Your Result Explained: Exact Takeoff Breakdown
A 20-foot-long by 3-foot-high terraced slope wall (60 sq ft of exposed face) sits right at the sweet spot for residential DIY construction: tall enough to turn a unusable slope into a flat lawn or patio, yet just below the 4-foot municipal engineering threshold in most cities. With standard 6" H × 16" W landscape blocks, 36 inches of exposed height equals 6 above-grade courses. Adding the mandatory 1 buried base course gives 7 total courses (42 inches total height).
Along a 20-foot (240-inch) length, each course requires 240 ÷ 16 = 15 blocks. Multiplying 7 courses by 15 blocks equals 105 net wall blocks (111 blocks with 5% cutting waste), plus 15 net top cap blocks (16 caps with 5% waste). For aggregate, the 6-inch leveling trench and 12-inch drainage column behind the wall require 78.75 cubic feet (2.92 cubic yards, or 3.94 tons) of crushed stone.
At 48 lbs per block, 111 wall blocks + 16 caps weigh approximately 5,840 lbs (2.92 tons, or roughly 2 full pallets), plus nearly 4 tons of stone—schedule a single flatbed/dump delivery.
Worked Math: 20 ft × 3 ft Exposed Wall (6"×16" Blocks + 1 Buried Row)
Step-by-step block, cap, and aggregate calculation for a 20-foot segmental retaining wall.
- Exposed Courses = 36 in ÷ 6 in block height = 6 courses.
- Total Courses (+1 Buried Row) = 6 + 1 = 7 courses.
- Blocks per Course = (20 ft × 12) ÷ 16 in block width = 15 blocks/course.
- Total Wall Blocks (+5% Waste) = 7 × 15 × 1.05 = 110.25 → 111 blocks.
- Cap Blocks (+5% Waste) = 15 × 1.05 = 15.75 → 16 cap blocks.
- Base Trench (20' × 1.5' × 0.5') + Backfill (20' × 1.0' × 3.0') × 1.05 = 78.75 cu ft = 2.92 yd³ (3.94 tons).
Bottom Line: Order 111 wall blocks (6"×16"), 16 cap blocks, 1.0 ton of crusher-run leveling base, and 3.0 tons of clean #57 drainage stone.
How This Scenario Is Calculated
Notice how important the buried course is: if you only calculated 60 sq ft × 1.5 blocks/sq ft = 90 blocks, you would be 15 blocks (one entire row) short on construction day!
We split your 2.92 cubic yards of gravel into 0.58 yd³ of compacted leveling base under the trench and 2.33 yd³ of clean #57 drainage stone behind the blocks.
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
Order Two Separate Stone Piles If Possible (Base vs. Drainage)
Use 1 ton of crusher run (with dust fines) for the 6-inch leveling trench under the wall, and 3 tons of washed #57 angular stone (no fines) behind the wall around the 4-inch drain pipe.
Rent a Gas Cut-Off Saw with a 14-Inch Diamond Blade
To maintain a running bond across 7 courses, you will need to cut 3 or 4 blocks in half so vertical seams never align.
Mistake Using small 4"×12" hollow garden blocks for a 3-foot-tall slope wall
Recommended Fix: Lightweight 22-lb garden border blocks lack the mass and depth (only 7"–8" deep) to hold back 3 feet of earth. Always use 48-lb 6"×16"×10" or 75-lb 8"×18"×12" structural SRW blocks for a 3-foot wall.
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
How many 8x18 blocks do I need for a 20 ft by 3 ft wall?
With 8" H × 18" W blocks (1.0 sq ft face area), 36 inches of exposed height takes 5 above-grade courses + 1 buried course = 6 courses of 14 blocks = 84 net blocks (89 blocks with 5% waste).
How wide and deep should I dig the base trench for a 20x3 wall?
For 10-inch-deep blocks, dig a trench 18 to 20 inches wide and 12 inches deep (6 inches for the compacted gravel leveling pad + 6 inches to bury the first block course).
How many tubes of masonry adhesive for 16 cap blocks?
One 10-oz cartridge of polyurethane landscape block adhesive glues roughly 8 to 10 cap blocks, so purchase 2 to 3 tubes (or 1 large 28-oz contractor tube).
Does a 3-foot retaining wall need geogrid?
On flat terrain with stable soil and no driveway load above the wall, a 3-foot exposed wall built with 6"×16" or 8"×18" blocks typically relies on gravity setback alone. If the slope above continues rising steeply or soil is soft clay, adding one layer of geogrid after course 3 adds immense stability.
How much does a 20x3 retaining wall weigh in total?
Between the 127 concrete blocks (~2.9 tons) and the base/backfill gravel (~3.9 tons), the finished wall structure weighs nearly 6.8 tons.