Footing calculator · United States

Concrete Footing Calculator

Q: What size rebar goes in a footing?

IRC R403.1.3 requires two No. 4 (1/2 inch / 12 mm) horizontal bars continuously through load-bearing wall footings with 3-inch clear cover from the bottom.

Q: How much concrete for a deck footing?

A 12-inch diameter sonotube 4 feet deep needs about 0.12 cubic yards. Six piers that size = 0.75 cubic yards. Round up to 1 yard when ordering.

Strip, pad, and pier footings — with frost depth guidance, rebar schedule, and the volume figure you order ready-mix against.

Footing Estimator · v3.2 · US

Dimensionsstrip footing

Quantity

Waste

Frost depth & siteauto-suggests by region

Region

Local frost depth: — · select region

Reinforcement

Rebar

Wall dowels

Concrete price ($ / yd³)

Materials Order

Order this much — cu yd — per footing × — qty

Raw volumebefore waste—
Cubic metressecondary unit—
80 lb bags~0.022 yd³ each—
60 lb bags~0.017 yd³ each—
Longitudinal rebar——
Transverse rebar——
Wall dowels——
Cost——

Footings total—

Per footing—

Excavation—

Frost depth req.—

This is a quantity estimator, not a structural design. Footing sizing depends on the building's load, the soil's bearing capacity, and local code. Use this calculator to order materials — but a P.E., CEng, CPEng, or P.Eng signs the actual footing schedule for any structural application.

— Three footing types

Strip, pad, and pier — when to use each.

Most residential and small-commercial footings fall into one of three categories. The geometry is different, but they all do the same job: spread the load from above onto soil that can carry it.

Continuous

Strip footing

A long, rectangular footing under a load-bearing wall. Most common residential type — supports perimeter walls of houses, garages, and outbuildings. Typically 16–24 inches wide (400–600 mm), 8–12 inches deep (200–300 mm), running the full length of the wall above.

Isolated

Pad footing

A discrete square or rectangle under a column or post. Common in post-frame buildings, deck post bases, and pergolas. Sized to spread a concentrated point load across enough soil area to stay below the soil's bearing capacity. Reinforced with a grid of bars in both directions.

Cylindrical

Pier / sonotube

A cylindrical footing poured in a tube form (Sonotube and similar). Used for deck posts, fence posts, mailbox bases, and any application where a discrete vertical column transfers load to soil below frost. Usually 8–16 inches diameter (200–400 mm), poured below local frost depth.

The three share the same rule: the bottom of the footing must rest on undisturbed soil, below the local frost line. Footings poured above frost depth heave in winter as the water in the soil beneath them freezes and expands — over multiple freeze-thaw cycles, that movement cracks foundation walls, leans deck posts, and tilts fences.

— Frost depth requirements

Where the bottom of the footing has to be.

US frost depths vary from the warm South (where freezing barely penetrates) to the upper Midwest and Northeast (where frost can drive 4 feet deep). The IRC R403.1.4 requires footing bottoms below frost line and at least 12 inches below grade.

Typical US frost depths by region
Region	States (examples)	Frost depth
Deep South	FL, southern TX, southern LA, AZ, southern CA	12" or none
South / mid-South	GA, AL, MS, NC, SC, TN, AR, NM	12–18"
Mid-Atlantic / OH Valley	VA, KY, MD, southern OH, southern IL	24–30"
Upper Midwest / Northeast	NY, PA, MA, MI, WI, IL, IA, MO	36–42"
Cold zone	MN, ND, SD, MT, ME, VT, NH	42–60"
Mountain / alpine	CO > 8,000 ft, WY, parts of UT, ID	48–72"+

The calculator above uses these ranges to suggest a minimum footing depth. Your local building department's published number is the governing one — always confirm before excavating.

Frost depth in the UK rarely exceeds 450 mm even in upland areas, but the more important driver is shrinkable clay near trees. The NHBC Standards Chapter 4.2 sets minimum trench depths for traditional strip footings based on soil type and proximity to vegetation. On normal stable ground, 750–900 mm is typical; near trees on shrinkable clay, depths can exceed 2.5 metres.

UK typical footing depths (NHBC indicative)
Site condition	Soil type	Trench depth
Rock / gravel	Stable, high bearing	450–600 mm
Firm clay, no trees	Stable shrinkable clay	750 mm
Firm clay, distant trees	Shrinkable clay, trees 10–20 m away	900–1200 mm
Shrinkable clay, near trees	Within tree influence zone	1500–2500 mm+
Made ground / fill	Variable bearing	To undisturbed soil
Upland / Scottish highlands	Frost-prone	600–750 mm (frost)

Approved Document A of the UK Building Regulations requires foundations on suitable bearing stratum below any zone affected by frost, vegetation, or made ground. Trial pits and soil investigation are essential on any clay site.

Most of Australia doesn't worry about frost. The defining factor for Australian residential footings is soil reactivity under AS 2870, which classifies sites from Class A (stable rock) through Class P (problem site). Each class drives footing depth and edge-beam dimensions.

