How this calculator works
This runs our retaining wall calculator. The number of courses is the wall height divided by the block height, and the blocks per course are the length divided by the block length. A cap row goes on top. Gravel goes in a compacted base trench and in a drainage strip behind the wall, with a perforated pipe along the bottom.
Worked example: a 20 ft long, 3 ft high wall of 12 × 4 × 10 inch blocks has 9 courses and takes 189 blocks with 5% waste, plus 20 caps. The gravel is 3.0 cubic yards (0.8 for the base and 2.2 behind the wall), with 20 ft of drain pipe and 120 ft² of fabric.
Materials by wall size (12 × 4 × 10 in blocks, 5% waste)
Gravel is the base trench plus the drainage strip behind the wall.
| Wall | Blocks | Caps | Gravel | Drain pipe |
|---|---|---|---|---|
| 10 ft long, 2 ft high | 63 | 10 | 1.1 yd³ | 10 ft |
| 20 ft long, 3 ft high | 189 | 20 | 3.0 yd³ | 20 ft |
| 30 ft long, 3 ft high | 284 | 30 | 4.6 yd³ | 30 ft |
| 40 ft long, 4 ft high | 504 | 40 | 7.6 yd³ | 40 ft |
| 50 ft long, 3 ft high | 473 | 50 | 7.6 yd³ | 50 ft |
Tips before you buy
- Drainage decides whether a wall lasts. Water pressure behind it is the main cause of failure, so use washed stone and a perforated pipe that drains to daylight.
- Bury the first course: about 1 inch for every foot of wall height, and at least 4 inches, on a level compacted base.
- Compact each layer of stone, and the block course, with a plate compactor as you go.
- Stagger the joints like brickwork, and step back (set back) each course if the blocks have a lip.
- Check local rules: height limits, setbacks and permits vary a lot.
Tools & materials for this project
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Frequently asked questions
How many blocks do I need for a retaining wall?
189 blocks of 12 × 4 × 10 inches for a 20 ft long, 3 ft high wall (9 courses), plus 20 cap blocks. A 4 ft high wall of the same length takes 252.
How much gravel do I need behind a retaining wall?
About 2.2 cubic yards of washed drainage stone for a 12 inch strip behind this wall, plus 0.8 cubic yards for the compacted base.
How high can I build a retaining wall without a permit?
It depends on where you live. Many places require a permit and an engineered design above 3 or 4 feet, and for any wall that holds a slope next to a building, a road or a neighbor. Ask your building department.
Do I need drain pipe behind a retaining wall?
Yes. A perforated pipe at the bottom of the drainage stone, sloping to an outlet, lets water out. Water pressure is the main reason walls fail.
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Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.