Retaining Wall Material & French Drain Calculator
Calculate total modular retaining wall blocks, cap stones, compacted base leveling gravel, 3/4" drainage gravel backfill, perforated French drain pipe, and filter fabric.
11. Wall Dimensions
The base course must be buried for structural stability (minimum 6 inches).
22. Block Dimensions & Accessories
Complete Material Bill of Quantities
Base Foundation & French Drain Materials
Drainage Defense: Why Retaining Walls Fail
Over 90% of retaining wall structural failures are caused by hydrostatic water pressure behind the wall, not weak blocks. Installing a 12-inch continuous column of 3/4" angular washed drainage stone wrapped in non-woven landscape filter fabric with a 4" perforated drain pipe allows water to evacuate immediately.
How it works
1
Enter your planned wall length and exposed finished height in feet.
2
Specify the buried base embedment depth (minimum 6 inches for structural toe lock).
3
Select your modular block style (standard garden 12x4", medium hardscape 16x6", or commercial 18x8").
4
Choose whether to include finished top cap stones and set your cutting waste percentage (10% recommended).
5
Review your comprehensive material takeoff: block units, cap stones, base crushed run gravel, washed drainage stone in cubic yards and tons, 4" pipe, and fabric.
Frequently asked questions
How many retaining wall blocks do I need per square foot?
Block count depends on face dimensions: standard 12" × 4" garden blocks require 3.0 blocks per square foot; 16" × 6" landscaping blocks require 1.5 blocks per square foot; large 18" × 8" commercial blocks require 1.0 block per square foot.
Why must the bottom course of blocks be buried?
The buried base course (toe embedment) locks the wall into the earth and prevents soil from sliding the base forward under lateral earth pressure. For walls up to 4 feet tall, standard practice requires burying at least 6 inches on top of a compacted 6-inch gravel base pad.
When does a retaining wall require building permits and an engineer?
In nearly all US building jurisdictions, any retaining wall with an exposed height of 4 feet or higher (measured from the bottom of the footing to the top of the wall) requires a municipal building permit and engineering drawings specifying geogrid reinforcement layers.
Why is gravel backfill essential behind a retaining wall?
Water-saturated clay or soil expands and creates enormous hydrostatic pressure. A continuous 12-inch column of 3/4" clean angular washed aggregate directly behind the wall relieves this hydrostatic force, directing water downward into the perforated collector pipe.
What role does geotextile filter fabric play in retaining walls?
Non-woven geotextile fabric lines the trench behind the wall, acting as a permanent physical separation barrier: it allows water to filter freely into the gravel drainage column while preventing fine dirt and silt particles from clogging the drainage rock and piping over time.
Is this tool free?
Yes. The site is supported by ads, so you never pay or need to create an account.
Does it work on mobile?
Yes. The page adapts to your screen and does not require installing any app.