Australian footing requirements by AS 2870 site class
Site class	Soil type	Typical footing
Class A	Stable rock	400 mm strip / standard pad
Class S	Slightly reactive	500 mm strip / standard waffle pod
Class M	Moderately reactive clay	600 mm edge beam, deepened corners
Class H1 / H2	Highly reactive clay	700–800 mm edge beam, stiffened raft
Class E	Extremely reactive	Engineered raft, 800+ mm beams
Class P	Problem site (uncontrolled fill, slope)	Individual engineering required

Alpine areas (Snowy Mountains, Tasmania highlands) do see frost — add 600–900 mm depth in those zones. Coastal sites within 1 km of surf carry an exposure-class requirement (B1, B2) under AS 3600 that drives cover and minimum grade.

Canadian frost depths are among the deepest in the world. The National Building Code of Canada sets minimum cover for foundations, and provincial amendments specify actual frost depths. The numbers below are typical residential ranges; municipal codes always govern.

Typical Canadian frost depths by region
Region	Examples	Frost depth
Pacific coast	Vancouver, Victoria, Lower Mainland	0.6–0.9 m
Southern Ontario	Toronto, Hamilton, Ottawa, London	1.2 m
Maritime provinces	Halifax, Moncton, Charlottetown	1.2–1.5 m
Quebec / Atlantic	Montreal, Quebec City, Trois-Rivières	1.5–1.8 m
Prairies	Calgary, Edmonton, Saskatoon, Winnipeg	1.5–1.8 m
Northern / arctic	Yellowknife, Whitehorse, Iqaluit	1.8–2.4 m+

For a frost-protected shallow foundation (FPSF) — an alternative to digging below frost depth — see CSA A23.1 Annex U for foam insulation and skirt requirements.

— Rebar schedule

What goes inside the footing.

Under the IRC, a load-bearing wall footing needs at least two continuous No. 4 bars running lengthwise, set 3 inches off the bottom of the trench on rebar chairs. Lap splices at corners and joints are 12 inches minimum (24 bar diameters for No. 4). Wider footings over 24 inches add transverse bars at 16-inch centers to control shear.

Strip footing — standard. 2 × No. 4 bars longitudinally, lap splices 12" at corners, supported on plastic chairs 3" off the bottom.
Strip footing — wider than 24". Add transverse No. 4 bars at 16" o.c. for shear control.
Pad footing. Grid of No. 4 bars at 12" o.c. each way, 3" cover bottom and sides.
Pier footing. 2–4 vertical No. 4 bars tied in a cage, centered in the tube with 2–3" cover.
Wall dowels. Vertical No. 4 dowels at 48" o.c. tied to longitudinal bars, extending up into the wall above. Required for any block or concrete wall on the footing.

All bar covered per ACI 318-19 §20.5.1.3.2: 3" minimum when cast against earth, 2" against forms.

UK domestic strip footings are typically reinforced with two 12 mm bars (or H10 high-tensile) running continuously, with cover requirements set by BS 8500 Table A.5 based on exposure class. For most foundations at XC2 exposure, 40 mm nominal cover applies.

Strip footing — domestic. 2 × 12 mm bars longitudinally, lap splices 500 mm at corners, 40 mm cover.
Strip footing — wider than 600 mm. Add transverse 12 mm bars at 400 mm centres.
Pad footing. Grid of 12 mm at 200 mm centres each way, 40 mm cover bottom and sides.
Pier / pile footing. 4 × 12 mm vertical bars in a cage with 8 mm links at 200 mm spacing.
Wall dowels / starter bars. 12 mm vertical bars at 600–800 mm centres, hooked into the longitudinal bars, projecting 500 mm into the wall above.

For larger foundations or commercial work, Eurocode 2 (BS EN 1992) governs the design.

Australian residential footings to AS 2870 typically use N12 deformed bars (12 mm Normal-ductility) at spacings driven by site class. Cover follows AS 3600 exposure class — 30 mm for A1 inland, 40–50 mm for B1/B2 coastal.

Strip footing — Class M / H. 2 × N12 longitudinally, ligatures (R8 stirrups) at 600 mm centres for stiffened raft edge beams.
Pad footing. N12 grid at 300 mm centres each way, ligatures around perimeter, 40 mm cover.
Pier / pile footing. 4 × N12 longitudinal bars in a cage with R6 helical ties at 100 mm pitch.
Starter bars to walls. N12 vertical bars at 600 mm centres for block walls, 400 mm for retaining walls.

For coastal sites (B2 exposure within 1 km of surf, or C1/C2 splash zones), cover increases to 45–65 mm and N16 may replace N12 to provide section ductility against chloride-induced corrosion.

Canadian footings use metric M-bar nomenclature — 15M bars (15 mm nominal diameter) are the residential standard, with cover per CSA A23.1. 75 mm minimum when cast against earth; 50 mm against forms.

