Footings and Foundations
Footing size calculators and foundation guides, from deck piers to fence posts and slabs. A footing spreads a concentrated load from a post, column or wall over enough soil that the soil can carry it without sinking. The tools here size footings from the load and the soil, and the guides cover depth, frost and when to get an engineer.
Size the footing
- Deck Footing Size Calculator – footing diameter from post load and soil
- Concrete Footing Calculator – concrete for pad and continuous footings
- Sonotube Concrete Calculator – concrete for round piers
- Concrete for Post Holes Calculator – bags per post hole
Rules and depth
- Deck Footing Requirements – size, depth and spacing for deck footings
- How Deep Should Fence Posts Be – post depth, frost and hole size
- Concrete Slab Thickness – slab thickness and base by use
- When Do You Need a Structural Engineer – cracks, settlement and bigger loads
Soil bearing values in the code
| Soil (IRC Table R401.4.1) | Presumptive bearing |
|---|---|
| Clay, sandy clay, silty clay, clayey silt | 1,500 psf |
| Sand, silty sand, clayey sand, silty gravel | 2,000 psf |
| Sandy gravel and gravel | 3,000 psf |
| Sedimentary rock | 4,000 psf |
| Crystalline bedrock | 12,000 psf |
Worked example: a deck post carrying 6 ft × 8 ft of deck at 50 psf takes 2,400 lb. On 1,500 psf clay it needs 2,400 ÷ 1,500 = 1.6 sq ft of footing, so an 18 inch round footing (1.77 sq ft) works.
Engineer's note: the footing size is only half the job; what it sits on matters as much. A footing must bear on undisturbed or properly compacted soil and below the frost line, not on the loose backfill around a basement or on topsoil. When in doubt, dig deeper rather than wider: a big footing on soft fill still settles.
Everything in this section
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Maintained by: Engr. Abdul Latif, civil and structural engineer. Every calculator here shows its formula and the code table it follows. For planning only; confirm with your local building department.