Footing Size Calculator estimates the required plan dimensions of isolated or strip footings based on applied loads, allowable soil bearing pressure, and safety factors. It supports square, rectangular, circular, and strip footings.
About footings. Footings are structural elements that spread the loads from columns or walls to the soil below, ensuring the soil bearing capacity is not exceeded. Correct sizing prevents excessive settlement and structural instability. This tool estimates the minimum plan dimensions based on vertical load and soil capacity, assuming uniform soil and vertical loading.
About bearing capacity. Allowable bearing capacity (qallow) represents the maximum soil pressure that can be applied safely without risk of shear failure or excessive settlement. Geotechnical investigations determine this value, and building codes often specify minimum safety factors. This calculator divides the allowable pressure by the safety factor to obtain a design bearing pressure for sizing.
About safety factor. A safety factor (FS) is applied to account for uncertainties in soil properties, load estimates, and construction quality. Typical FS values range from 2.0 to 3.0 for ultimate bearing capacity or 1.0 to 1.5 when using already reduced allowable bearing capacity values. Increasing the FS increases the footing size.
About footing shapes. Footings can be square, rectangular, circular, or strip (continuous under walls). For square and circular footings, the calculator solves for side or diameter from the required area. For rectangular footings, it uses a given width and solves for the length. For strip footings, it solves for width per meter length.
Calculation. The basic principle is that footing area must be large enough so that soil pressure does not exceed the allowable value divided by the safety factor. The design bearing pressure is:
quse = qallow / FS
Square footing: A = P / quse → side = √A
Circular footing: A = P / quse → D = √(4A/π)
Rectangular footing: A = P / quse → L = A / B (given width)
Strip footing: B = w / quse (per meter length)
- Example 1: P = 1000 kN, qallow = 200 kPa, FS = 1.5 → side ≈ √(1000/(200/1.5)) = 2.74 m
- Example 2: P = 800 kN, qallow = 250 kPa, FS = 2.0 → D ≈ √(4×(800/(250/2))/π) = 2.28 m
- Example 3: w = 150 kN/m, qallow = 180 kPa, FS = 2.0 → B = 150/(180/2) = 1.67 m
Corresponding tools. Use the Slope Stability Calculator to assess soil stability, the Retaining Wall Load Calculator for nearby structures, and the Earth Volume Calculator to estimate excavation quantities.
