Concrete Footing Calculator
Strip & Pad Footings
Calculate the exact cubic yards, 80lb bag count, and material cost for any concrete footing. Handles both continuous strip footings for foundation walls and isolated pad footings for posts and columns. Includes IRC frost depth table and ACI bearing requirements.
Know your footing volume before you break ground. Enter footing type and dimensions to get cubic yards, bag count, and a ready-mix vs bagged cost comparison instantly.
Runs continuously under a foundation wall
Total linear feet of continuous footing
Footing width, IRC minimum 12" for 1-story
Depth below grade, must clear local frost line
ACI recommends 10% for all below-grade pours
National avg $165–$210/CY. Footings often need a small-load premium.
Enter footing dimensions
to calculate cubic yards and bags
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The concrete footing formula
Concrete footings are measured in cubic yards, just like slabs and columns. The calculation converts your footing dimensions to cubic feet first, then divides by 27. Strip footings require one calculation for the total run. Pad footings multiply a single footing volume by the count.
Strip Footing Volume
CY = (Length ft × Width in ÷ 12 × Depth in ÷ 12) ÷ 27 × 1.10
Pad Footing Volume
CY = (Width in ÷ 12 × Length in ÷ 12 × Depth in ÷ 12 × Count) ÷ 27 × 1.10
The 1.10 multiplier represents the ACI-recommended 10% overage for below-grade pours. Form faces are never perfectly parallel and the subgrade is rarely perfectly flat: the extra volume covers these variances and prevents a short pour. Use the concrete volume calculator if your project combines footings with slab or column pours in a single order.
A quick field check: a typical residential continuous footing 40 feet long, 12 inches wide, and 18 inches deep requires 2.96 CY base volume. With 10% overage, order 3.26 CY. That is approximately 147 bags of 80lb mix , clearly a ready-mix job. The break-even between ready-mix and bagged concrete is around 1 to 1.5 cubic yards once short-load delivery fees are factored in. The calculator above runs that comparison automatically.
Frost depth by region
The single most critical dimension for any footing is depth below the frost line. Soil water expands approximately 9% when it freezes, generating enormous upward force, frost heave. A footing above the frost line moves with the frozen soil every winter and returns to grade every spring, cracking anything attached to it. The frost depth varies dramatically by climate and must match your local building code, not a national average.
| Region | Typical Frost Depth | Representative States |
|---|---|---|
| Deep South / Gulf Coast | 0–6" | FL, LA, coastal TX, coastal GA |
| South / Southwest | 6–12" | TX, AZ, NM, inland GA, SC |
| Mid-Atlantic / Pacific NW | 12–24" | VA, NC, TN, OR, WA coast |
| Midwest / Mid-South | 24–36" | OH, IN, IL, MO, KS, CO |
| Upper Midwest / Mountain | 36–54" | MN, WI, MI, IA, ND, SD, ID, WY |
| Northern New England / Alaska | 54–72"+ | ME, VT, NH, MT, AK |
These are regional ranges: your local building department has the code-required depth for your specific jurisdiction. The IRC 2024 provides a national frost depth map (Figure R403.3(1)) but local amendments often set higher minimums. Always confirm with your building department before digging. Frost depths in urban heat islands can be shallower than surrounding rural areas, but building codes do not account for this, use the code-required depth regardless.
IRC minimum footing dimensions
IRC 2024 Table R403.1 specifies minimum footing widths and thicknesses based on the number of stories the footing supports and the assumed soil bearing capacity. The table assumes 1,500 psf soil bearing: the default for most residential sites without a soils report. If your soil is weaker (soft clay, fill, organic soils), wider footings are required and a geotechnical report is strongly recommended.
| Stories Supported | Min Width | Min Thickness | Min Rebar |
|---|---|---|---|
| 1-story | 12" | 6" | 2 × No. 4 bars |
| 2-story | 15" | 7" | 2 × No. 4 bars |
| 3-story | 18" | 8" | 2 × No. 4 bars |
The footing must be at least as wide as the wall it supports and always at least as wide as the footing is thick. A 6-inch concrete wall cannot sit on a 6-inch wide footing: the footing must project at least 2 inches beyond each face of the wall per ACI 318-25. For stem walls and basement walls, the footing projection creates the bearing ledge. Use the concrete slab calculator if you are combining the footing pour with a structural slab.
