Retaining Wall Block Calculator
Estimate the wall blocks, cap blocks, drainage gravel, base material and landscape fabric for a segmental retaining wall.
A 30-ft retaining wall 3 feet high built from 16 × 6 inch blocks with one buried course has 7 courses of 23 blocks, so about 138 wall blocks plus 23 cap blocks before waste. Add at least 12 inches of drainage gravel behind the wall.
Your numbers
Result
How to calculate a retaining wall
A segmental retaining wall is built in horizontal rows ("courses") of interlocking blocks on a buried base. Count the rows from the wall height and the block height, then the blocks in each row from the wall length and the block length.
blocks per course = wall length ÷ block length
wall blocks = (courses − 1) × blocks per course × (1 + waste %)
cap blocks = blocks per course (final row)
The last course is a cap block, glued with masonry adhesive, so we subtract one course from the wall-block count and add the caps separately.
Blocks needed (16 × 6 in face, 1 buried course, before waste)
| Wall | Courses | Wall blocks | Cap blocks |
|---|---|---|---|
| 10 ft × 2 ft | 5 | 32 | 8 |
| 20 ft × 2 ft | 5 | 60 | 15 |
| 30 ft × 3 ft | 7 | 138 | 23 |
| 40 ft × 3 ft | 7 | 180 | 30 |
| 50 ft × 4 ft | 9 | 304 | 38 |
| 100 ft × 3 ft | 7 | 450 | 75 |
Drainage and base
- Base: a compacted trench of crushed stone, typically 6 inches deep and about twice the block depth wide, buried so the first course is partly below grade.
- Backfill: at least 12 inches of free-draining gravel behind the wall, with a perforated drain pipe at the bottom.
- Geogrid: reinforcing fabric layers tie taller walls into the slope.
Use the gravel calculator to refine drainage and base quantities with your own widths and densities.
Tiered walls and setback
Instead of one tall wall, two or three shorter walls separated by a level terrace are safer, cheaper and easier to build. As a rule of thumb, set the upper wall back from the lower wall by at least twice the height of the lower wall so that it doesn't load it. Each tier needs its own base, drainage and cap.
When a retaining wall needs an engineer
Walls that hold back a slope next to a building, a driveway, a pool or a property line, walls on soft or wet soil, and any wall taller than your local limit (commonly 3–4 feet) generally need a stamped design. Gravity walls rely on the weight and setback of the blocks; reinforced walls add geogrid layers spaced every one or two courses.
How tall can a DIY wall be?
Many jurisdictions require engineering or a permit above 3–4 feet, and walls that hold soil under a driveway, a building or a slope need a professional design. For a free-standing garden wall (no retained soil), see the concrete block calculator.
Frequently asked questions
How many blocks do I need for a retaining wall?
Multiply the number of courses (height ÷ block height) by blocks per course (length ÷ block length), subtract the cap row, then add waste and caps.
How tall can a DIY retaining wall be?
Many jurisdictions require engineering or a permit above 3–4 feet. Always check local code, and for tall walls hire a pro.
How much gravel goes behind a retaining wall?
A column at least 12 inches wide behind the wall, as tall as the wall, plus a drain pipe at the base to carry water away.
Do I need to bury the first course?
Yes. Bury at least one full course (typically about 6 inches), and more for taller walls, following your block manufacturer's instructions.
How many blocks for a 30-foot wall 3 feet high?
With 16×6 in blocks and one buried course, a 30 ft × 3 ft wall has 7 courses of 23 blocks, so about 138 wall blocks plus 23 caps before waste.
How much does a retaining wall cost?
Block material is only part of the cost; excavation, base, drainage and labor are often more. Get local bids for walls over 3 feet.
Do I need geogrid?
Taller walls (often over 3–4 feet, depending on the system) need geogrid layers tied back into the soil. Follow the block manufacturer's specifications.
Can I use pavers or bricks for a retaining wall?
Only for low, free-standing walls. Retaining walls need heavier interlocking blocks designed for the load.
Sources and further reading
- National Concrete Masonry Association — NCMA: segmental retaining wall resources
- Interlocking Concrete Pavement Institute — ICPI: paver, base and bedding guidance
- Call 811 before you dig — Common Ground Alliance
These references support the general guidance on this page. The calculator applies the standard formulas shown above to the numbers you enter.
Related calculators
. Estimates only. Always confirm quantities with your supplier or a qualified contractor before ordering, and add extra for waste and site conditions.