Strip footing — residential. 2 × 15M bars longitudinally, lap splices 400 mm at corners, 75 mm cover bottom.
Strip footing — wider than 600 mm. Add 15M transverse bars at 400 mm centres.
Pad footing. 15M grid at 300 mm centres each way, 75 mm cover all around.
Pier / sonotube footing. 4 × 15M vertical bars in a cage centered in the tube.
Wall dowels. 15M vertical bars at 600 mm centres minimum, hooked into the longitudinal bars.

For exterior footings exposed to de-icing salts (C-class exposure), epoxy-coated rebar is sometimes specified — usually in commercial work or where salt-spray distance is short. Residential below-grade footings at F-class exposure use standard rebar with proper cover.

— Common footing mistakes

Where DIY footings go wrong.

Pouring above the frost line. The single most consequential error in DIY foundation work. A footing above frost will heave 1–4 inches in winter, then settle unevenly in spring. Over several freeze-thaw cycles, that movement cracks walls, leans deck posts, and tilts fence posts.
Pouring on disturbed soil. The bottom of the trench must rest on undisturbed soil (or properly compacted engineered fill). Loose backfill at the bottom will settle and the footing will crack regardless of how much concrete is in it.
Rebar resting on the soil. Rebar needs concrete cover beneath it, not just above. Plastic chairs or concrete dobies hold bars 3 inches off the soil. Rebar in direct contact with earth rusts and breaks the bond with the surrounding concrete.
Skipping the gravel base. A 4–6 inch compacted gravel layer under the footing provides drainage and a stable platform. Without it, even good soil settles unevenly during freeze-thaw cycles and the footing tilts.
Tube footings without a flared base. A 12-inch sonotube has 0.78 sq ft of bearing area — that's a lot of load per square foot. A flared concrete base (or a 24-inch pad at the bottom) spreads the load over enough soil to stay below the safe bearing capacity on most residential soils.
Forgetting wall dowels. If a block or concrete wall is going on the footing, vertical dowels must be set into the wet concrete before it cures. Drilling them in later is possible but expensive and structurally inferior.

The cost of a bad footing. Replacing a failed footing means tearing out everything above it — the wall, the slab, sometimes the structure. A 4-foot deck post on a heaved sonotube can pull joists out of hangers across the entire span. Pour it deep enough, on solid undisturbed soil, with proper rebar and dowels, the first time.

Reviewed by Jordan Mireles, P.E.Licensed civil engineer · 14 years residential and light commercial concrete. Footing dimensions and rebar schedules verified against IRC 2024 R403, ACI 318-19 §13 (Foundations) and §20.5 (Cover), and ACI 332-20 residential code. Last reviewed May 2026.

— Common footing questions

Quick answers.

How deep should a concrete footing be?

Below the local frost line and on undisturbed soil. Southern US: 12–18 inches. Northern US: 36–48 inches. Most of the UK: 750–900 mm (deeper near trees on shrinkable clay). Canada: 1.2 m in southern Ontario, 1.5–1.8 m across the Prairies and Quebec, up to 2.4 m in the North. Australia: above frost depth in most regions, with site-class footings per AS 2870.

How wide should a footing be?

Footing width depends on wall thickness, load, and soil bearing capacity. IRC default: 12 inches wide for one-story, 15 inches for two-story, 18 inches for three-story on 2000 psf soil. Rule of thumb: footing width is typically twice the wall thickness — 8" wall sits on a 16" footing, 10" wall on a 20" footing.

What size rebar goes in a footing?

IRC R403.1.3 requires a minimum of two No. 4 (1/2 inch / 12 mm) horizontal bars placed continuously through all load-bearing wall footings, with 3 inch (75 mm) clear cover from the bottom. Pad footings use a grid of bars in both directions. Wider footings over 24 inches add transverse bars at 16–18 inch spacing.

How much concrete for a deck footing?

A 12-inch diameter sonotube footing 4 feet deep needs about 0.12 cubic yards (3.14 cubic feet). For a deck with 6 piers at that size, plan on about 0.75 cubic yards. Round up to 1 yard when ordering ready-mix to account for short-load fees — extra goes into a fence post hole or pier you have ready.

What is the difference between a footing and a foundation?

The footing is the wider concrete base at the very bottom that distributes load to the soil. The foundation wall sits on top of the footing and rises to support the building. Footings are always reinforced and sized for soil bearing; foundation walls are sized for vertical and lateral loads from the structure and any soil pressure from outside.

Can I pour a footing in winter?

Yes, with precautions. ACI 306R-16 requires that concrete be maintained above 50°F (10°C) for at least 7 days after placement. In practice that means heating the formwork and subgrade before pouring, using accelerating admixtures or Type III cement, and insulating immediately after the pour. Never pour on frozen ground — the subgrade thaws and settles, cracking the footing.

— Related calculators

Concrete Footing Calculator

Materials Order Live

Strip, pad, and pier — when to use each.

Strip footing

Pad footing

Pier / sonotube

Where the bottom of the footing has to be.

What goes inside the footing.

Where DIY footings go wrong.

Quick answers.

Estimating something specific?

Slab

Yard / Volume

Block

Cost

Materials Order