Estimating retaining wall materials means calculating how much block, gravel, capstone, geogrid, and drainage components your project requires. Getting this wrong in St. Louis can cost you literally. Between our expansive clay soil, heavy rainfall, and elevation changes, precision matters more than in most regions.
In this guide, we’ll help you calculate material quantities correctly for St. Louis conditions. You’ll learn how slope, wall type, soil movement, and code requirements affect your calculations and how working with a local supplier can prevent costly errors.
Smart Planning Tips for Missouri Retaining Walls
- Wall height, length, and slope drastically affect material needs. Shorter walls use fewer blocks and less fill, while taller walls need reinforcement.
- Missouri’s clay-rich soil demands additional backfill, geo-fabric, and drainage. This helps prevent wall shifting and soil saturation.
- Over-ordering costs money, but under-ordering delays your build. You risk inconsistent color batches or emergency reorders.
- Local suppliers offer accurate takeoffs based on regional terrain. Their advice accounts for soil swelling, slope, and drainage patterns.
Each of these tips can save you time, money, and unnecessary headaches during your project.
Why Accurate Material Estimation Matters in Missouri
What risks come with poor material planning?
Underestimating or overestimating materials can derail your project.
- Underordering causes project delays and inconsistent block batches
- Overordering ties up your budget and leads to excess waste
- Last-minute orders cost more due to rushed delivery or supply mismatches
These setbacks increase your overall project costs and timeline.
Why St. Louis soil conditions demand precise planning
St. Louis sits on highly expansive clay soils. According to the USDA NRCS Soil Survey, these soils swell significantly when wet placing intense lateral pressure on walls.
Rainfall averages 41 inches annually. Combined with freeze-thaw cycles, this causes:
- Soil expansion and contraction, which shifts your wall over time
- Drainage issues behind walls if backfill is too dense
- Erosion of backfill if the wrong gravel or fabric is used
Key Factors That Affect Material Quantities
How do wall height and length affect material needs?
Longer and taller walls need exponentially more materials.
- One standard 6″x16″ block covers ~1.33 ft² of wall face
- For a 3 ft tall x 40 ft long wall, you’ll need ~90–100 blocks, plus caps and backfill
- Add 5–10% for waste, cuts, or future repairs
Precise measurements help reduce surprises during installation.
How does your slope or setback influence materials?
Sloped yards or step-up walls require more caps, geo-grid, and fill.
- Steeper grades often mean higher backfill depth
- Walls built in stepped sections may need more interlocking components
These changes must be accounted for in your initial material list.
What’s the difference between gravity and reinforced walls?
Reinforced walls require more layered materials.
- Gravity walls: heavier blocks, deep footer, compacted base
- Reinforced walls: lighter block but need geo-grid, drainpipe, and extra base depth
Knowing your wall type guides how much base, gravel, and support you’ll need.
Material Types You Need to Estimate
Retaining wall blocks
The backbone of your wall. Choose blocks rated for freeze-thaw conditions.
- Most St. Louis walls use modular concrete blocks (6×16 or 8×12 inches)
- Order 5–10% more than calculated for breakage and odd cuts
This ensures uniformity and prevents delays if some blocks are damaged.
Capstones
For finishing the top of the wall. Typically match block length.
- Needed for visual finish and structural sealing
- Mitered cuts for corners require extra quantity
Don’t overlook these they’re both functional and aesthetic.
Gravel and base fill
Used for leveling, drainage, and load-bearing.
- Base: 4″–6″ crushed rock compacted under the wall
- Backfill: 12″ layer of #57 gravel behind wall
- Rule of thumb: For 50 ft wall, you may need ~1.5–2 tons of gravel
Gravel choice impacts drainage performance, especially on clay soils.
Geogrid and filter fabric
Prevents wall movement and soil intrusion.
- Required for walls over 3–4 ft
- Installed every 2 courses, extending back into slope
- Use landscape fabric to separate gravel and native clay
Skipping these components can lead to long-term wall failure.
Missouri-Specific Considerations
Does Missouri require engineered plans for retaining walls?
Yes. In St. Louis County, any wall over 4 feet or near structures/driveways needs:
- Engineered drawing
- Permit with city or county
- IBC-compliant design stamped by a Missouri PE (Professional Engineer)
See St. Louis County Residential Code Amendments for specifics.
Can local suppliers help with takeoffs?
Yes. Many Missouri-based vendors do custom takeoffs using your site layout.
- Adjust calculations for soil type, rain impact, and drainage
- Prevent overbuying or delays due to wrong quantities
- Some offer bundled materials and delivery schedules
We recommend working with trusted pros like retaining wall contractor st louis in St. Louis, who specialize in estimating and sourcing walls for local terrain.
Tools and Resources to Make Estimating Easier
What tools or calculators can I use?
You can estimate block and gravel needs using:
- Allan Block Wall Estimator
- Unilock Wall Calculator
- Simple spreadsheet formulas for base and cap totals
Each tool helps provide a solid estimate but still needs to be adjusted for soil and slope.
Can I do this myself or should I get help?
Yes and no. If your wall is under 2 ft and on flat ground, you can probably DIY.
But if you have:
- Sloped yard
- Clay-heavy soil
- Walls over 3 ft
…it’s best to get professional guidance.
Plan with Precision, Build with Confidence
Proper material estimation is the first step in building a long-lasting retaining wall. Missouri’s soils are unforgiving. Drainage, pressure, and freeze cycles can wreck even good designs if the materials aren’t sized and matched properly.
If you’re unsure where to begin, talk to a supplier that truly understands our region. At Retaining Wall & Paving Solutions, we combine estimating, sourcing, and full-service retaining wall installation in St. Louis designed specifically for local conditions. Let us help you plan smarter. See how we estimate and deliver better wall systems for Missouri yards.
Resources
- USDA NRCS Soil Survey Missouri
- https://stlouiscountymo.gov/st-louis-county-departments/transportation-and-public-works/codes-and-ordinances/
- https://allanblock.com/Tools/Tools.html
- https://contractor.unilock.com/wall-calculator/