Concrete Footing Calculator

What Is a Concrete Footing?

A concrete footing (also called a footer) is the lowest element of a foundation — a widened concrete pad or continuous strip that spreads the building's load across enough soil to keep the bearing pressure within what the ground can carry. The footing bears directly on undisturbed soil; the stem wall, grade beams, and slab built on top of it make up the rest of the foundation. Two things make or break a footing: it must reach below the frost line, and it must be wide enough for the soil beneath it.

IRC R403.1 sets the residential minimums — a footing at least 6 inches thick, generally twice the width of the wall it supports, bearing at least 12 inches below grade and below the local frost line. Footings are normally 3,000 PSI concrete with two continuous #4 rebar in the lower third. This calculator estimates the volume, bag count, weight, and cost for strip, pier, and pad footings; for a combined slab-and-footing pour, use our slab and footing calculator.

Frost Depth: How Deep Must a Footing Go?

Frost heave is the number-one cause of footing failure. When soil moisture freezes it expands and lifts anything bearing above the frost line; thaw drops it back unevenly, cracking walls and floors over a few winters. The fix is simple — bear the footing below the depth at which the ground freezes.

• Deep South (Gulf Coast, Florida) — frost depth near 0 in; footings still bear a minimum 12 in below grade on undisturbed soil
• Mid-South / lower Midwest — roughly 12–24 in
• Northern tier (upper Midwest, Northeast) — roughly 36–48 in
• Coldest regions (northern MN, ND, northern New England) — 60 in or more

There is no national frost-depth figure — your local building department publishes the design value for your jurisdiction, and that is the depth your excavation must beat. Always bear on undisturbed native soil, never on loose fill.

Soil Bearing Capacity and Footing Width

Footing width comes from a simple balance: the load pressing down divided by the footing's contact area must stay below the soil's safe bearing capacity. IRC Table R401.4.1 gives presumptive load-bearing values you can use without a soils report:

• Soft clay / silt — about 1,500 psf
• Sandy or stiff clay, clayey gravel — about 2,000 psf
• Sand, silty/clayey gravel mixes — about 2,500 psf
• Gravel, sandy gravel — about 3,000 psf

Weaker soil or heavier loads need a wider footing to keep pressure in check. As a starting point under IRC R403.1, a 16-inch-wide footing carries a standard 8-inch foundation wall, and a 20-inch footing carries a 10-inch wall — widen from there as the load demands. Size the steel for that width with our rebar calculator.

Stepped Footings on a Sloped Lot

On sloping ground you don't pour the footing at an angle — you step it. Each run stays level and bears at or below frost depth, then drops to the next level with a vertical step where they lap together.

• Maximum ground slope — IRC R403.1.5 limits the slope of the bottom surface to 1 unit vertical in 2 horizontal (a 2:1 grade)
• Step height — keep individual vertical steps to about 24 inches or less
• Lap the corners — overlap the upper and lower footing at each step and carry the rebar through so the steps act as one continuous footing
• Bear undisturbed — every horizontal section must sit on undisturbed soil, never on fill that was placed to level a step

Footing Construction Tips

• Dig below the frost line — the single most important rule; confirm the local design frost depth before excavating
• Bear on undisturbed soil — never pour on disturbed or filled ground without engineering
• Place the rebar correctly — at least two #4 bars in the bottom third with 3 inches of cover from the soil, plus vertical dowels to tie the wall above
• Wet the excavation — lightly dampen dry soil before pouring so it doesn't pull water out of the fresh concrete
• Protect the cure — cover footings and keep them moist for at least 7 days before building on them
• Build the wall on top — once footings cure, estimate the foundation wall volume with our concrete wall calculator and order both pours together

How Much Does a Concrete Footing Cost?

Concrete footing costs depend on the footing type, size, and whether you hire a contractor or DIY. For materials alone, ready-mix concrete runs $130–$165 per cubic yard delivered, while 80 lb bags cost about $5.50–$7.00 each (roughly $185–$235 per cubic yard when buying bags). A typical residential strip footing for a 40-foot wall — roughly 2.2 cubic yards with waste — costs $285–$365 in concrete alone. Five round pier footings for a deck average $35–$55 in bagged concrete.

Labor is the biggest cost multiplier. Professional footing installation runs $6–$12 per linear foot for strip footings and $75–$200 per pier, including excavation, forming, rebar, and concrete. A 120-linear-foot perimeter footing for a small addition typically costs $1,500–$3,500 fully installed, plus $150–$400 for a permit and inspection in most jurisdictions. Use our concrete cost calculator for a detailed material breakdown.