What is the static pile capacity formula?

Ultimate pile capacity Q_ultimate = Q_point + Q_skin, where Q_point = A_tip × q_unit (tip resistance) and Q_skin = Perimeter × L × f_unit (shaft friction). Allowable capacity = Q_ultimate / Factor of Safety.

What is a typical factor of safety for piles?

A factor of safety of 2.5 is standard when pile capacity is determined from soil investigation data alone. FS = 2.0 may apply when load tests are performed; FS = 3.0 is used for highly variable soils or critical structures.

How do I get q_unit and f_unit?

These unit values come from a site-specific geotechnical investigation. For driven piles in sand, q_unit is often estimated from SPT N-values using correlations (e.g., Meyerhof). f_unit for clays is typically α × Su (adhesion factor × undrained shear strength). Consult a geotechnical engineer for site-specific values.

Does this tool handle layered soils?

No. The calculator assumes uniform unit skin friction over the full pile length. For layered profiles, compute Q_skin for each layer separately and sum them; enter Q_point from the tip layer only. A geotechnical specialist should analyse complex stratigraphy.

What is the difference between driven and bored piles?

Driven piles displace soil and tend to have higher skin friction (especially in sands). Bored (cast-in-place) piles may have lower friction due to stress relief during drilling. The formula structure is the same; the unit values differ based on installation method and soil type.

Pile Capacity Calculator

Compute ultimate and allowable pile capacity from tip resistance and skin friction for circular or square piles in metric or imperial units.

Pile Parameters

Unit System

Pile Type

Diameter

Pile Length

Unit Point Bearing (q_unit)

kPa

Unit Skin Friction (f_unit)

kPa

Factor of Safety

Results

Tip Area (A_tip)

—

m²

Perimeter

—

Point Resistance (Q_point)

—

Skin Friction (Q_skin)

—

Ultimate Capacity (Q_ult)

—

Allowable Capacity (Q_allow)

—

Formula Reference

Q_point= A_tip × q_unit

Q_skin= Perimeter × L × f_unit

Q_ultimate= Q_point + Q_skin

Q_allow= Q_ultimate / FS

Circular: A_tip = π·d²/4, Perimeter = π·d

Square: A_tip = s², Perimeter = 4·s

Test case (metric): Circular D=0.5 m, L=10 m, q=500 kPa, f=40 kPa, FS=2.5 → Q_ult ≈ 726.5 kN, Q_allow ≈ 290.6 kN

Summary

Compute ultimate and allowable pile capacity from tip resistance and skin friction for circular or square piles in metric or imperial units.

How it works

Select pile type (circular or square) and unit system (metric or imperial).
Enter pile dimensions: diameter or side length, and pile length.
Enter unit point bearing (q_unit) and unit skin friction (f_unit) from soil investigation.
Set a factor of safety (default 2.5) for the allowable capacity calculation.
The calculator computes tip area, perimeter, Q_point, Q_skin, Q_ultimate, and Q_allowable.
Results update instantly; verify against the worked test case shown below the output.

Use cases

Preliminary sizing of driven or bored piles for building foundations.
Comparing capacity of circular versus square piles for the same soil profile.
Checking pile adequacy against a structural column load during early design.
Educational exercises on static pile capacity analysis.
Estimating pile group requirements by dividing total load by single-pile allowable capacity.
Quick sanity-check of results from detailed geotechnical software.

Frequently Asked Questions

Last updated: 2026-09-20 · Reviewed by Nham Vu