Concrete Pile Foundation Calculator

Estimate concrete volume and rebar for bored or screw pile foundations. Includes a smart grid planner to suggest pile spacing based on building area.

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About

Pile foundations are essential when the surface soil is too weak to support a strip or slab foundation, requiring loads to be transferred to deeper, more stable strata. They are also ideal for slopes or areas with high water tables. However, calculating the volume of a pile is not as simple as measuring a cylinder; many piles feature 'belling' (expanded bases) to increase load capacity, and wastage during boring can be significant.

This calculator handles the geometry of standard cylindrical and belled piles. It helps you determine the total concrete volume required for your grid and estimates the steel reinforcement cage weight. The built-in Grid Planner assists in estimating how many piles you might need based on standard spacing rules for your building's footprint.

pile foundation concrete piles bored piles foundation grid

Formulas

The volume of a standard cylindrical pile is calculated using the circular area formula:

Vpile = π × r2 × h

If the pile has a belled base (frustum of a cone), the additional volume is:

Vbell = π × hb3 (R2 + Rr + r2)

Where R is the radius of the bell base and r is the radius of the pile shaft. Total project volume:

Vtotal = Vpile × Ncount × (1 + SafetyFactor)

Reference Data

Pile TypeTypical DiameterMax Depth (Res.)Spacing RuleRebar Config
Bored Pile (Mini)200 - 300 mm3 - 6 m3x Diameter4x φ10mm
Bored Pile (Std)300 - 500 mm10 - 20 m2.5x - 3x Diam6x φ12mm
Screw Pile (Helix)76 - 114 mm shaftVariable1.5m - 3mInternal Grout
Driven Pile200 - 400 mmVariable3x DiameterPre-stressed
Belled PileStem: 300mm, Bell: 600mm5 - 15 m3x Bell DiamComplex Cage
Micro Pile100 - 250 mmUp to 30 mClose clusterCentral Bar

Frequently Asked Questions

A general rule of thumb for pile spacing is 3 times the diameter of the pile (center-to-center). For example, if you are using 300mm diameter piles, they should be spaced at least 900mm apart. Placing them too close creates 'group action' where the soil stresses overlap, reducing the total capacity of the group.
When drilling into earth, the hole walls are rarely perfect cylinders. Soil can collapse (overbreak), creating voids that fill with concrete. It is standard practice to order 10-15% more concrete than the theoretical geometric volume to account for this overbreak.
Belled piles are used in cohesive soils (like clay) where the soil is firm enough to hold the bell shape without collapsing. The expanded base significantly increases the bearing capacity by distributing the load over a larger area and providing resistance against uplift (pull-out) forces.
A standard residential pile cage consists of 4 to 6 vertical bars and spiral or circular ties (stirrups). This calculator estimates the weight of the vertical bars based on the pile depth. Professional engineering drawings are required for the exact spacing of the spiral ties.