Isolated Footing Sizing Calculator: Pad Dimensions from Bearing Pressure

A column concentrates the weight of a building into a shaft a few hundred millimeters across; the soil under it can typically carry a few hundred kilopascals before it either fails in shear or settles more than the structure can tolerate. The isolated footing is the translator between those two facts: a pad of reinforced concrete wide enough that the concentrated load, spread over the pad area, arrives at the soil as a pressure it can live with. Sizing one is honest arithmetic, and this calculator runs the loop the way it is done at a design desk.

The loop

• Required area = total load (column plus footing self-weight plus overburden) divided by the allowable bearing pressure
• Convert area to dimensions: square root for a square pad, or a chosen aspect ratio where property lines or grade beams force a rectangle
• Round up to a buildable dimension, since nobody forms a footing at 2.309 m
• Recheck the actual pressure with the rounded size and report the utilization

Worked example

An 800 kN column on soil with 150 kPa allowable bearing needs 5.33 m2. A square footing wants 2.31 m sides; rounding to 2.35 m gives an actual pressure of 145 kPa at 97 percent utilization. Comfortable, buildable, done. Change the soil to a soft 75 kPa clay and the same column suddenly needs a 3.3 m square pad, which is the point where a designer starts weighing isolated footings against a combined footing, a raft, or piles: bearing pressure quietly drives the entire foundation concept.

Where the allowable pressure comes from, and what this check is not

The allowable bearing value deserves more respect than it usually gets, because it smuggles two different criteria into one number: the ultimate bearing capacity divided by a safety factor of about 3, and the pressure that keeps settlement within tolerable limits, whichever is lower. On stiff soils, strength governs; on the soft and the compressible, settlement almost always does, and a footing that passes a strength-based number can still crack the building above it as consolidation proceeds. This calculator handles the bearing side; the settlement side lives with the consolidation calculation, and the concrete side, punching shear around the column perimeter and bending steel in both directions, follows in the structural design. In practice punching shear is what usually sets the footing thickness, which is why real pads are far thicker than the bearing arithmetic alone would ever suggest.

Related Calculators and Reference

See also the Bearing Pressure Converter for getting an allowable pressure into the units your report quotes before sizing the pad.

The allowable bearing pressure feeding this comes from the Terzaghi bearing capacity calculator, the settlement criterion is checked with the consolidation settlement calculator, and the reinforcement inside the pad is quantified by the rebar weight and spacing calculator. A clear open overview is the shallow foundation article.