18-Inch Sonotube & Pier Footing Calculator
Verified volume tables, ready-mix cubic yardage, 80 lb and 60 lb bag counts, vertical rebar cage specifications, and allowable soil bearing loads for an 18-inch round cardboard form tube.
Interactive Round Pier & Hole Calculator
Use our interactive estimator to calculate custom hole diameters and post volume displacement.
Net concrete volume based on entered dimensions. Base calculation excludes waste/order allowance.
Volume Breakdown Per Hole
Calculation Steps
For 1 hole measuring 10 in diameter × 36 in depth with a 4 in square post buried 36 in:
- Normalize Hole Dimensions to Canonical Base Units (Meters)
Convert hole diameter and depth to meters using exact NIST international conversion factors.
Diameter: 10 in = 0.254 m | Depth: 36 in = 0.9144 m→ Radius: 0.127 m | Depth: 0.9144 m - Calculate Cylindrical Hole Volume Per Hole
Calculate volume of the circular cylinder using π × r² × depth.
π × (0.127)² × 0.9144→ 0.046333331888675194 m³ - Calculate Square Post Displacement Volume Per Hole
Calculate displacement of the buried square post using width² × buriedDepth.
(0.1016)² × 0.9144→ 0.009438948863999999 m³ - Calculate Net Concrete Volume Per Hole
Subtract buried post displacement from cylindrical hole volume.
0.046333331888675194 - 0.009438948863999999→ 0.036894383024675195 m³ - Multiply by Number of Holes
Multiply per-hole net concrete volume by the total quantity of holes.
0.036894383024675195 × 1→ 0.036894383024675195 m³ - Convert Total Volume to Common Construction Units
Convert canonical cubic meters to cubic feet and cubic yards.
0.036894383024675195 m³ = 1.3029128404113506 cu ft = 0.04825603112634632 cu yd→ 0.036894383024675195 m³ | 1.3029128404113506 cu ft | 0.04825603112634632 cu yd
Concrete Volume & Bag Requirements by Frost Depth
Local building codes specify footing depth based on the regional frost line to prevent frost heave. The table below lists exact volume, recommended volume (+10% safety buffer for rough excavation), and bag requirements for a single 18-inch tube:
| Depth | Volume (cu ft) | Cubic Yards (+10%) | 80 lb Bags | 60 lb Bags | 50 lb Bags | Cured Weight |
|---|---|---|---|---|---|---|
| 24″ (2 ft) | 3.89 cu ft | 0.14 yd³ | 7 | 9 | 11 | ~564 lbs |
| 36″ (3 ft) | 5.83 cu ft | 0.22 yd³ | 10 | 13 | 16 | ~846 lbs |
| 42″ (3.5 ft) | 6.8 cu ft | 0.25 yd³ | 12 | 16 | 19 | ~987 lbs |
| 48″ (4 ft) | 7.78 cu ft | 0.29 yd³ | 13 | 18 | 21 | ~1127 lbs |
| 60″ (5 ft) | 9.72 cu ft | 0.36 yd³ | 17 | 22 | 26 | ~1409 lbs |
Standard Uses for an 18-Inch Sonotube & Pier Footing
- High-capacity commercial building columns and pole structures
- Highway and municipal overhead sign foundations
- Heavy manufacturing equipment bases and vibration dampener piers
- Subgrade caisson tops and retaining wall anchor piers
Reinforcement Schedule & Tie Spacing
Pier footings supporting elevated structures experience bending, shear, and uplift forces that unreinforced concrete cannot resist:
Maintain 1.5 inches of clear concrete cover inside cardboard form tubes, or 3 inches if poured against unformed native earth (per ACI 318).
Allowable Load-Bearing Capacity by Soil Type
The total vertical weight an 18-inch cylindrical pier footing can safely support depends on the contact surface area (1.767 sq ft) and the underlying soil bearing capacity:
| Subgrade Soil Classification | Estimated Bearing Pressure | Safe Allowable Vertical Load |
|---|---|---|
| Soft Clay / Silt | 1,500 psf | 2,651 lbs |
| Medium Stiff Clay / Loose Sand | 2,000 psf | 3,534 lbs |
| Dense Sand & Gravel / Hardpan | 3,000 psf | 5,301 lbs |
Note: Safe loads are based on standard dead + live design loads. Consult local building officials or a licensed geotechnical engineer for authoritative site soil tests.
Frequently Asked Questions: 18-Inch Sonotube & Pier Footing
How much concrete does an 18-inch Sonotube hold?
An 18-inch Sonotube holds approximately 1.77 cubic feet of concrete per linear foot of depth. At a standard 4-foot (48-inch) frost depth, a single tube requires 0.29 cubic yards of concrete (with a 10% safety margin), which equals 13 bags of 80 lb concrete.
How many bags of concrete for an 18-inch Sonotube at 3 feet?
At a depth of 3 feet (36 inches), an 18-inch Sonotube needs 10 bags of 80 lb concrete or 13 bags of 60 lb concrete including a 10% waste buffer.
Does an 18-inch pier footing require rebar?
Yes. Standard building codes generally require vertical reinforcement (at least 6 #4 bars) tied with lateral hoops to resist frost heave uplift and horizontal seismic/wind shear forces on elevated posts.