Footing Size Calculator

Footing sizing is the process of determining the minimum plan dimensions required for a foundation footing so that the pressure it exerts on the soil does not exceed the allowable bearing capacity. A footing that is too small risks excessive settlement, structural cracking, or even catastrophic bearing failure. One that is oversized wastes concrete and excavation labour. Getting the size right balances safety with economy.

The calculator evaluates four standard load combinations prescribed by building codes such as ASCE 7 and Eurocode 0. Each combination applies different safety factors to dead load (DL), live load (LL), wind load (WL), and seismic or earthquake load (EL). The governing case — the one that demands the largest footing area — is automatically selected, ensuring the design satisfies every code-mandated scenario.

Footing shapes available include square, rectangular, and circular footings. Square footings are the most common for isolated column supports because they distribute load equally in both directions and simplify formwork. Rectangular footings are used when the column sits near a property line or when the column transmits unequal moments in two axes. Circular footings are preferred for round columns or when aesthetic considerations demand a round plan.

Beyond area, the calculator estimates the required depth, concrete volume, and the actual bearing pressure relative to the allowable soil pressure. A utilisation ratio below 100 % confirms that the designed footing safely spreads the applied loads to the ground without exceeding soil capacity.

Building codes define several load combinations that must be checked during design. The four primary combinations used by this calculator are:

• Combo 1 (Gravity only): DL + LL — the most common case for typical service conditions.
• Combo 2 (Gravity + Wind): DL + LL + WL — with a 33 % increase in allowable soil pressure permitted when wind loads are transient.
• Combo 3 (Gravity + Seismic): DL + LL + EL — same 33 % allowable increase applies.
• Combo 4 (Uplift / Overturning): 0.9 DL + EL — checks stability when dead load alone must resist seismic overturning.

The required area for each combination is the factored load divided by the (possibly increased) allowable soil pressure. The governing area is the largest of the four values.

Once the required plan area is established, the footing dimensions depend on the chosen shape:

Computed dimensions are rounded up to the nearest 50 mm (0.05 m) to align with standard formwork modules and construction tolerances. A minimum dimension threshold prevents impractically small footings in low-load situations. The concrete depth is estimated at 15 % of the minimum plan dimension, with a floor of 300 mm to ensure adequate shear capacity.

Follow these steps to size a footing:

1. Enter Applied Loads: Input the dead load, live load, wind load, and seismic load in kilonewtons (kN). These are the unfactored service loads acting on the column above the footing.
2. Set Allowable Soil Pressure: Enter the net allowable bearing capacity of the soil in kN/m². This value comes from a geotechnical report or local code tables.
3. Choose Footing Type: Select square, rectangular, or circular. For rectangular footings, specify the length-to-width ratio.
4. Set Minimum Dimension: The default minimum of 0.6 m ensures constructability. Increase it if required by the foundation type or local code.
5. Review Results: The calculator shows the governing load combination, required and actual footing dimensions, soil pressure utilisation, estimated depth, and concrete volume.

The results panel presents several key metrics. The governing combination tells you which load case controls the design — this is the case that requires the largest footing area. The total load for that case is shown in kN.

Footing dimensions (length and width) are the plan sizes that satisfy all code checks. The actual area is the provided area after rounding, which may be slightly larger than the theoretical minimum.

The soil pressure utilisation ratio compares the actual bearing pressure under gravity loads to the allowable soil capacity. A value below 100 % indicates the footing is adequate; a value above 100 % signals that the footing is undersized for the given loads or soil conditions.

The estimated depth is a preliminary value for concrete thickness. Final depth should be verified for punching shear, one-way shear, and development length requirements per the applicable reinforced concrete design code.

Footing sizing is a fundamental step in the design of nearly every building and structure. Residential homes typically use isolated square or rectangular spread footings bearing on competent soil or engineered fill. Commercial buildings may employ combined footings, strap footings, or mat foundations when columns are closely spaced or column loads are very high.

In bridge engineering, pier footings must resist not only vertical loads but also horizontal forces from braking, wind, and seismic events. Industrial facilities such as storage tanks and silos use ring footings or circular footings that distribute the load uniformly around a cylindrical base.

Geotechnical engineers provide the allowable soil bearing capacity based on site investigations, including Standard Penetration Tests (SPT), cone penetration tests, and laboratory analysis. The structural engineer uses this data to size footings that safely transfer building loads into the ground without excessive settlement or bearing failure.