Our Retaining Wall Block Calculator tells you exactly how many blocks you need per course, total courses, and overall block count for your retaining wall project — enter your wall length, height and block size.
Retaining Wall Block Calculator
Retaining Wall Block Formula
Total Blocks = Blocks per Course × Number of Courses × (1 + Waste %)
Blocks per course = wall length ÷ block length. Number of courses = wall height ÷ block height. Add 5-10% waste for cuts at corners, curves and breakage.
What’s Underneath a Retaining Wall?
Every retaining wall needs a compacted gravel base trench (typically 6-12 inches deep, depending on wall height and frost depth) below the first course, plus a gravel backfill zone (usually 12 inches deep) behind the wall for drainage. Skipping proper drainage is the #1 cause of retaining wall failure.
Recommended Base Depth by Wall Height
| Wall Height | Base Depth | Embedment (Buried Courses) |
|---|---|---|
| Up to 2 ft | 4-6 in | None to 1 course |
| 2-4 ft | 6-8 in | 1 course |
| Over 4 ft | 8-12 in (engineer review recommended) | 1-2 courses + geogrid |
Tips for Building a Retaining Wall
- Check local codes — walls over 3-4 feet often legally require an engineer’s design and a building permit.
- Step the wall back slightly with each course (batter) for walls over 2 feet — this adds stability against soil pressure.
- Install drainage pipe at the base behind the wall to relieve hydrostatic pressure that causes walls to bulge or fail over time.
- Use geogrid reinforcement tied into the backfill for taller walls — it dramatically increases load capacity.
FAQs
How many blocks do I need for a 20 ft retaining wall?
It depends on block size and wall height — for an 18-inch block at 3 ft (6 courses) height, a 20 ft wall needs roughly 80 blocks. Use the calculator above with your exact dimensions.
Do I need a permit for a retaining wall?
Many jurisdictions require a permit for walls over 3-4 feet tall, or any wall holding back a slope near a structure. Check your local building department before starting.
How deep should the base trench be?
4-6 inches for short walls (under 2 ft), 6-8 inches for medium walls (2-4 ft), and 8-12+ inches with possible engineering review for taller walls.
What happens if I skip drainage behind the wall?
Water builds up hydrostatic pressure behind the wall, which is the most common cause of bulging, leaning and eventual wall failure — always include gravel backfill and a drain pipe or weep holes.