Foundation Size Calculator

Work out foundation size instantly with clear inputs, formula shown and shareable results.

About Foundation Size Calculator

A pad footing spreads the column load until the pressure on the soil is within the allowable value, so the required area is total load divided by safe bearing capacity. An allowance of about ten percent is added for the footing's own weight and the soil above it, and the side length is rounded up to a buildable 50 mm increment.

Formulas

Required area

A = column load x (1 + self weight allowance) / safe bearing capacity

Square footing

Side = sqrt(A), rounded up to a practical dimension

Worked example

With the default values below, Foundation Size Calculator returns Square footing side of 2.45. Change any input above and every figure here is recalculated in your browser.

Inputs used

Column service loadminimum 1 kN: 800 kN
Safe bearing capacityminimum 1 kPa: 150 kPa
Allowance for footing self weightaccepts 0–30 %: 10 %

Results

Square footing side: 2.45 m
Bearing area required: 5.867 m²
Area of the chosen footing: 6.003 m²
Bearing pressure achieved: 146.6 kPa

Step by step

1Values used
2Required area
3Square footing
4Square footing side= 2.45 m
5Bearing area required= 5.867 m²
6Area of the chosen footing= 6.003 m²
7Bearing pressure achieved= 146.6 kPa

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Preliminary estimate only. Foundation design must be completed by a qualified structural engineer including settlement, thickness and reinforcement design.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Foundation design must be completed by a qualified structural engineer including settlement, thickness and reinforcement design.

Frequently Asked Questions

How is the Foundation Size figure worked out?

A = column load x (1 + self weight allowance) / safe bearing capacity. A pad footing spreads the column load until the pressure on the soil is within the allowable value, so the required area is total load divided by safe bearing capacity.

Does this size the reinforcement too?

No. This is the serviceability bearing check only. Thickness and reinforcement come from factored loads with bending, one-way shear and punching shear checks.

What values do I need to enter?

This calculator takes 3 inputs: Column service load, Safe bearing capacity, Allowance for footing self weight. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.