Pile Capacity Calculator helps you estimate the axial load capacity of a single pile based on soil properties and pile dimensions. It supports both cohesive (clay) and cohesionless (sand) soil methods.
About unit systems. This tool works in a single SI state internally (meters, kN, kPa) and allows toggling between metric (m, kN, kPa) and imperial (ft, kip, ksf) units. When you switch the unit system, all values are converted from the SI state for consistent results without rounding errors. Results are shown in the primary unit system you select, with secondary values displayed for reference.
About clay (α-method). The α-method is used for cohesive soils such as clay, where the skin friction is calculated using the adhesion factor (α) and undrained shear strength (cᵤ). Typical values for α range from 0.3 to 1.0 depending on soil consistency. This method assumes that the pile develops most of its resistance from adhesion along the shaft.
About sand (β & Nq-method). For cohesionless soils like sand, shaft resistance is estimated using the β-method (which relates to effective vertical stress and soil friction angle φ), and end bearing is calculated using the Nq bearing capacity factor. The friction angle φ is crucial for both β and Nq values, and typically ranges from 25° to 40° for sands.
About factor of safety (FS). The factor of safety accounts for uncertainties in soil properties, construction quality, and design assumptions. The tool divides the ultimate pile capacity by FS to produce the allowable design load. Typical FS values range from 2.0 to 3.0 depending on codes and reliability requirements.
Calculation. The tool computes both shaft resistance (Qs) and end bearing (Qp), then sums them for ultimate capacity (Qult), and divides by the factor of safety to give the allowable capacity (Qall).
Formulas:
- Clay (α-method): Qs = α · cu · perimeter · L (shaft resistance); Qp = 9 · cu · Ap (end bearing)
- Sand (β & Nq-method): Qs = β · σavg · perimeter · L (shaft resistance); Qp = Nq · σtip · Ap (end bearing)
Where:
- perimeter = π · D (pile perimeter)
- area Ap = πD² / 4 (pile tip cross-sectional area)
- σavg = γ′ · (Df + L/2) (average vertical effective stress)
- σtip = γ′ · (Df + L) (tip vertical effective stress)
Examples:
- Example 1 — Clay: D = 0.6 m, L = 12 m, α = 0.7, cᵤ = 60 kPa ⇒ Qs ≈ 950 kN, Qp ≈ 1500 kN, Qall ≈ 980 kN (FS = 2.5)
- Example 2 — Sand: D = 0.6 m, L = 12 m, β = 0.25, φ = 30°, γ′ = 9 kN/m³ ⇒ Qall ≈ 1100 kN (FS = 2.5)
- Example 3 — Changing FS from 2.5 to 2.0 increases allowable capacity by ≈ 25%
Corresponding tools. To size concrete piles, use the Concrete Volume Calculator. To calculate soil bearing pressure under footings, use the Soil Bearing Capacity Calculator.
