BuildCalc Pro

Retaining Wall Calculator

Calculate blocks, backfill gravel, drain pipe, and total cost for your retaining wall project.

Written & reviewed by Michael FairUpdated June 2026Based on U.S. codes & pricingHow we calculate

Planning and Estimating a Retaining Wall Project

Retaining walls are essential for managing slopes, preventing erosion, and creating level terraces in landscaping. Proper material estimation is critical since retaining wall blocks are heavy (30-80 lbs each) and delivery logistics require accurate quantities.

The most common retaining wall materials are interlocking concrete blocks, natural stone, timber, and poured concrete. Interlocking blocks are the most popular DIY option because they require no mortar and stack easily with built-in lip or pin alignment systems.

Standard retaining wall blocks are approximately 12 inches wide × 4 inches tall × 8 inches deep. At this size, you need about 3 blocks per linear foot per course (row). A 4-foot tall wall has 12 courses and requires 36 blocks per linear foot of wall length.

Backfill material is often overlooked in cost estimates. You need drainage gravel behind the wall (at least 12 inches deep), landscape fabric to prevent soil migration, and a perforated drain pipe at the base. These materials typically add 25-40% to block costs.

Walls over 4 feet tall usually require engineering and permits. Taller walls experience significantly more lateral earth pressure, requiring geogrid reinforcement, deeper bases, and potentially battered (tilted back) designs. Always consult local codes before starting.

How to Use This Calculator

1. Select Imperial or Metric units. 2. Enter wall length and desired height in feet. 3. Choose a block size. 4. Adjust the waste percentage (10% standard). 5. Click "Calculate" for a complete materials list. Tip: For walls over 4 feet, consult a structural engineer and check local building codes for permit requirements.

Understanding Your Result

Your result shows the number of retaining wall blocks, cap blocks, courses (rows), backfill gravel, and base gravel needed for your wall. Block count includes your chosen waste factor for cuts at corners and ends. The estimate assumes standard interlocking blocks — if you are using natural stone or timber, the quantities and pricing will differ.

Key Formulas

Wall Blocks Needed

(Wall Length × Wall Height) ÷ Block Face Area = Number of Blocks

Divide total wall face area by the exposed face area of one block. Add 5-10% for cuts and waste.

Footing Concrete

Footing Length × Footing Width × Footing Depth ÷ 27 = Cubic Yards

Retaining walls over 4 feet tall typically need a concrete footing 2× the wall width and 12" deep minimum.

Backfill Gravel

Wall Length × Drain Zone Width × Wall Height × 0.7 ÷ 27 = Cubic Yards

The drain zone behind the wall is typically 12" wide. The 0.7 factor accounts for gravel settling.

Worked Example

A 20-foot long, 3-foot tall retaining wall using standard blocks (approximately 3 blocks per linear foot per course, 4 inches tall).

  1. 1Number of courses: 36 inches ÷ 4 inches per block = 9 courses.
  2. 2Blocks per course: 20 ft × 3 blocks/ft = 60 blocks.
  3. 3Total blocks: 9 × 60 = 540 blocks; add 10% waste = 594 blocks.
  4. 4Cap blocks (top row): 60 cap units.
  5. 5Backfill gravel (12 inches deep behind wall, 3 ft tall): 20 × 1 × 3 × 0.7 ÷ 27 = 1.6 cu yd. Base gravel (6 inches deep, 18 inches wide): 20 × 1.5 × 0.5 ÷ 27 = 0.6 cu yd. Total gravel ≈ 2.2 cu yd.

Order 594 wall blocks, 60 cap blocks, and about 2.2 cubic yards of drainage gravel, plus 20 feet of 4-inch perforated drain pipe. At $2–$5 per block, block cost alone runs $1,190–$2,970. Gravel at $40–$65 per cubic yard adds roughly $90–$145. Total materials for this wall: approximately $1,400–$3,300 before any concrete footing.

Assumptions Behind This Estimate

  • Standard interlocking concrete blocks approximately 4 inches tall with a face rate of about 3 blocks per linear foot — larger blocks (6 or 8 inches tall) need fewer courses and a different count.
  • A waste factor of 5–10% covers cuts at wall ends and corners; curved walls or stepped walls may need 15%.
  • Backfill gravel is 12 inches wide behind the wall and runs the full wall height; the 0.7 factor accounts for gravel compaction and settling.
  • Walls 4 feet or shorter typically do not require a concrete footing — a compacted gravel base 6 inches deep is sufficient. Taller walls usually need engineered footings.
  • The estimate does not include geogrid reinforcement, which is required for walls over 3–4 feet tall depending on soil conditions and surcharge loads.

Before You Buy: Practical Advice

  • Bury the first course below grade — this locks the base in place and prevents the wall from sliding forward under soil pressure.
  • Compact the gravel base thoroughly with a plate compactor before laying the first course; an uneven or soft base causes the entire wall to lean or settle over time.
  • Install landscape fabric between the backfill gravel and the retained soil to prevent fine soil from migrating into the gravel and clogging the drainage system.
  • Walls over 4 feet tall typically need a building permit and may require engineering drawings — check with your local building department before ordering materials.
  • Place the perforated drain pipe at the base of the wall (holes down) with daylight at one end so water exits rather than pooling behind the wall, which is the leading cause of retaining wall failure.

Frequently Asked Questions

How many blocks do I need for a retaining wall?

Multiply wall length (in feet) by wall height (in feet), then multiply by approximately 9 blocks per square foot for standard-size blocks. A 20-foot long, 3-foot tall wall needs about 540 blocks plus 10% waste (594 blocks total).

How much does a retaining wall cost per foot?

DIY retaining walls cost $10-$25 per square foot of wall face for materials. Professional installation runs $25-$50 per square foot. A 20-foot long, 3-foot tall wall costs $600-$1,500 DIY or $1,500-$3,000 professionally installed.

Do I need a permit for a retaining wall?

Most jurisdictions require permits for retaining walls over 4 feet tall (measured from the bottom of the footing to the top of the wall). Some areas require permits for walls over 3 feet. Walls holding surcharges (driveways, structures) may have stricter requirements.

How deep should the base be for a retaining wall?

Bury the first course 1-2 inches below grade. The gravel base should be 6 inches deep and extend 2-3 inches beyond the front and back of the blocks. Compact the base thoroughly to prevent settling.