Size concrete footings by finding the building's size, construction materials and the location's soil bearing capacity on a concrete footing width chart. The footing sizes on these charts are only recommendations, and ideal footing sizes can vary between different locations with similar soils and even slightly different building materials.
Soil bearing capacity is a rough measure of how much weight different types of soil can withstand before subsiding. Igneous bedrocks like granite and diorite have the highest soil bearing capacity, while loose soils like sand and clay have the lowest capacities. The lower a soil's weight bearing capacity, the wider concrete footings must be to support the building.
The size of the building and the materials used in its construction are just as important as soil bearing capacity when determining ideal concrete footing widths. One-story traditional timber framed buildings have the lowest weight requirements, and taller solid masonry or concrete buildings have the highest. For example, a one-story wood framed house built on coarse gravel requires only 6-inch wide concrete footings, while a three-story solid brick home built on sand requires much larger 30-inch wide footings.
These charts only provide general estimates, and determining truly ideal footing sizes requires having an engineer add up the weight of all building materials and size the required footings based on this value and the area's soil bearing capacity.