Geotechnical
Footing Size Calculator (Bearing Area)
A spread footing spreads a column load over enough soil that the bearing pressure stays below the allowable value. Enter the service load and allowable bearing pressure to get the footing area and a square footing size. For estimation only; a licensed PE must design.
Formula
Required area A = P ÷ qa, where P is the unfactored (service) load in lb and qa is the allowable bearing pressure in psf. A square footing side = √A. IRC Table R401.4.1 presumptive values: 1,500 psf for clay, sandy clay, silty clay and clayey silt; 2,000 psf for sand, silty sand, clayey sand, silty gravel and clayey gravel; 3,000 psf for sandy gravel and gravel; 4,000 psf for sedimentary rock; 12,000 psf for crystalline bedrock.
Worked example
A post carrying 30,000 lb on silty sand (2,000 psf presumptive): A = 30,000 ÷ 2,000 = 15.0 ft². Side = √15 = 3.87 ft, about 46.5 in, so round up to a 4 ft by 4 ft footing.
Assumptions and limits
- Ignores the weight of the footing and overlying soil, which reduces net capacity; add it to P for a closer estimate.
- Concentric load only; eccentric loads, moments, settlement, frost depth and punching shear are not checked.
- Presumptive values apply only where the building official accepts them; a geotechnical report governs otherwise.
- For estimation only. A licensed Professional Engineer (PE) must design and stamp any structural, foundation or life-safety element.
Frequently asked questions
What is allowable bearing pressure?
The pressure the soil can carry with an adequate safety factor against bearing failure and without excessive settlement. It comes from a geotechnical investigation or presumptive code values.
Can I use 2,000 psf without a soil test?
Under the IRC the building official may accept presumptive values, and 1,500 psf is often assumed when soil type is unknown. Expansive, organic or fill soils require investigation.
How thick should a spread footing be?
Thickness is governed by one-way and two-way (punching) shear in ACI 318 and by reinforcement development. The IRC minimum for plain concrete footings is 6 in, but column footings are typically thicker.