and hides the uneven base. Each course needs the wall length divided by the face length of one block, again rounded up. Five percent is added for broken units and the cut blocks at ends and corners.
Blocks = (⌈height ÷ block height⌉ + 1) × ⌈length ÷ block length⌉ × 1.05Gravel goes in two places. The levelling pad under the first course is taken as 15 cm deep and 60 cm wide (6 in × 24 in) of compacted crushed stone. The drainage zone is a band of clean angular stone about 30 cm (12 in) wide behind the blocks, running the full height of the wall. Both are increased by 10% for compaction, and the weight uses 1.6 tonnes per cubic metre. A perforated drain pipe normally runs along the back of the bottom course for the whole length.
Worked example
A garden wall 6 m long and 0.6 m high built from blocks with a 40 × 15 cm face needs ⌈0.6 ÷ 0.15⌉ = 4 visible courses plus one buried, so 5 courses of ⌈6 ÷ 0.40⌉ = 15 blocks. That is 75 blocks, or 79 with the allowance, plus 16 caps for the top. The blocks stand 0.75 m tall in total, so the gravel is 6 × (0.15 × 0.60 + 0.30 × 0.75) = 1.89 m³, or 2.08 m³ once compacted, about 3.3 tonnes.
Blocks per unit of wall face
| Block face | Blocks per m² | Blocks per 10 ft² | Typical use |
|---|---|---|---|
| 30 × 15 cm | 22.2 | 20.6 | Low edging and bed walls |
| 40 × 15 cm | 16.7 | 15.5 | Garden terraces up to knee height |
| 40 × 20 cm | 12.5 | 11.6 | Heavier landscape walls |
| 12 × 4 in | 32.3 | 30.0 | Tree rings, small borders |
| 16 × 6 in | 16.1 | 15.0 | Standard garden wall blocks |
| 18 × 8 in | 10.8 | 10.0 | Larger units for taller walls |
Block depth, from front to back, matters for stability but not for the count. Deeper and heavier blocks resist soil pressure better, which is why maximum heights differ from one product line to the next; the limit printed by the manufacturer applies to their block only.
Before you build
- Height limit. A wall higher than about 1 m (3–4 ft), or any wall holding back a slope, a driveway, a building or a pool, needs an engineered design with geogrid or reinforcement, and many areas require a permit. Have it designed by a qualified engineer and check your local code.
- Drainage is not optional. Water trapped behind a wall adds far more pressure than the soil itself. Wrap the drainage stone in filter fabric and give the pipe an outlet to daylight.
- Level the first course perfectly. Any error in the base course is copied, and magnified, in every row above it.
- Allow for setback. Most blocks step back a little with each course. This does not change the count, but it moves the top of the wall back into the slope.
- Curves and corners. Tight curves need extra cut blocks; add another 5% for a curved wall.
Frequently asked questions
How many retaining wall blocks do I need for a 20 ft wall?
With 16 × 6 in blocks, a 20 ft wall takes 15 blocks per course. At 3 ft exposed height that is 6 courses plus one buried, 105 blocks, or 111 with 5% extra, plus 16 caps.
How deep should the first course of a retaining wall be buried?
A common rule is one full block or about one eighth of the wall height, whichever is more, so at least 15 cm (6 in) for most garden walls.
How much gravel do I need behind a retaining wall?
Allow a band of drainage stone about 30 cm (12 in) wide for the full wall height, plus a 15 cm (6 in) base pad. A 20 ft wall 3 ft high needs about 3.7 cubic yards including the pad.
How high can I build a retaining wall without an engineer?
Many jurisdictions require an engineered design or permit above about 1 m (3–4 ft), and sooner if the wall carries a load such as a driveway. Check with your local building office before you start.
Do I need caps on a retaining wall?
Caps are not structural, but they finish the top, shed water and stop the last course being knocked loose. Glue them down with a masonry adhesive.
Last reviewed: 2026-09-27 · How we calculate