How deep should footings be?

Footings must extend below the frost line to prevent frost heave. Minimum depth varies by climate zone — from 12 inches in southern states to 48+ inches in northern states (IRC Table R301.2). Footings also need a minimum 12-inch bearing on undisturbed soil.

How wide should a foundation footing be?

Footing width must be at least as wide as the wall above (for strip footings). IRC R403.1 requires minimum widths of 12 inches for 1-story, 15 inches for 2-story, and 23 inches for 3-story structures with standard soil bearing capacity of 1,500 psf.

Do footings need rebar?

IRC allows plain (unreinforced) concrete for footings in some conditions. Reinforcement is required for engineered footings, high-load applications, or when soil conditions dictate it. Minimum is typically 2 #4 bars running continuously in strip footings, with stirrups for deeper footings.

🏛️

Footing Calculator

Concrete volume, rebar schedule, and soil bearing for strip, spread & pier footings

0.00 Cubic yards

0 80-lb bags

0 Rebar (lin ft)

OK Bearing check

Footing Type

Type

Dimensions

Total length

Footing width

Footing depth

Rebar & Load

Rebar bar size

Rebar rows

Applied load

lbs

Soil bearing capacity

Results

0.00 yd³

0.0 Cubic feet

0 80-lb bags

0 60-lb bags

0 Rebar (lin ft)

0.0 Bearing area (ft²)

0 Soil pressure (psf)

Calculating…

Cross-Section Diagram

Soil Bearing Capacity by Type

Soil Class	Bearing Capacity (psf)	Description	IRC Presumptive
Crystalline bedrock	12,000	Granite, basalt, hard limestone	—
Sedimentary rock	4,000	Layered rock, soft sandstone	—
Sandy gravel / gravel	3,000	Well-graded, densely packed	Yes
Sand / gravel sand	2,000	Coarse sand, dense	Yes
Sandy clay / silty sand	1,500	Mixed soils, moderately stiff	Yes (default)
Clay	1,000	Stiff to medium clay	Yes
Soft clay / silt	500–750	Low cohesion, high plasticity	Requires testing
Organic / filled	<500	Peat, debris fill, uncompacted	Not presumptive

Note: IRC Table R401.4.1 allows presumptive bearing values without soil testing for most residential construction. Soil testing (geotechnical report) is required for high loads, unusual soil conditions, or engineered designs.

Frost Depth by Climate Zone (Approximate)

Region	Frost Depth	Example Cities
Deep South / Hawaii	0 in	Miami, New Orleans, Honolulu
South / Southwest	6–12 in	Atlanta, Dallas, Phoenix
Mid-Atlantic / Pacific NW	12–24 in	DC, Baltimore, Seattle, Portland
Midwest / Great Plains	24–42 in	Kansas City, Denver, Chicago
Northeast / Great Lakes	36–48 in	New York, Cleveland, Detroit
Northern / Mountain	48–60 in	Minneapolis, Boston, Denver
Alaska / extreme north	60–100+ in	Anchorage, Fairbanks

IRC R403 Footing Requirements

Section	Requirement
R403.1	Footings below frost line — depth per Table R301.2
R403.1.1	Min 12 in wide for 1-story, 15 in for 2-story, 23 in for 3-story (1,500 psf soil)
R403.1.2	Footing projection each side ≥ 2 in (T-footing rule)
R403.1.3	Plain concrete footings allowed in seismic zone D0 and lower
R403.1.3.1	Reinforcement required in SDC D0–D2: 2 continuous #4 bars
R403.1.4	Min 6 in clearance from soil to treated wood above grade
R403.1.6	Isolated posts: footing area to support post loads + soil bearing
R403.3	Sloped sites: step footings at max 1:10 slope per step

IRC Minimum Footing Width by Stories

Stories	1,500 psf soil	2,000 psf	3,000 psf
1	12 in	12 in	12 in
2	15 in	12 in	12 in
3	23 in	17 in	11 in

Concrete Mix — Footings

Application	Min f'c	W/C ratio
Interior footings	2,500 psi	0.58
Exterior footings (no freeze)	2,500 psi	0.50
Footings in freeze-thaw	3,000 psi	0.45
High-sulfate soil	4,000 psi	0.40

Concrete Volume Reference

Bag Size	Yield (ft³)	Bags/yd³
40 lb	0.30	90
60 lb	0.45	60
80 lb	0.60	45
Ready-mix	27 ft³/yd³	Ordered by CY

Ready-mix is more economical above ~1 yd³. Minimum ready-mix orders are typically 1 yd³ (some suppliers charge short-load fees under 5 yd³).

