Retaining Wall Block Calculator

About the Retaining Wall Block Calculator

Segmental retaining walls use interlocking concrete blocks to hold back soil on slopes, create terraces, and define landscape features. These gravity walls rely on their mass and the interlocking system to resist the pressure of the retained soil.

This calculator estimates the number of retaining wall blocks and cap stones needed for your project based on wall length, wall height, and block face dimensions. Standard landscape blocks (like Allan Block, Versa-Lok, or similar) typically have a face area of about 1.17 square feet (approximately 18"×12" face for a standard large block).

Retaining walls over 4 feet tall generally require engineering design and may need geogrid reinforcement. Always check local building codes and consider soil conditions before building.

When This Page Helps

Retaining wall blocks are sold by the piece and are heavy (30–80+ lbs each). Accurate count prevents multiple delivery trips and ensures you have enough matching blocks from the same production lot for consistent color.

How to Use the Inputs

1. Enter the total wall length in feet.
2. Enter the exposed wall height in feet.
3. Select the block face size or enter custom dimensions.
4. Review the block count for face blocks and cap stones.
5. Add 5–10% waste for cutting at corners and ends.

Example Calculation

Tips & Best Practices

• Excavate a level trench 6" deep and wider than the block by 8–12" for the base course.
• Use compacted gravel (road base) for the first 6" of the trench — blocks should never sit on bare soil.
• Set the first course partially below grade for stability; bury at least one full block height.
• Backfill behind the wall with 12" of 3/4" crushed stone for drainage.
• Install perforated drain pipe at the base of the wall to channel water away.
• Walls over 4 feet typically need engineering and geogrid soil reinforcement layers.

Types of Retaining Wall Blocks

Gravity blocks use mass alone to resist soil pressure. Segmental blocks have lips or pins that interlock courses for added stability. Hollow-core blocks can be filled with gravel or concrete for extra weight. Large-face blocks cover more area with fewer courses.

Geogrid Reinforcement

For walls over 3–4 feet, geogrid layers extend back into the retained soil to increase the effective mass of the wall system. Geogrid is placed between block courses at specified intervals (typically every 2–3 courses). The length of geogrid behind the wall should be at least 60% of the wall height.

Cost Considerations

Standard retaining wall blocks cost $3–$8 each. Cap stones cost $5–$12 each. Base gravel and drain rock add $200–$500 for a typical wall. Installed cost is $20–$35 per face square foot for walls under 4 feet.

Frequently Asked Questions

• How tall can a retaining wall be without an engineer?

  Most building codes allow gravity retaining walls up to 4 feet (exposed height) without an engineer's design. Walls taller than 4 feet, or walls supporting structures, driveways, or slopes, typically require engineering and a permit.

• What is the setback on retaining wall blocks?

  Most segmental blocks have a built-in setback (batter) of 1/2" to 1-1/4" per course. This leans the wall back into the retained soil, significantly increasing its resistance to overturning.

• Do I need drainage behind a retaining wall?

  Yes, always. Water pressure (hydrostatic pressure) is the number one cause of retaining wall failure. Install 12" of crushed stone behind the wall and a perforated drain pipe at the base to channel water away.

• What base material should I use?

  Use 6" of compacted 3/4" minus crushed stone (road base) for the leveling pad. This provides a stable, well-drained foundation. Never use sand alone — it can wash out over time.

• How are cap stones attached?

  Cap stones are glued to the top course with construction adhesive (like PL Premium or manufacturer-recommended adhesive). Apply two beads of adhesive to the top of the wall blocks before setting the caps.

• How much does a retaining wall block weigh?

  Standard landscape retaining wall blocks weigh 30–80 lbs each. Larger commercial blocks can weigh 80–125 lbs. A typical 3-foot-high, 30-foot-long wall uses about 80–100 blocks weighing 3,000–5,000 lbs total.