Isolated pad footings for deck posts and columns do not have a prescriptive size in the IRC, they must be engineered based on the post load and soil bearing capacity. A common residential rule of thumb is a 20-inch square footing, 8 to 10 inches thick, for a standard 4×4 or 6×6 deck post in 1,500 psf soil. For heavier loads, consult a structural engineer.
Rebar requirements for footings
IRC R403.1.3 requires that all continuous footings for load-bearing walls contain a minimum of two No. 4 (1/2-inch diameter) horizontal reinforcing bars, placed continuously and lapped at corners. This applies to all footings: not just those in seismic or wind zones. The rebar controls thermal cracking and prevents the footing from splitting along its length under differential settlement.
| Requirement | IRC / ACI Standard | Practical Note |
|---|---|---|
| Minimum bar size | No. 4 (1/2" dia.) | No. 5 recommended for 2-story and above |
| Minimum bar count | 2 continuous bars | One each side of footing centerline |
| Clear cover (bottom) | 3 inches minimum | Use dobies or chairs, never lay on soil |
| Clear cover (sides) | 2 inches minimum | Check with your form width |
| Corner laps / splices | 24 bar diameters | 12" minimum for No. 4 bar |
| Seismic Design Category C–F | Additional hooks required | Confirm with local engineer |
A common field error is placing rebar directly on the soil at the bottom of the form. Rebar in contact with the ground corrodes within a few years, losing tensile strength. Concrete cover is the only corrosion protection, always support rebar on plastic chairs or concrete dobies to maintain the 3-inch minimum bottom cover required by ACI 318-25 Section 26.6.1.
For T-intersections where interior walls meet perimeter footings, IRC requires the rebar to be lapped and tied: not just butted together. A common detail is bending one bar at 90° around the corner and overlapping the straight bar by 24 diameters minimum. Rebar chairs should be spaced no more than 4 feet apart to prevent the bar from deflecting below the minimum cover under the weight of the fresh concrete.
Footing cost benchmarks
Footing cost varies significantly by volume, access, and local labour markets. Material-only estimates are straightforward from the calculator above. Installed footing cost involves excavation, forming, pouring, and finishing, each step adds labour. The benchmarks below reflect national averages for standard residential continuous footings; complex projects, remote sites, and difficult soil add cost.
| Footing Type | Materials Only | Installed (incl. labour) | Notes |
|---|---|---|---|
| Strip footing, 12" × 8" | $3–$5/lf | $8–$14/lf | Shallow residential, good access |
| Strip footing, 16" × 12" | $6–$9/lf | $14–$22/lf | 2-story or wider foundation wall |
| Pad footing, 20" × 20" × 10" | $18–$25 each | $55–$90 each | Deck or porch post, includes tube form |
| Pad footing, 24" × 24" × 12" | $28–$38 each | $90–$140 each | Heavier column or multi-story post |
Short-load fees are a common cost surprise on footing projects. Most ready-mix plants charge $50 to $200 when you order under 3 to 5 cubic yards: the plant minimum. A typical residential perimeter footing for a small addition runs 2 to 4 cubic yards, which is below the minimum at most plants. Either pay the short-load fee, combine your pour with the slab, or switch to bagged concrete for volumes under 1.5 cubic yards. The calculator shows the break-even automatically. Use the concrete cost calculator to model the full project cost including delivery and short-load fees. The concrete footing calculator guide covers the complete formula, IRC frost depth requirements, and rebar minimums with worked examples.
Reading your footing result
Your result shows the concrete volume for the footing dimensions entered. Cross-check against the IRC 2024 Table R403.1 minimum footing widths: one-story buildings on medium soils (1,500 psf bearing capacity) require a minimum 12-inch-wide continuous footing; two-story construction requires 15 inches; three-story, 23 inches. If your footing is narrower than the minimum for your building type, the result is code-non-compliant regardless of soil conditions , widen before ordering.