Rebar for Footings (ASTM A615)

Bar	Diameter	lbs/ft	Typical footing use
#3	⅜ in	0.376	Light piers, shallow footings
#4	½ in	0.668	Standard residential strip footings
#5	⅝ in	1.043	Heavy loads, commercial footing
#6	¾ in	1.502	Engineered / high-load footings

How to Size a Footing

Determine frost depth — footings must sit below the local frost line (IRC Table R301.2) to prevent frost heave lifting the foundation.

Estimate total load — sum all loads (dead load + live load + snow) from the structure above in pounds. Use tributary area × design load (typically 40–50 psf for residential).

Verify soil bearing — divide total load by footing area. The resulting pressure (psf) must be less than the presumptive or tested bearing capacity.

Calculate concrete volume — length × width × depth in feet, divide by 27 for cubic yards. Add 10% waste.

Schedule rebar — IRC R403.1.3.1 requires 2 continuous #4 bars in most reinforced footings. Place bars 3 inches from bottom for cover per ACI 318.

Key Terms

Strip Footing — A continuous concrete footing running beneath a foundation wall. Width and depth are sized to spread building loads over the soil. The most common footing type in residential construction.

Isolated Footing — A discrete pad footing beneath a single column or post. Sized based on column load divided by soil bearing capacity. Typically square or round in plan view.

Soil Bearing Capacity — The maximum load per square foot that soil can support without excessive settlement. Ranges from ~1,500 psf for clay to 8,000+ psf for bedrock. IRC Table R401.4.1 provides presumptive values for common soil types.

Frost Depth — The depth to which soil freezes in winter. Footings must bear below this depth to avoid frost heave. Required depth ranges from 0 inches in the Deep South to 60+ inches in northern states.

f'c (Compressive Strength) — Specified 28-day compressive strength of concrete in psi. IRC requires minimum 2,500 psi for interior footings and 3,000 psi for footings in freeze-thaw exposure zones.

Rebar Cover — The minimum thickness of concrete between the rebar and the nearest surface. ACI 318 requires 3 inches of cover for footing bars cast against earth — this protects steel from moisture and corrosion.

Frequently Asked Questions

How thick should a residential footing be?

IRC R403.1 requires footing thickness to be at least as thick as the projection of the footing beyond the wall face — and never less than 6 inches thick. Most residential strip footings are 8–12 inches thick. Deeper footings are used in poor soil or for heavier loads.

When do I need a geotechnical (soil) report?

IRC R401.4 allows presumptive bearing values for most residential construction without testing. A geotech report is needed when: the site has fill, organic soil, or unusual conditions; the structure is unusually heavy or tall; the AHJ (building department) requires it; or the design uses engineered foundations.

Can I pour footings in cold weather?

Concrete must be kept above 50°F for proper curing (ACI 306). In cold weather, use accelerators, insulated blankets, or heated enclosures. Never pour on frozen ground. IRC and ACI cold-weather concreting practices apply below 40°F ambient temperature.

What rebar cover is required in footings?

ACI 318 Section 20.6.1.3 requires 3 inches of concrete cover for reinforcement cast against and permanently exposed to earth (like footings). This means the rebar must be at least 3 inches from the bottom of the footing.

What size footing do I need for a deck post?

IRC R507.3.1 provides prescriptive deck footing sizes based on load area. A typical 4×4 post supporting 80 sq ft of deck area needs at least a 12-inch diameter × 12-inch deep footing in 1,500 psf soil. Larger footings are needed for 6×6 posts or heavier loads.