Concrete Footing Calculator

How the footing calculator works

A concrete footing is a continuous strip or pad that spreads a building's load onto the soil. The calculator uses the same rectangular prism formula as a slab: length × width × thickness, with the thickness converted from inches to feet (÷ 12). For strip footings, length is the wall length and width is the footing width. For pad footings (under a single post or column), the "length" input is 1 (single pad) or the number of pads.

Standard footing dimensions are 16 inches wide × 8 inches thick for a single-story wood-framed wall on typical soil, 24 inches wide × 12 inches thick for a two-story or load-bearing wall. The width is determined by the soil's bearing capacity — sandy soil at 2000 psf wants a wider footing than gravelly soil at 4000 psf. Your local code dictates minimum widths; never pour narrower than code allows.

A worked example: a 20-foot strip footing at 16″ wide and 8″ thick. Volume = 20 × (16/12) × (8/12) = 17.78 cubic feet, or 0.66 cubic yards before waste. With 10% waste, gross = 0.73 yards, rounded up to 0.75 yards (ready-mix minimum). In bags: 30 eighty-pound bags, 40 sixty-pound bags, or 60 forty-pound bags.

Footing types — strip, pad, and combined

Strip footings run continuously under a wall and are the most common residential footing type. They're economical because the form work is simple (a 2×8 form on each side of the trench) and the pour is continuous. Standard strip footing is 16 inches wide × 8 inches thick; wider versions (24 inches) for two-story or load-bearing walls.

Pad footings (also called spread footings or isolated footings) support a single point load — a post, pier, or column. They're typically square: 12×12, 16×16, 24×24, or 30×30 inches, and 8–12 inches thick. The size is determined by the column load and soil bearing capacity. A typical residential deck post sits on a 12×12×8 pad footing; a two-story house column may need a 24×24×12 pad.

Combined footings support two or more columns close together where individual pads would overlap. They're L-shaped, trapezoidal, or rectangular and engineered to distribute the loads evenly. Combined footings are common in commercial construction and large residential projects; they require engineering design and are beyond the scope of this calculator.

For any footing carrying a structural load (house foundation, retaining wall, deck support), the dimensions must be specified by an engineer or stamped on the building permit. The calculator here gives you a materials estimate once you know the dimensions; it does not tell you what those dimensions should be. Footings are inspected before pouring in most jurisdictions.

Rebar, drainage, and what the footing estimate excludes

Most modern codes require rebar in footings — typically two #4 bars continuous for residential footings, or two #5 bars for load-bearing walls. The bars sit on rebar chairs 3 inches from the bottom of the footing (1/3 of footing depth for an 8-inch footing). For a 20-foot strip footing, you need 40 linear feet of #4 bar — about $25 of rebar plus $5 of chairs. Use the rebar calculator on this site for any footing or slab rebar estimate.

Form work is the second-largest exclusion. Strip footings require a 2×8 form on each side of the trench, braced every 4 feet with stakes and kickers. A 20-foot strip footing uses 4 2x8x10 boards ($24), 10 stakes ($10), and 20 kicker braces ($15). Pad footings are often poured in excavated holes without forms if the soil is cohesive; otherwise use 2× forms.

Other exclusions: excavation (renting a mini-excavator at $200/day or hiring a backhoe at $75/hour), soil compaction (plate compactor $40/day), rebar tying wire ($5), form-release agent ($10), anchor bolts or post bases (set into wet concrete), inspection fees ($50–200 depending on jurisdiction), and disposal of excavated soil ($200–500 per load). A realistic all-in footing budget is 2× the concrete cost for materials-only estimate.

Concrete for footings should be a 3000–4000 psi mix with 4–6% entrained air in freeze-thaw climates. Specify the mix design when ordering ready-mix — the batch plant will provide a delivery ticket with the design strength and any admixtures. For bagged concrete, use a crack-resistance or fiber-reinforced mix for footings; the fibers reduce shrinkage cracking during the long cure.

Common mistakes and how to avoid them

The most dangerous footing mistake is pouring on undisturbed or organic soil. Topsoil, fill dirt, and buried vegetation decompose and settle over time; a footing on this material will drop unevenly and crack the wall above. Always excavate to solid bearing soil — clay, sand, or gravel — and never pour on topsoil. If you encounter soft spots during excavation, dig deeper and backfill with compacted gravel or concrete. The cost of over-excavation is small; the cost of a cracked foundation is enormous.

A second common mistake is undersized footing width for the soil bearing capacity. Sandy soil at 2000 psf requires a wider footing than gravelly soil at 4000 psf for the same wall load. A typical single-story wood-framed wall exerts 1000-1500 pounds per linear foot; on 2000 psf soil that needs 6-8 inches of width per foot of wall, or 16-24 inches total. On 4000 psf soil the same wall needs 8-12 inches. When in doubt, pour wider — the additional concrete is cheap compared to a footing failure.

Rebar placement is the third common error. IRC R403.1.2 and ACI 318 require rebar in residential footings — typically two #4 bars continuous. The bars must sit on rebar chairs 3 inches from the bottom of the footing, not on the soil and not in the middle of the pour. Rebar on the soil does nothing; rebar in the middle of an 8-inch footing is at the neutral axis and provides half its potential strength. Buy rebar chairs — they cost $0.10 each — and set the bars correctly before the pour.

Finally, do not pour a footing without forms unless the soil is cohesive clay that holds a vertical edge. Sandy or rocky soil will cave into the footing trench, contaminating the concrete and reducing the effective width. Use 2×8 lumber forms on both sides of the trench, braced every 4 feet with stakes, and you will get a footing of the correct width and thickness. Forms also make it easier to strike off the top of the pour level.