Your Result Explained: Exact Takeoff Breakdown
A 40-foot-long wall holding back 4 feet (48 inches) of vertical grade (160 sq ft of exposed face) is a major structural earth-retention project. At this scale, contractors use commercial-grade 8" H × 18" W × 12" D segmental retaining wall (SRW) units, where each block covers exactly 1.0 square foot of face area and weighs ~75 lbs. Forty-eight inches of exposed height equals 6 above-grade courses; adding 1 buried base course gives 7 total courses (56 inches total wall height).
Along a 40-foot (480-inch) span, each course takes 480 ÷ 18 = 26.67 → 27 blocks per row. Seven courses × 27 blocks = 189 net wall blocks; with a 10% allowance for step-downs and cuts, order 208 wall blocks (or 199 with 5% waste; 208 at 10% waste) plus 30 cap blocks. The 6-inch leveling pad (20" wide trench) and 12-inch drainage zone behind the 40×4 wall require 235.13 cubic feet—equal to 8.71 cubic yards or 11.76 short tons of crushed stone.
Notice the structural warning: because total height (including the buried course) is 4.67 feet, this wall will almost certainly require 1 to 2 layers of biaxial geogrid soil reinforcement and a local building permit check.
Worked Math: 40 ft × 4 ft Structural Wall (8"×18" Blocks + 10% Margin)
Calculating courses, 8"×18" SRW units, cap units, and aggregate tonnage for a 160 sq ft exposed retaining wall.
- Exposed Courses = 48 in ÷ 8 in block height = 6 courses.
- Total Courses (+1 Buried Course) = 6 + 1 = 7 courses (4.67 ft total height).
- Blocks per Course = Ceiling((40 ft × 12) ÷ 18 in) = Ceiling(26.67) = 27 blocks/row.
- Net Wall Blocks = 7 courses × 27 blocks = 189 blocks.
- With 10% Waste = Ceiling(189 × 1.10) = 208 wall blocks + 30 cap blocks.
- Base Trench + 12" Drainage Backfill (+10%) = 235.13 cu ft = 8.71 yd³ (11.76 tons).
Bottom Line: Order 208 structural 8"×18" SRW blocks, 30 cap blocks, 11.76 tons (8.71 yd³) of crushed stone, 40 ft of 4" perforated drain pipe, and biaxial geogrid.
How This Scenario Is Calculated
If you build this same 40×4 ft wall with medium 6"×16" blocks (1.5 blocks/sq ft) instead of 8"×18" blocks, you will need 9 courses (8 exposed + 1 buried) of 30 blocks = 270 net blocks (297 blocks with 10% waste).
Hollow-core 8"×18" SRW blocks also require clean #57 stone to fill the hollow cores as each course is stacked—our 10% aggregate overage helps cover core-fill stone.
Blocks = Ceiling([(Ceiling(Height ÷ BlockH) + BuriedRow) × Ceiling(Length ÷ BlockW)] × (1 + Waste%)) Gravel Cu Ft = [Length × TrenchW × BaseDepth + Length × BackfillW × ExposedHeight] × (1 + Waste%) Project-Specific Tips & Common Mistakes
Install Biaxial Geogrid Reinforcement into the Backfill Soil Zone
For a 4-foot wall, excavate the soil bank back 3 to 4 feet behind the blocks, lay biaxial geogrid between courses 2 and 3 (and courses 4 and 5) extending back into the compacted backfill zone, and compact in 6-inch lifts.
Rent a Walk-Behind Trench Compactor ("Jumping Jack" or Narrow Plate)
A 40-foot trench requires compacting nearly 12 tons of aggregate and backfill soil; mechanical compaction in lifts is mandatory to prevent settlement.
Mistake Driving heavy equipment within 3 feet of the top of the wall during backfill
Recommended Fix: Keep skid steers and dump trucks at least 3 to 4 feet back from the wall face; use only walk-behind plate compactors directly behind the blocks so you do not push the upper courses forward.
Reference Coverage Table (#57 Crushed Stone (3/4" Clean Angular Limestone/Granite))
Compare how coverage changes across common depths for #57 Crushed Stone (3/4" Clean Angular Limestone/Granite) (default planning density 2700 lbs/yd³):
| Depth | 1 Cubic Yard Covers | 1 Ton Covers | Yards / 100 sq ft | Tons / 100 sq ft | Bags (0.5 ft³) / 100 sq ft |
|---|---|---|---|---|---|
| 4 in | 81 sq ft | 60 sq ft | 1.23 yd³ | 1.66 tons | 67 bags |
| 6 in Active | 54 sq ft | 40 sq ft | 1.85 yd³ | 2.5 tons | 100 bags |
| 8 in | 40.5 sq ft | 30 sq ft | 2.47 yd³ | 3.33 tons | 134 bags |
| 12 in | 27 sq ft | 20 sq ft | 3.7 yd³ | 5 tons | 200 bags |
Reference Source: FHWA Aggregates Reference & ASTM C29 bulk density standard (95–105 lbs/ft³).
Frequently Asked Questions
Does a 4-foot-tall retaining wall require a building permit?
In most US cities and counties (following the International Building Code), any retaining wall over 4.0 feet measured from the bottom of the footing to the top of the wall requires a permit and engineered geogrid plan.
How many pallets is 208 blocks of 8x18 retaining wall block?
Large 8"×18"×12" blocks weigh ~75 lbs each and are stacked 32 to 36 blocks per pallet, so 208 blocks equals roughly 6 pallets weighing ~15,600 lbs (7.8 tons).
How much gravel goes in the base trench vs. behind a 40x4 wall?
Of the 8.71 total cubic yards (11.76 tons), about 1.35 cubic yards (1.9 tons) goes into the 6-inch compacted crusher-run leveling trench, and 7.36 cubic yards (9.9 tons) of washed #57 stone goes into the 12-inch drainage zone and block cores.
How far back do you have to dig for geogrid on a 4-foot wall?
Standard NCMA residential charts typically specify a geogrid embedment length of 70% to 80% of the total wall height—roughly 3.5 to 4.0 feet measured from the front face of the wall.
Where should the 4-inch drain pipe exit on a 40-foot wall?
Pitch the 4-inch perforated drain pipe downward at 1%–2% so water exits to daylight around both ends of the wall (or through a solid PVC outlet pipe every 30 to 40 feet).