Frost depth is the other dimension that must be verified locally. The IRC requires footings to extend below the frost depth for your climate zone: ranging from 12 inches in the deep South to 48–60 inches in northern states. A footing that meets the minimum width but sits above the frost line will heave in winter and cause structural cracking. Confirm your local frost depth with your building department or the IRC Figure R403.3(1) contour map before finalising your footing depth input.
Frequently asked questions
How deep should a concrete footing be?
Footings must extend below the local frost line to prevent frost heave. IRC 2024 Section R403.1.4 requires footings to be placed at least 12 inches below undisturbed ground surface and below the frost penetration depth. Frost depths range from zero in the deep South to 72 inches or more in northern New England and Minnesota. Always confirm the required depth with your local building department, local code amendments sometimes set depths higher than the IRC map.
What is the minimum footing width for a load-bearing wall?
IRC 2024 Table R403.1 requires minimum widths of 12 inches for 1-story structures, 15 inches for 2-story, and 18 inches for 3-story, assuming 1,500 psf soil bearing capacity. The footing must always be at least as wide as the wall it supports and must project at least 2 inches beyond each face of the wall per ACI 318-25. For soft or fill soils, wider footings are required and a soils report is recommended.
What is the difference between a strip footing and a pad footing?
A strip footing (continuous footing) runs without interruption under a foundation wall, spreading wall load along its length. It is used for load-bearing walls, basement perimeters, and porch walls. A pad footing (isolated footing) is a discrete square or rectangle placed under a single column or post, carrying a concentrated point load. The two types use the same concrete but differ in geometry and rebar layout. Strip footings use longitudinal bars running the length of the footing; pad footings use a grid of bars in both directions.
Can I pour a concrete footing in cold weather?
Yes, with precautions. ACI 306R-16 requires that concrete be maintained above 50°F for at least 7 days after placement. In practice this 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 will thaw and settle, causing the footing to crack. If overnight lows are expected below 40°F, protect the fresh concrete with insulating blankets. Cold-weather concrete surcharges from ready-mix plants typically add $10 to $25 per CY.
What size rebar do I need in a concrete footing?
IRC R403.1.3 requires a minimum of two No. 4 (1/2-inch diameter) horizontal bars placed continuously through all load-bearing wall footings. Rebar must have a minimum 3-inch clear cover from the bottom of the footing and 2-inch cover from the sides per ACI 318-25. Bars must be lapped a minimum of 24 bar diameters at corners and splices, 12 inches for No. 4 bar. Rebar must be supported on plastic chairs or concrete dobies, never placed on soil.
Do I need a footing for a deck post?
Yes. Most jurisdictions require a concrete footing below the frost line under every deck post. A common residential approach is a 10 to 12-inch diameter tube form poured to the frost depth with a 20-inch flared base or footing pad. The post sits on a post base connector anchored into the concrete. Some regions allow precast concrete deck piers on non-frost areas, but poured footings are the most reliable and widely accepted method. Confirm depth requirements with your local building department before digging.
What happens if a footing is not below the frost line?
A footing above the frost line will heave upward as soil freezes each winter and settle back as it thaws each spring. This annual cycle cracks foundation walls, misaligns door frames, and fatigue-cracks structural connections. After several seasons the damage compounds to the point where doors no longer close, windows rack out of square, and visible cracks open in foundation walls and attached slabs. Frost heave damage is typically not covered by homeowner insurance because it is classified as a maintenance failure. The repair requires excavating and re-pouring the footing to the correct depth.
References
American Concrete Institute. (2025). ACI 318-25: Building Code Requirements for Structural Concrete. ACI.
American Concrete Institute. (2016). ACI 306R-16: Guide to Cold Weather Concreting. ACI.
International Code Council. (2024). International Residential Code for One- and Two-Family Dwellings. ICC.
RSMeans. (2025). Residential Cost Data. Gordian.