Boulder Wall Calculator
A boulder retaining wall is priced by its face area (length × height). Enter the wall's dimensions to estimate the tonnage of boulders, the compacted gravel base beneath it and a typical installed cost. Open Advanced options to set boulder depth, density and your local price.
Your measurements
Face height above grade. Walls over ~3–4 ft often need an engineer.
Face area
90 ft²
Boulders
7 tons
≈ 5 yd³ · ~1.4 t/yd³
Gravel base
1.7 tons
2 ft × 6 in footing
Estimated cost
- Installed
- $3,600.00($40.00/ft²)
- Typical range
- $2,250.00 – $5,400.00($25–$60/ft²)
Estimates only. Verify against your supplier's coverage figures before ordering.
Footprint
Footprint to scale
The formula
Face area = Length × Height · Boulder tons = (Face area × depth ÷ 27) × density
Example: A 30 ft × 3 ft wall = 90 face ft². At ~1.5 ft average boulder depth that's about 5 yd³ ≈ 7 tons of boulders, and roughly $2,500–$5,400 installed.
How it works
- 1Find the wall's face area: length × exposed height, in feet.
- 2Multiply by the average boulder depth and divide by 27 to get cubic yards of rock.
- 3Multiply cubic yards by the boulder density (~1.3–1.5 tons/yd³) to order boulders by the ton.
- 4Add a compacted gravel/crushed-stone footing the full length, set below grade, plus drainage stone behind the wall.
- 5Estimate installed cost from the face area at roughly $25–$60 per square foot.
Frequently asked questions
How many boulders do I need for a retaining wall?
Boulders are bought by the ton, not the piece, because sizes vary. Estimate the rock volume (face area × average depth ÷ 27 for cubic yards) and multiply by about 1.4 tons per cubic yard. A landscape yard can convert tonnage into a rough piece count for the boulder size you choose.
How much does a boulder retaining wall cost?
Plan on roughly $25–$60 per face square foot installed, driven by boulder size, delivery distance and machine time. A 30 ft × 3 ft wall (90 ft²) runs about $2,500–$5,400 installed; DIY material-only is less, but you'll need equipment to set large stones.
Does a boulder wall need a gravel base?
Yes. Set the first course on a compacted crushed-stone base below grade so the wall doesn't settle, and pack drainage stone behind it so water escapes instead of pushing the boulders over.