Footing Size Calculator

Structural Foundation Engineering & Geotechnical Sizing (IRC Table R403.1 & ACI 318)

Concrete footing design distributes structural superstructure weight across the soil:

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• Geotechnical Bearing Area Formula (Allowable Stress Design)**:

A_min = (P) / (q_a)

Where:

• P: Total factored axial load

• q_a: Allowable soil bearing capacity in Pounds per Square Foot (PSF per IRC Table R401.4.1)

• A_min: Minimum bottom contact area in square feet

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• Footing Dimension Formulas**:

• Square Pad Footing: Side (in) = √(A_min) × 12

• Round Sonotube Pier: Diameter (in) = √(4 · A_min)π × 12

• Continuous Strip Footing: Width (in) = (Load per Linear Foot (PLF)q_a) × 12

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• IRC Soil Bearing Classes (Table R401.4.1)**:

• 1,500 PSF: Clay, sandy clay, silty clay

• 2,000 PSF: Sand, silty sand, loam (Standard Residential Default)

• 3,000 PSF: Dense compact gravel-sand mixture

• 4,000+ PSF: Sedimentary rock / cemented hardpan

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• Concrete Volume & Bag Estimation**:

Total Concrete (cu yd) = Volume (cu ft) ×27

Bags (80 lb) = Gross Volume (cu ft)0.60 cu ft/bag

How to Use This Calculator

Enter your Footing Structural Type — choose from Isolated Column / Post Pad Footing (Square Pad), Round Sonotube Pier Footing (Cylindrical Below Frost Line), Continuous Wall Strip Footing (Linear Foundation Wall), Total Supported Load (Dead + Live) (e.g. 6000 in lbs), Soil Bearing Capacity (PSF - IRC R401.4.1) — choose from 1,500 PSF (Clay, Sandy Clay, Silty Clay - Standard Minimum), 2,000 PSF (Sand, Silty Sand, Loam - Most Common Residential), 3,000 PSF (Dense Compact Sand & Gravel Mixture), Footing Thickness / Depth (e.g. 10 in inches), Number of Footings / Pier Locations (e.g. 6), Concrete Waste & Excavation Factor — choose from 5% (Formed Poured Concrete), 10% (Recommended - Earth Formed / Over-Excavation), 15% (Rough Excavation / Irregular Trench), and Structural Footing Benchmark — choose from Custom Structural Load, Elevated Deck (6 Piers, 6,000 lb load each, 2000 PSF) → 22" Pad / 0.68 cu yd (31 Bags), Pole Barn Foundation (10 Posts, 8,000 lb load each) → 24" Pad / 1.63 cu yd. Measure twice before entering dimensions — small measurement errors scale up significantly in material estimates.

The Formula

Diameter in inches = sqrt(4 Area / PI) 12

Strip width in inches = (PLF / PSF) * 12

Isolated square pad: Side in inches = sqrt(Area) * 12

Worked Example

A structural builder designing 6 deck post piers under 6,000 lbs tributary load in 2,000 PSF soil computes minimum bearing area as A = 6,000 / 2,000 = 3.00 sq ft, selecting a 22" × 22" pad with 10" depth.

Understanding Your Result

Construction estimates are starting points — site conditions, material specifications, and local code requirements will affect final quantities and costs. Always obtain at least three contractor bids for major projects. Local building departments can confirm permit requirements and inspection milestones. Add waste factors appropriate to the material type and complexity: 10% for simple rectangular shapes, 15–20% for complex geometries or custom cuts.