Retaining Wall Calculator
Estimate interlocking retaining wall blocks, buried base courses, top cap stones, drainage gravel backfill, and perforated pipe for landscaping and structural walls.
Calculation Results
Total interlocking segmental blocks (including buried foundational row and cut waste).
Protective top cap coping stones to seal the top course.
Tons of clear #57 crushed drainage stone placed 12 inches behind the wall.
Tons of crushed aggregate for the 6-inch deep leveling foundation trench.
Linear feet of 4-inch perforated French drain pipe with fabric sleeve.
How This Calculator Works
Plan your retaining wall installation with these steps:
- Input Wall Length & Visible Height: Measure along the planned slope contour.
- Select Block Type: Standard 16x6 segmental blocks with rear locking lips are standard for DIY walls up to 4 feet.
- Keep Buried Course Checked: Never build a retaining wall on top of bare soil. The first course must be buried in a compacted gravel trench.
- Review Takeoff: The calculator outputs block counts, cap stones, base leveling stone, drainage gravel backfill, and perforated pipe.
Formula & Calculations
Retaining wall calculations must account for subterranean stability, hydraulic pressure relief, and segmental block dimensions (NCMA standards):
1. Buried Base Course (Toe Resistance)
A gravity retaining wall must have its bottom course embedded beneath the ground (minimum 6 inches or 1 full block height for walls up to 4ft tall). This provides critical toe resistance against lateral earth pressure sliding.
Total Wall Height = Visible Height + Buried Course Depth
2. Block Count Formula
Blocks = ceil([Wall Length × Total Height / Block Face Area] × (1 + Waste / 100))
3. Drainage Gravel & Hydrostatic Pressure Relief
Water accumulation behind a wall creates massive hydrostatic pressure, the #1 cause of wall blowouts. Building codes require a continuous 12-inch wide vertical chimney of #57 crushed drainage stone directly behind the wall blocks, coupled with a 4-inch perforated drain pipe routed to daylight.
Step-by-Step Calculation Examples
Backyard Terraced Wall (30ft long x 3.5ft visible height)
Using standard 16x6 blocks with buried bottom course, cap stones, and 12-inch drainage gravel zone.
- wall_length: 30
- wall_height: 3.5
- block_preset: 16x6
- buried_course: true
- include_caps: true
- waste: 10
- total_blocks: 198
- cap_stones: 24
- drainage_gravel: 6.60 ton
- base_trench_gravel: 1.24 ton
- drain_pipe_length: 32 ft
Frequently Asked Questions
Why must the first course of retaining wall blocks be buried?
Embedding the bottom course of blocks 6 inches below ground level anchors the wall against the tremendous horizontal pressure of the retained soil. If you build on ground level, the soil pressure will push the base forward, causing immediate wall failure.
At what height does a retaining wall require an engineer?
In virtually all municipal building codes, any retaining wall exceeding 4 feet in total height (including the buried base course) requires building permits and structural engineering design with geogrid soil reinforcement tiebacks.
How much drainage gravel is needed behind a retaining wall?
Standard masonry building codes require at least a 12-inch wide column of clean, washed aggregate (#57 crushed stone) placed directly behind the rear of the blocks extending from the base pipe up to 6 inches from the top.
What is geogrid and when is it needed?
Geogrid is a high-tensile polyester synthetic mesh laid horizontally between block courses and extending back into the soil slope. It mechanically ties the wall blocks to the retained earth mass, and is generally required for any wall taller than 3 to 4 feet or walls supporting sloped driveways.
Do I need mortar or glue between retaining wall blocks?
Interlocking segmental retaining wall blocks feature a rear lip or fiberglass pin system that dry-stacks without mortar. However, specialized exterior concrete landscape block adhesive should be applied to secure the top cap stones in place.
What is the best foundation base for a retaining wall?
A trench dug 12 inches deep and 18 to 24 inches wide, filled with 6 inches of densely compacted crushed gravel (crusher run or road base) leveled within 1/8 inch. Never set retaining wall blocks on concrete or bare soil.