Foundation Footing Size Calculator

Calculate the minimum required footing area and dimensions based on structural load and allowable soil bearing capacity.

Total Column / Wall Load (kips or kN)

Load Unit

Allowable Soil Bearing Capacity (ksf or kPa)

Bearing Capacity Unit

Estimated Footing Self-Weight (% of column load)

Footing Shape

1. Total Design Load

P_total = P_column × (1 + SW%/100)

where SW% is the estimated self-weight of the footing as a percentage of the column load.

2. Required Footing Area

A_req = P_total / q_allow

where q_allow is the allowable soil bearing capacity.

3. Footing Dimensions

Square: B = √A_req → rounded up to nearest 3-in increment
Rectangular (2:1): B = √(A_req/2), L = 2B → each rounded up to nearest 3-in increment
Circular: D = 2√(A_req/π) → rounded up to nearest 3-in increment

4. Net Bearing Pressure Check

q_net = P_total / A_actual ≤ q_allow

The footing is assumed to be a shallow spread footing (not a deep/pile foundation).
The allowable soil bearing capacity (q_allow) must be determined by a geotechnical engineer from a soil investigation report; typical values range from 1–4 ksf (50–200 kPa) for cohesive soils and 2–8 ksf (100–400 kPa) for granular soils.
Self-weight of the footing is approximated as a percentage of the column load (typically 5–10%); a more precise value requires knowing the footing depth and concrete unit weight (≈ 150 pcf / 23.6 kN/m³).
Dimensions are rounded up to the nearest 3-inch (0.25 ft) increment per standard construction practice.
This calculator addresses bearing capacity only; additional checks for punching shear, one-way shear, bending, and settlement are required per ACI 318 / ASCE 7 / Eurocode 7.
Unit conversions: 1 kip = 4.44822 kN; 1 ksf = 47.88 kPa; 1 ft² = 0.0929 m².
References: ACI 318-19 (Building Code Requirements for Structural Concrete); ASCE 7-22 (Minimum Design Loads); Das, B.M. — Principles of Foundation Engineering, 9th ed.

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