Footing Size Calculator — Isolated Footing from Load & SBC

Enter the column's service load and the soil's safe bearing capacity, and get the footing plan size — with the bearing pressure checked against the SBC. It sizes the pad; the depth and steel come from a full design.

Isolated Footing Size Estimator

Area = (load + 10% self-weight) ÷ SBC · pad rounded up to 50 mm · pressure checked

Isolated footing

2.45 × 2.45 m · 147 kN/m² on soil

Service axial load P (kN)

From the column — service (un-factored) load

Safe bearing capacity (kN/m²)

Soft soil ~100 · medium ~150–200 · hard/rock 300+

Footing plan size

2.45 × 2.45 m

6 m² provided · gross pressure 147 kN/m² ≤ SBC ✓

Footing size

2.45 × 2.45 m

Required area

5.87 m²

Gross pressure

147 kN/m²

Factored net (design)

200 kN/m²

2.45 × 2.45 m gives 6 m², so the soil sees 147 kN/m² against its 150 kN/m² capacity — within bearing, with 2% to spare.

This sizes the pad and checks the pressure — not the depth or steel. The thickness comes from one-way and two-way (punching) shear, and the reinforcement from bending under the factored net pressure (200 kN/m²). Get a real SBC from a soil test, not a table, and keep the footing at least 1 m below ground / below the frost or shrinkage zone.

How it works

Add ~10% to the column service load for the footing's own weight and the soil over it.
Required area = load ÷ safe bearing capacity (SBC).
Plan size = √area for a square footing, rounded up to the next 50 mm.
Check the gross pressure (load ÷ provided area) stays within the SBC.

Worked example

A 800 kN column on soil of 150 kN/m² SBC:

With 10% self-weight, load = 880 kN → area = 880 ÷ 150 = 5.87 m²
A square footing rounds up to 2.45 × 2.45 m = 6.00 m²
Gross pressure = 880 ÷ 6.00 = 147 kN/m² ≤ 150 ✓

The factored net pressure for designing the depth and steel is 1.5 × 800 ÷ 6.00 ≈ 200 kN/m².

Typical safe bearing capacity

Soil	SBC (kN/m²)
Soft clay / filled ground	50–100
Medium clay / sandy soil	150–200
Dense sand / gravel	250–450
Hard rock	1,000+

Always confirm with a soil test — these are indicative only.

Frequently asked questions

How do I calculate footing size? Divide the column load (plus ~10% self-weight) by the soil's safe bearing capacity to get the area, then take the square root for a square footing. An 800 kN column on 150 kN/m² soil needs about 2.45 × 2.45 m.

What is safe bearing capacity? The pressure the soil can carry safely without excessive settlement or shear failure — typically 150–200 kN/m² for medium soils, confirmed by a site soil test.

Does this give the footing depth and steel? No. This sizes the pad and checks the bearing pressure. The thickness comes from one-way and punching shear, and the reinforcement from bending under the factored net pressure — both need a full design.

How deep should a footing be? The base should sit on firm strata, usually at least 1 m below ground and below any loose fill or seasonal moisture-movement zone.