Pile Capacity Calculator
Find the ultimate and allowable capacity of a driven pile from its diameter, length, and SPT N-values using Meyerhof's simplified static method.
🪵 What is Pile Capacity?
Pile capacity is the maximum load a driven pile foundation can safely carry, combining two separate contributions, end bearing resistance at the pile tip and skin friction resistance along the embedded shaft. This calculator uses Meyerhof's simplified SPT-N based static method, a widely taught preliminary approach that estimates both contributions directly from standard penetration test results without requiring detailed soil strength parameters.
Geotechnical and structural engineers reach for a quick SPT-based capacity check at the earliest stage of deep foundation sizing, right after borehole logs come back with SPT N-values at different depths. A structural engineer sizing piles beneath a column load compares the allowable capacity against the actual column load before finalizing pile diameter and length. A geotechnical consultant reviewing several trial pile lengths for a site runs the same calculation across each option to recommend the shortest, most economical pile that still clears the required factor of safety.
A common misconception is that a longer pile always costs proportionally more for proportionally more capacity. In reality, the balance between end bearing and skin friction shifts with pile geometry, a short, wide pile in dense soil may get most of its capacity from end bearing alone, while a long, slender pile accumulates most of its capacity from skin friction along the shaft, meaning the most economical length depends on the specific soil profile at the site.
This calculator takes the pile diameter, embedded length, tip and average shaft SPT N-values, and factor of safety, and returns the end bearing, skin friction, ultimate, and allowable capacity, plus a chart showing how allowable capacity changes as pile length changes.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Typical Driven Pile in Medium-Dense Sand
D = 0.4 m, L = 10 m, Ntip = 30, Navg = 15, FS = 2.5
Example 2 — Larger Diameter, Longer Pile in Dense Sand
D = 0.5 m, L = 15 m, Ntip = 40, Navg = 20, FS = 2.5
Example 3 — Short Pile, Higher Factor of Safety
D = 0.3 m, L = 8 m, Ntip = 20, Navg = 10, FS = 3.0
❓ Frequently Asked Questions
🔗 Related Calculators
What is pile capacity and why does it matter?
Pile capacity is the maximum load a driven pile can safely carry without the surrounding soil failing in shear or settling excessively. It combines end bearing at the pile tip and skin friction along the embedded shaft, each estimated here from SPT N-values using Meyerhof's simplified correlation, then divided by a factor of safety to give an allowable working load.
How is pile capacity calculated from SPT N-values?
End bearing is Qp = 40 x Ntip x Ap, where Ntip is the SPT N-value at the pile tip and Ap is the tip cross-sectional area. Skin friction is Qs = 2 x Navg x As, where Navg is the average SPT N-value along the shaft and As is the shaft surface area. Ultimate capacity Qu = Qp + Qs, and allowable capacity Qa = Qu divided by the factor of safety.
What is the difference between end bearing and skin friction?
End bearing is the resistance the soil directly beneath the pile tip provides, similar to a shallow footing bearing on soil. Skin friction is the resistance from friction and adhesion along the full embedded surface of the pile shaft. Most piles rely on a combination of both, with the balance depending heavily on pile length and diameter.
What factor of safety should I use for pile capacity?
A factor of safety of 2.5 against ultimate capacity is a common default for driven piles under typical static loads, and is this calculator's default. Piles supporting critical structures, or sites without load test data to confirm the SPT correlation, often use FS = 3.0 or higher for additional conservatism.
Why do the coefficients 40 and 2 appear in the formula?
These are Meyerhof's simplified, widely used SPT correlation coefficients for driven piles in cohesionless or mixed soil: 40 x Ntip (in consistent kN and m2 units) for end bearing and 2 x Navg for skin friction per unit shaft area. They are empirical approximations calibrated against pile load test data, not derived from first-principles soil mechanics.
Does this calculator work for bored or cast-in-situ piles?
No. This calculator is scoped to driven piles using Meyerhof's simplified SPT correlation. Bored (cast-in-situ) piles typically develop lower skin friction and end bearing per unit SPT N due to stress relief and disturbance during boring, and use different, generally lower empirical coefficients not covered here.
Why does allowable capacity increase with pile length?
A longer pile has more embedded shaft surface area (As = pi x D x L grows linearly with L), which directly increases the skin friction contribution Qs. This is why the allowable capacity versus length chart on this page slopes upward, even though the end bearing term Qp stays constant as length changes for a fixed tip N-value.
What SPT N-value should I use if the soil layer varies with depth?
Use the actual N-value recorded at or near the pile tip elevation for Ntip, and average the N-values recorded along the pile's embedded shaft length for Navg. A real borehole log will show N-values changing with depth, and this calculator expects you to have already read off the tip value and computed the shaft average from that log.
How accurate is Meyerhof's simplified SPT method?
It is a widely taught, quick preliminary estimate accurate to within a moderate margin for typical cohesionless and mixed soils, useful for early-stage sizing and comparing trial pile geometries. Final design should always be checked against a code-compliant static analysis and, where feasible, an actual pile load test, since SPT correlations carry meaningful scatter.
What happens to pile capacity if the pile diameter increases?
A larger diameter increases both the tip area Ap (which grows with D squared, boosting end bearing) and the shaft perimeter (which grows linearly with D, boosting skin friction per unit length). Because Ap grows faster than the shaft perimeter, increasing diameter tends to help end bearing proportionally more than it helps skin friction.