Segmental retaining walls — the interlocking concrete blocks used for garden terraces and landscape walls — go up quickly, but most of their strength comes from what is below and behind them: a buried first course, a compacted gravel leveling pad and a drainage zone of clean stone. This calculator counts the blocks and caps from the wall dimensions and block size, then estimates the gravel and drain pipe that a durable wall needs.
How to use the retaining wall calculator
- Choose US or Metric units.
- Enter the wall length and the exposed height above finished grade.
- Enter the block’s face length, height and depth from the product sheet.
- Set the number of courses below grade — at least one.
- Choose whether to include caps and enter the cap length.
- Check the leveling pad depth and the width of drainage gravel behind the wall.
Retaining wall formulas
Gravel tons use a typical 1.4 tons per cubic yard; the pad includes 20% for compaction.
Worked example: a 30 ft garden wall, 3 ft high
Blocks 16 in long, 6 in high, 12 in deep. Courses: ⌈ 36 ÷ 6 ⌉ + 1 buried = 7.
Blocks per course: ⌈ 360 ÷ 16 ⌉ = 23. Blocks: 7 × 23 = 161; with 5% → 170 blocks. Caps: 24.
Leveling pad: 30 ft × 2 ft × 0.5 ft × 1.2 = 36 ft³ = 1.33 yd³ ≈ 1.87 tons.
Drainage stone: 30 ft × 1 ft × 3.5 ft = 105 ft³ = 3.89 yd³ ≈ 5.44 tons, plus 30 ft of 4 in perforated pipe.
Typical block sizes
| Block type | Face (L × H) | Depth | Blocks per ft² of face |
|---|---|---|---|
| Small garden wall block | 12 × 4 in | 7–8 in | 3.0 |
| Medium wall block | 16 × 6 in | 10–12 in | 1.5 |
| Large wall block | 18 × 8 in | 12 in | 1.0 |
Check your product’s exact dimensions; small differences matter over a long wall. Corner units and caps are often sold separately.
Building a wall that stays put
- Excavate a trench wide enough for the leveling pad and deep enough for the pad plus the buried course.
- Compact and level the pad carefully. The first course decides how straight the whole wall will be.
- Set each course back as the block’s lip or pins dictate. That small batter leans the wall into the soil it holds.
- Backfill as you go with drainage stone directly behind the blocks and compacted soil behind that, every course or two.
- Wrap the drainage zone in landscape fabric so soil does not clog it, and run the drain pipe to daylight or a dry well.
- Use geogrid where the manufacturer’s charts call for it — usually on taller walls, walls with slopes, driveways or buildings above, and in clay soils.
Walls over 4 feet, or walls supporting a driveway, building or steep slope, need an engineered design; the International Residential Code (R404.4) requires it for walls retaining more than 48 inches of unbalanced fill. Check with your building department before you start.
Order the base and drainage stone by the ton with the gravel calculator, and level the terrace behind the wall with soil from the topsoil calculator. For a patio on top, the paver calculator covers pavers, base and sand.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block face length to get blocks per course, and the total height (including buried courses) by the block height to get courses, then multiply. A 30 ft wall of 16 × 6 in blocks, 3 ft exposed with one buried course, takes 7 courses of 23 blocks: 161 blocks, or 170 with 5% extra.
How deep should the first course of a retaining wall be buried?
Bury at least one full course, and more on taller walls; a common guideline is about one inch of embedment for every eight inches of wall height, with a minimum of about six inches. The block maker's installation guide gives the exact figure for its system.
How tall can a block retaining wall be without an engineer?
Many block systems allow gravity walls of roughly 3 to 4 feet without reinforcement. The International Residential Code requires retaining walls holding back more than 48 inches of unbalanced fill to be designed for stability, and many towns require a permit at lower heights or when there is a slope or driveway above.
Do retaining walls need drainage?
Yes. Water pressure behind a wall is a leading cause of failure. A zone of clean crushed stone at least 12 inches wide behind the blocks, with a perforated drain pipe at the base that outlets to daylight, relieves that pressure.
What goes under a retaining wall?
A leveling pad of compacted crushed stone, typically about 6 inches deep and about 12 inches wider than the block depth, so the first course sits on a firm, level, well-drained base.