Footing / Pier Size by Soil Bearing
Size a spread footing or pier from the load it carries and the allowable soil bearing — with a buildable size and the actual pressure it produces.
Total unfactored load on the footing
15.0 sf1.39 m²
Required area
4' square1,875 psf actual
Buildable size
Square pad · 6% bearing margin
Assumptions & notes
- Required bearing area = load ÷ allowable bearing = 30,000 lb ÷ 2,000 psf.
- Square pad side rounded up to the next 3 in.
How footing sizing works
A spread footing spreads a concentrated load out over enough soil that the
pressure under it stays below what the soil can safely carry. The size you need
is just required area = load ÷ allowable bearing pressure. From that
area you get the pad side (√area), the pier diameter
(√(4·area / π)), or the second side of a rectangle
(area ÷ width).
The soil presets are the presumptive load-bearing values from IRC Table R401.4.1 — reasonable defaults when you don't yet have a geotechnical report, from 1,500 psf for clay up to 12,000 psf for bedrock. Enter a custom value (psf or kPa) when you do have a soils report.
This is the sizing companion to the tributary-area load takedown: use that to find the load a column brings down, then size its footing here. Remember it sizes bearing area only — the footing's thickness, reinforcement, frost depth, and overturning still have to be designed and reviewed.
Official source
Opens the official standard in a new tab — always verify against your adopted code edition and local authority (AHJ).
Frequently asked questions
How do I size a footing from the soil bearing capacity?
Divide the load by the allowable soil bearing pressure to get the required bearing area: area = load ÷ bearing. A 30,000 lb load on 2,000 psf soil needs 15 sq ft — about a 4' square pad. Then round up to a buildable size and check the actual pressure.
What allowable soil bearing value should I use?
The presets are the presumptive load-bearing values from IRC Table R401.4.1 (1,500 psf clay up to 12,000 psf bedrock). They're conservative defaults for when you don't have a soils report — always verify against a geotechnical report and your local building department.
Is this a footing design?
No. It sizes the bearing area only — the footprint that keeps soil pressure under the allowable. It does not design the footing's thickness, reinforcement, punching shear, overturning, or frost depth. Use it for quick sizing and checking, then have the footing designed and reviewed.
What's the difference between the required and buildable size?
The required dimension is the exact square root of the area; the buildable size rounds it up to a practical increment (3 in for pads, 2 in for piers), so the actual bearing pressure drops a little below the allowable — the tool shows that margin.
Related tools
- Tributary Area & Load TakedownFloor area a column carries and the load it takes down — interior, edge, or corner.
- Beam Span Quick-ReferenceRough member depth from a span using typical span-to-depth ratios (rule of thumb).
- Concrete TakeoffCubic yards, cubic metres, and bag counts for slabs, footings, walls, columns, and round piers — with a waste allowance.