# Pile Capacity Calculator

N value for sand, cu for clay: shaft and base worked separately, then divided by a factor of safety.

- Page: https://jianzhujisuan.com/en/pile-capacity
- Category: Foundations (https://jianzhujisuan.com/en/c/foundations)
- Chinese version: https://jianzhujisuan.com/zh/pile-capacity.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Pile diameter 0.6 m, Pile length 20 m, Soil type Sand (SPT), SPT N value 20 blows/300mm, Undrained shear strength cu 50 kPa, Factor of safety 2.5): **Allowable capacity 1508 kN**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Pile diameter | number | 0.6 | m |  |
| Pile length | number | 20 | m |  |
| Soil type | select | Sand (SPT) | — |  |
| SPT N value | number | 20 | blows/300mm |  |
| Undrained shear strength cu | number | 50 | kPa |  |
| Factor of safety | number | 2.5 | — |  |

## Formula

- **Perimeter**: u = π·D
- **Base area**: Ab = π·D²/4
- **Shaft, sand**: fs = 2N (kPa, cap 100)
- **Base, sand**: qb = 400N (kPa, cap 15000)
- **Shaft, clay**: fs = 0.5·cu
- **Base, clay**: qb = 9·cu
- **Ultimate**: Qu = fs·u·L + qb·Ab
- **Allowable**: Qa = Qu / FS

## Worked example

600 mm diameter, 20 m long, sand with N = 20, factor of safety 2.5: perimeter 1.885 m, base area 0.2827 m², fs = 40 kPa giving 1508 kN shaft, qb = 8000 kPa giving 2262 kN base, 3770 kN ultimate and 1508 kN allowable, with the base carrying 60%.

## Questions

### How do I calculate the capacity of a single pile?

Two parts. Shaft capacity is unit shaft friction times perimeter times length; base capacity is unit end bearing times base area. Add them for ultimate capacity and divide by the factor of safety, normally 2.5, for the allowable value. Unit values come from soil type: for sand use SPT and take fs = 2N kPa with qb = 400N kPa; for clay use fs = 0.5cu and qb = 9cu. Always benchmark the result against a load test before it goes into a construction drawing.

### Is a 60% base share realistic?

For a short pile founded on a stiff layer, yes. For a long pile with a slenderness ratio above 25 it is a warning sign, usually meaning the N value is too high or the base sits on a sand layer with soft clay beneath. Check two things: at least 3D of competent soil below the base, otherwise the end bearing is lost to the soft layer beneath; and for high slenderness, reduce the base term for settlement rather than taking the full 400N.

## Related tools

- [Bearing Capacity Calculator](https://jianzhujisuan.com/en/bearing-capacity): The three bearing factors follow from the friction angle; divide the ultimate value by a factor of safety.
- [Retaining Wall Stability Calculator](https://jianzhujisuan.com/en/retaining-wall): Soil and surcharge produce moments at different heights; both factors of safety must pass.
- [Earth Pressure Calculator](https://jianzhujisuan.com/en/earth-pressure): All three coefficients at once; the gap between active and passive is where the money is.

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