Retaining Wall Block & Base Calculator
Estimate interlocking segmental retaining wall blocks, capstones, leveling pad gravel, and drainage rock.
Wall Dimensions & Block Sizing
Retaining Wall Blocks Quick Reference (16" × 6" Block)
Common wall sizes with one buried base course, 5% waste included, and a 12-inch drainage chimney behind the wall. All figures match the calculator above.
| Wall (L × Exposed H) | Courses | Blocks / Course | Total Blocks | Caps | Base Gravel | Backfill Gravel |
|---|---|---|---|---|---|---|
| 20 × 2 ft | 5 | 15 | 79 | 16 | 0.56 yd³ | 1.48 yd³ |
| 30 × 3 ft | 7 | 23 | 170 | 25 | 0.83 yd³ | 3.33 yd³ |
| 40 × 3 ft | 7 | 30 | 221 | 32 | 1.11 yd³ | 4.44 yd³ |
| 50 × 4 ft | 9 | 38 | 360 | 40 | 1.39 yd³ | 7.41 yd³ |
| 60 × 4 ft | 9 | 45 | 426 | 48 | 1.67 yd³ | 8.89 yd³ |
Gravel quantities convert to tons at 1.4 tons per cubic yard — e.g., a 30 × 3 ft wall needs about 1.2 tons of base gravel plus 4.7 tons of drainage backfill.
Worked Example: 30 ft × 3 ft Wall
1. Courses: 3 ft × 12 = 36" ÷ 6" block height = 6 exposed courses, plus 1 buried course = 7 courses total.
2. Blocks per course: 30 ft × 12 = 360" ÷ 16" block width = 22.5, rounded up to 23 blocks.
3. Wall blocks: 7 × 23 = 161, plus 5% waste = 170 blocks. Caps: 23 × 1.05 = 25 capstones.
4. Leveling pad: 30 ft × 1.5 ft wide × 0.5 ft deep = 22.5 cu ft = 0.83 cu yds (1.17 tons) of compacted crusher run.
5. Drainage chimney: 30 ft × 3 ft × 1 ft = 90 cu ft = 3.33 cu yds (4.67 tons) of clean #57 stone behind the wall, with a perforated 4" pipe at the base daylighting to discharge.
Frequently Asked Questions
How deep should the base trench be for a retaining wall?
The base trench should be excavated at least 12 inches deep and 18 to 24 inches wide. Fill with 6 inches of compacted angular crushed stone (#57 or crusher run), and bury the first course of blocks completely underground for lateral stability.
How high can a DIY retaining wall be built without an engineer?
Most local building codes allow gravity block retaining walls up to 4 feet in total height (including the buried base course) without requiring a stamped engineered plan or geogrid soil reinforcement.
Why do retaining walls need gravel backfill behind them?
Water trapped behind a wall is the #1 cause of retaining wall failure due to hydrostatic pressure. A 12-inch wide vertical chimney of clean drain gravel (along with a perforated drain pipe at the base) allows water to drain freely away.
How many retaining wall blocks do I need per linear foot?
It depends on block width. A 12-inch-wide block needs 1 block per foot of wall, a 16-inch block needs 0.75 blocks per foot (23 blocks for a 30-foot course), and an 18-inch block needs 0.67 blocks per foot. Multiply by the number of courses, then add 5% for cuts and waste.