📍 Pile Capacity

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

How a deep foundation pile carries load through a combination of side friction and end bearing.

When soil near the surface is too weak or compressible to support a footing directly, piles carry the load down to stronger soil or rock — through two mechanisms working together. Skin friction is the shear resistance mobilized along the pile's entire embedded surface; end (tip) bearing is the direct bearing resistance at the pile's base, similar in concept to a very deep, narrow footing.

The relative contribution of each depends heavily on soil type and pile length — in soft clay, skin friction often dominates; for a pile driven to bedrock, tip bearing can dominate instead.

Key formula

Skin friction: Qs = (perimeter × L) × f_s
End bearing: Qp = A_tip × q_p
Ultimate capacity Qu = Qs + Qp
Allowable capacity = Qu / FS  (commonly FS = 2 to 3)

Variables

f_s

unit skin friction (soil- and pile-material-dependent)

q_p

unit end bearing pressure at the pile tip

L

embedded pile length

Notes & limitations

• Real pile capacity design uses soil-specific methods (alpha method for clay, beta method for sand, or direct correlation with CPT/SPT data) to estimate f_s and q_p. This calculator takes f_s and q_p as inputs you supply rather than deriving them from soil parameters, and the placeholder defaults are not a substitute for a geotechnical report.
• Pile capacity is normally confirmed in the field by load testing or driving-resistance formulas, since soil variability makes purely analytical capacity estimates uncertain for anything beyond preliminary sizing.