How wide and how deep should the footing be for a house? This guide covers the International Residential Code (IRC) minimums, the soil bearing values the code allows without a soil test, how footing width is worked out from the load on the wall, and a free calculator for a preliminary size.
| Minimum width | 12 in (IRC R403.1.1) |
|---|---|
| Minimum thickness | 6 in (IRC R403.1.1) |
| Minimum depth | 12 in below undisturbed ground (R403.1.4) |
| Cold climates | Below the frost depth (R403.1.4.1) |
| Default soil (sand, SM/SC) | 2,000 psf (Table R401.4.1) |
IRC minimum footing rules
- Width and thickness (R403.1.1): at least 12 in wide and 6 in thick for concrete footings. The footing must also project at least 2 in beyond each face of the foundation wall, and the projection cannot be more than the footing thickness.
- Depth (R403.1.4): the bottom of an exterior footing must be at least 12 in below undisturbed ground.
- Frost (R403.1.4.1): footings must go below the frost line set by your jurisdiction, unless they are frost-protected by another approved method.
- Concrete strength (Table R402.2): at least 2,500 psi for footings.
- Seismic (R403.1.3): in Seismic Design Categories D0, D1 and D2, footings and stem walls need reinforcement.
- Prescriptive widths (Table R403.1 series): the IRC tables give minimum widths for light-frame and masonry houses by number of stories, foundation type, snow load and soil. Use the table in the edition your area has adopted.
Soil bearing values allowed without a soil test
| Soil type (Unified Soil Classification) | Presumptive load-bearing value |
|---|---|
| Crystalline bedrock | 12,000 psf |
| Sedimentary and foliated rock | 4,000 psf |
| Sandy gravel and gravel (GW, GP) | 3,000 psf |
| Sand, silty sand, clayey sand, silty gravel, clayey gravel (SW, SP, SM, SC, GM, GC) | 2,000 psf |
| Clay, sandy clay, silty clay, clayey silt, silt, sandy silt (CL, ML, MH, CH) | 1,500 psf |
How footing width is calculated
A footing spreads the weight of the house over enough soil that the soil is not overloaded. For a strip footing under a wall:
- Footing width (ft) = total load on the wall (lb per linear foot) ÷ allowable soil bearing (psf)
- Tributary width is the strip of roof and floor that the wall carries. For an outside wall it is about half the span to the next support; for a center bearing wall it is about half of each span on either side.
- The calculator above adds roof, floor, wall and foundation weights using the typical design loads in the table below.
| Load | Value used | Basis |
|---|---|---|
| Roof dead load | 15 psf | Typical light-frame roof with shingles |
| Roof snow or live load | 20–70 psf | Use your local design snow load |
| Floor dead load | 10 psf | Typical wood floor |
| Floor live load | 40 psf | IRC living areas (sleeping rooms are 30 psf, so this is conservative) |
| Light-frame walls | 10 psf × 9 ft per story | Studs, sheathing and finishes |
| Foundation wall | 150 / 300 / 800 lb per ft | Slab edge / 3 ft crawl wall / 8 ft basement wall, 8 in concrete |
Worked example: two-story house with a crawl space
- House 28 ft deep, so each outside wall carries a tributary width of about 14 ft
- Roof: (15 + 30 psf snow) × 14 ft = 630 lb/ft
- Two floors: 2 × (10 + 40 psf) × 14 ft = 1,400 lb/ft
- Walls: 2 stories × 9 ft × 10 psf = 180 lb/ft; crawl-space wall 300 lb/ft; footing allowance 75 lb/ft
- Total load = 2,585 lb per foot of wall
- On 2,000 psf sand: 2,585 ÷ 2,000 = 1.29 ft = 15.5 in → 16 in wide; projection (16 − 8) ÷ 2 = 4 in, so 6 in thick works
- On 1,500 psf clay: 20.7 in → 21 in wide × 7 in thick (thickness grows because the projection is now 6.5 in)
Because this estimate uses full floor live loads everywhere, it is usually a little more conservative than the IRC prescriptive tables. The table value, or an engineer’s design, governs.
How deep should a house footing be?
- At least 12 in below undisturbed soil everywhere.
- Below the frost depth your building department sets. In much of the northern US that is 36–48 in or more; in warm southern areas it can be 12 in.
- On firm, undisturbed soil. Never pour on loose fill or topsoil; dig through it or have it engineered.
When you need an engineer
- Expansive clay, collapsible soil, organic soil, or fill under the footing
- Heavy loads: masonry or concrete walls, point loads from beams or posts, very large spans
- Houses beyond the scope of the IRC prescriptive tables, or on steep slopes
- Seismic Design Categories D0–D2, high-wind areas, or a building official who asks for a design
New to the terms? See footing vs foundation: the difference explained.
FAQ
What is the minimum footing size for a house?
Under the IRC, concrete footings must be at least 12 in wide and 6 in thick, with the bottom at least 12 in below undisturbed ground and below the frost line. Many houses need wider footings depending on stories, snow load and soil.
How wide should a footing be for a 2-story house?
It depends on the soil and loads. A typical two-story light-frame house with a crawl space needs roughly 16 in on 2,000 psf sandy soil and about 21 in on 1,500 psf clay. Check the IRC Table R403.1 for your edition.
How deep should house footings be?
At least 12 in below undisturbed ground and below the local frost depth, which can be 36–48 in or more in northern states.
What soil bearing capacity can I assume?
Without a soil test, IRC Table R401.4.1 allows 1,500 psf for clays and silts, 2,000 psf for sands and silty or clayey gravels, and 3,000 psf for sandy gravel and gravel.
How thick should a footing be?
At least 6 in, and at least as thick as the footing projects beyond the wall on each side.
References: International Residential Code (IRC) Sections R401.4.1, R402.2, R403.1.1, R403.1.3, R403.1.4 and R403.1.4.1. Loads in the calculator are simplified for preliminary sizing of light-frame houses. Final footing sizes must follow the code edition adopted where you build, or an engineered design.
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