Helical Pile Capacity Calculator
Convert final installation torque to helical pile capacity, or work backward from a design load to the torque you need, using AC358 Kt factors.
Inputs
Torque correlation per ICC-ES AC358 and IBC 1810.3.3.1.9: ultimate capacity = Kt × final installation torque.
Allowable capacity
| Kt used | |
| Ultimate capacity | |
| Allowable capacity | |
| In kips (allowable) |
Torque correlation is an estimate. Load tests to ASTM D1143 or D3689 verify it. An engineer signs off the design.
How the calculation works
As a helix screws into soil, the torque needed to keep it advancing rises with soil strength. Multiplying the final installation torque by an empirical torque correlation factor (Kt) gives the pile's ultimate axial capacity. The International Building Code accepts a documented torque correlation as one way to set capacity (Section 1810.3.3.1.9) and takes the allowable load as half the ultimate, a factor of safety of 2.
| Shaft | AC358 default Kt (ft⁻¹) | Torque for 10 kips allowable (FS 2) |
|---|---|---|
| 1.5 to 1.75 in. square bar | 10 | 2,000 ft-lb |
| 2-7/8 in. round | 9 | 2,222 ft-lb |
| 3.0 in. round | 8 | 2,500 ft-lb |
| 3-1/2 in. round | 7 | 2,857 ft-lb |
Worked example
Supportworks' published design example puts a 30 kip working load on a 2-7/8 in. pile: 30,000 × 2 = 60,000 lb ultimate, and 60,000 ÷ 9 = 6,667 ft-lb, specified as 6,700 ft-lb minimum. Choose "Torque needed for a load" above to reproduce it.
Limits
- The shaft's rated installation torque caps what you can prove by torque. CHANCE's 2018 data sheet rates the RS2875.203 at 7,000 ft-lb (63 kips ultimate); the May 2026 ESR-2794 lists 6,710 ft-lb for its lead section, so always use the current report.
- AC358 defaults apply to conforming systems (true helix plates 8 to 14 in., 3 in. pitch, helices spaced 2.4 to 3.6 diameters apart).
- Torque correlation does not cover lateral load, buckling in very soft soil or group effects.
Full explanation and a printable chart: helical pile torque to capacity chart.
Frequently asked questions
How do you calculate helical pile capacity from torque?
Multiply final installation torque (ft-lb) by the torque correlation factor Kt (ft⁻¹) to get ultimate capacity in pounds, then divide by the factor of safety, usually 2, for the allowable load.
What Kt should I use?
AC358 defaults are 10 for 1.5 to 1.75 in. square bar, 9 for 2-7/8 in., 8 for 3.0 in. and 7 for 3-1/2 in. round shafts. A manufacturer ESR or project load test can justify a different value.
Why is the factor of safety 2?
IBC Section 1810.3.3.1.9 sets the allowable axial load at half the ultimate capacity. Some evaluation reports require 2.5 when there is no geotechnical report.
Can torque prove any capacity I want?
No. You cannot install past the shaft rated torque, so the shaft rating caps the capacity torque can prove. Bigger loads need a larger shaft or more piles.
Sources
- ICC-ES AC358 acceptance criteria
- IBC helical pile provisions (UpCodes)
- Hubbell: Origin of the Kt factor
- Supportworks: Helical pile design examples
- CHANCE RS2875.203 data sheet
- ICC-ES ESR-2794 (CHANCE)
Related guides and tools
Last reviewed October 2026 by the Helical Pile USA editorial team. Estimates are for planning only; a licensed engineer or installer sets the final design. Found an error? Tell us.
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