How do you calculate pile capacity?

Ultimate pile capacity = skin friction + end bearing. Skin friction integrates unit side resistance (adhesion α·c in clay; K·σ'ᵥ·tanδ in sand) over the shaft surface; end bearing is 9·c·A in clay or σ'ᵥ·Nq·A in sand, with a limiting depth. Divide each part by its factor of safety for the allowable load.

What factor of safety is used for pile capacity?

Common allowable-stress practice applies FS ≈ 2 on side resistance and FS ≈ 3 on end bearing (end bearing needs much more settlement to mobilize), or FS = 2 to 3 on the total depending on the reliability of soil data and whether a load test is performed.

Why is capacity limited below a certain depth?

Measured unit side friction and end bearing in sand stop increasing roughly below 20 pile diameters — the limiting (critical) depth. The NAVFAC procedure caps effective-stress growth with limiting-depth ratios for side and tip resistance, and the calculator exposes both ratios; 20 diameters is the default, not a hidden constant.

Free Pile Capacity Calculator — Skin Friction & End Bearing

How to use it

Describe the pile

Choose driven or drilled, enter length and diameter — the tool sets displacement behavior and bearing factors accordingly.

Describe the soil

Pick clay (undrained strength) or sand (friction angle), enter the unit weight and the water-table depth.

Set factors of safety

Defaults follow common practice — FS = 2 on side resistance and FS = 3 on end bearing.

Read the capacity breakdown

The tool integrates side friction over the shaft with a limiting depth, adds end bearing, and reports ultimate and allowable capacity in compression and uplift.

The method behind the numbers

Ultimate axial capacity is the sum of two mechanisms — skin friction and end bearing. This calculator follows the NAVFAC DM-7.02 procedure: in clay, unit side resistance is the adhesion α·c (Tomlinson-style α from the strength) and end bearing is 9·c over the tip area; in sand, side resistance is K·σ′ᵥ·tanδ and end bearing σ′ᵥ·Nq, with the effective stress capped at the limiting depth of 20 diameters. The water table enters through buoyant unit weight below it. Uplift capacity takes the side resistance plus the pile's effective weight, each with its own factor of safety.

What the simplifications are

One uniform soil layer, a prismatic pile, and editable factors of safety (defaulting to 2 on side, 3 on tip). Real profiles are layered, and effects like downdrag, battered piles, rock sockets and belled bases change the answer — the full axial analysis handles all of them and adds the t-z load–settlement curve you see plotted below the results. Method details and every parameter definition are in the axial capacity documentation.