Isolated Footing Load Calculator helps you estimate the allowable column load based on footing size, soil capacity, and safety factors. This tool is intended for preliminary geotechnical sizing of isolated footings.
About isolated footings. Isolated footings (also called pad footings) are used to support individual columns. They distribute structural loads to the soil beneath. Footing design depends on soil bearing capacity, column loads, footing size, and depth of embedment.
About unit systems. The calculator supports both Metric (SI) and Imperial (US customary) units. In Metric, lengths are in meters (m), pressures in kilopascals (kPa), and weights in kilonewtons (kN). In Imperial, lengths are in feet (ft), pressures in kips per square foot (ksf), and weights in kips (kip) or pounds per cubic foot (pcf) for unit weight.
About soil bearing capacity. Soil capacity can be defined directly as an allowable bearing pressure (qallow) or calculated using soil parameters such as cohesion (c), friction angle (φ), and soil unit weight (γ) based on Terzaghi’s bearing capacity theory. The calculator provides both workflows—direct qallow or full soil parameter input.
About self-weight of footing. The calculator optionally includes the self-weight of the concrete footing in the load balance. The self-weight is calculated using footing thickness (t), plan area (B×L), and concrete unit weight (γc).
Calculation. The calculator estimates the net allowable column load as the total soil reaction minus footing self-weight.
Formulas:
- Area = B × L (plan area of footing)
- qult = c·Nc·sc·dc + γ·Df·Nq·sq·dq + 0.5·γ·B·Nγ·sγ·dγ (ultimate bearing capacity from soil parameters)
- qallow = qult / FS (allowable bearing pressure after applying factor of safety)
- R = qallow × Area (total allowable soil reaction on the footing)
- Wf = B × L × t × γc (self-weight of the concrete footing)
- P = R − Wf (net allowable column load)
Examples:
- Example 1: B = 1.5m, L = 1.8m, qallow = 200kPa, FS = 3 → R = 180kN, Wf = 39kN → P ≈ 141kN
- Example 2: B = 2.0m, L = 2.0m, soil: c = 25kPa, φ = 30°, γ = 18kN/m³, Df = 1m, FS = 3 → qult ≈ 780kPa → qallow ≈ 260kPa → R ≈ 1040kN
- Example 3: Same as above, with t = 0.6m, γc = 24kN/m³ → Wf = 58kN → P ≈ 982kN
Corresponding tools. To size rectangular or circular columns, use Column Load Calculator. To estimate soil bearing pressure from plate load tests, use Soil Bearing Capacity Calculator.
