Static formulaQu = Qs + QpFree

Pile Load
Capacity

Ultimate and allowable axial capacity of a single circular pile from shaft friction plus end bearing, using the static capacity equation.

Who it's for: geotechnical and foundation engineers sizing a single driven or bored pile. You'll need soil-report values for unit skin friction and end bearing — this is a design-level tool, not a preliminary estimate.

Qu = fs·(πD·L) + qp·(πD²/4)

Shaft resistance Qs = average unit skin friction × shaft surface area; base resistance Qp = unit end bearing × tip area. Allowable load Qa = Qu / FS (typically 2.5–3.0). Enter the design unit friction and end bearing from your soil report (α/β or N-value methods).

Pile Capacity

How to use this calculator

Enter the pile diameter

The outer diameter of the pile shaft in metres, from the pile schedule or design drawings.

Enter the embedded length

The depth from the pile cut-off level to the pile tip in metres. Use the net embedded length, not total pile length.

Enter average unit skin friction fs

The average unit shaft resistance along the embedded length in kPa, from your soil report. Use the α-method for cohesive soils or the β-method for cohesionless or drained conditions.

Enter unit end bearing qp

The unit resistance at the pile tip in kPa, calculated from bearing capacity factors or correlated from CPT tip resistance (qc) or SPT N-values.

Set the factor of safety

Typically 2.5–3.0 for a static design without a pile load test. A maintained load test to failure can justify reducing this to 2.0.

Read the allowable load Qa

The result shows Qa in kN — the axial load the pile may carry. Compare against the column or structural load; deduct pile self-weight for large-diameter piles.

The formula

Single-pile capacity combines two sources of resistance: shaft friction distributed along the embedded length and end bearing concentrated at the pile tip. The allowable load is then the ultimate capacity divided by a factor of safety.

Qu = Qs + Qp = f_s·(πDL) + q_p·(πD²/4)
Qa = Qu / FS

Q_u = ultimate pile capacity (kN)
Q_s = f_s × A_s = shaft friction resistance (kN)
Q_p = q_p × A_p = end bearing resistance (kN)
f_s = average unit skin friction over the shaft (kPa)
A_s = πDL = pile shaft surface area (m²)
D = pile outer diameter (m)
L = embedded pile length (m)
q_p = unit end bearing at pile tip (kPa)
A_p = πD²/4 = pile tip cross-section area (m²)
FS = factor of safety (typically 2.5–3.0)

Worked example

Example

A 600 mm diameter bored pile is embedded 15 m in stiff clay. The soil report gives average unit skin friction f_s = 50 kPa and unit end bearing q_p = 4,000 kPa. With FS = 2.5: shaft area A_s = π × 0.6 × 15 = 28.3 m²; tip area A_p = π × 0.6²/4 = 0.28 m². Shaft friction Q_s = 50 × 28.3 = 1,414 kN; end bearing Q_p = 4,000 × 0.28 = 1,131 kN. Ultimate Q_u = 1,414 + 1,131 = 2,545 kN. Allowable Q_a = 2,545 ÷ 2.5 = 1,018 kN.

When this estimate will be off

Assumes uniform unit skin friction over the full embedded length. In layered soils, split the pile into segments per layer and sum Qs contributions separately.
Pile self-weight is not deducted. For large-diameter or long piles in water, subtract the submerged pile weight from Qa before comparing to the structural demand.
Group efficiency is not modelled. A pile group may carry less than the sum of individual pile capacities due to overlapping shear zones; evaluate group capacity separately.
Negative skin friction (downdrag) from consolidating fill or soft layers is not included. Where present, treat it as an additional downward load acting on the pile.

Frequently asked questions

What is the difference between shaft friction (Qs) and end bearing (Qp)?

Shaft friction is resistance generated by friction and adhesion between the pile surface and surrounding soil along the embedded length. End bearing is resistance at the pile tip against the soil or rock below. Long slender piles in clay tend to be shaft-friction-dominated; short piles driven to dense gravel or rock carry most load in end bearing.

How do I find the design values for fs and qp?

Unit skin friction fs comes from a site investigation. Use the α-method (undrained shear strength × adhesion factor) for clay, or the β-method (effective overburden × K·tan δ) for sand and drained conditions. Unit end bearing qp is calculated from bearing capacity factors or correlated from CPT tip resistance or SPT N-values. Your geotechnical report should state design values for each layer.

What factor of safety should I use?

Traditional static design applies FS = 2.5–3.0 when no pile load test is performed. A maintained load test to failure supports reducing FS to 2.0. LRFD codes (ASCE 7, EN 1997) replace FS with resistance factors φ and partial factors γ — this calculator uses the global FS approach, so confirm with the governing specification.

Does this calculator account for group efficiency?

No — it calculates single-pile capacity only. In a pile group, overlapping shear zones reduce total capacity below the sum of individual piles. Group efficiency depends on pile spacing relative to diameter, soil type, and pile installation method. Evaluate group capacity separately using the perimeter block shear method or an equivalent raft approach.

Sources

FHWA NHI-16-009 — Design and Construction of Driven Pile Foundations — covers static capacity calculation by unit skin friction and end bearing methods
EN 1997-1:2004 §7.6 — Eurocode 7 pile design by calculation — shaft and base resistance components

This free Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.