Concrete Footing Calculator

Footing Types and When to Use Each

A footing is the widened concrete base that spreads a structural load over enough soil that the ground can carry it without settling. The footing is always wider than the wall or column it supports, which is what lets a thin wall sit on weak soil. Three types cover almost all residential and light-commercial work:

• Continuous (strip) footing — a long, rectangular footing that runs unbroken beneath a load-bearing wall, foundation stem wall, or CMU block wall. This is the most common type and the default for perimeter foundations.
• Spread (pad/isolated) footing — a square or rectangular pad beneath a single point load: a steel column, a porch post, a deck post, or a beam pocket. Sized individually to the load it carries.
• Stepped footing — a strip footing that drops in level "steps" to follow a sloped lot or to reach consistent bearing soil, keeping each section level and below frost depth without over-excavating the high side.

T-shaped (spread-and-stem) footings combine a wide base with an integral stem and are common where frost depth is deep. Enter length, width, and depth below to get cubic yards, 80-lb bag count, and ready-mix volume with a 10% waste buffer included.

Footing Width by Wall Load and Soil (IRC R403.1)

Footing width is set by two things: the load coming down the wall (number of stories and roof) and the soil's load-bearing value. The 2021 International Residential Code Table R403.1(1) gives minimum widths for footings on soil with a presumptive load-bearing value of 1,500 psf — a conservative baseline for ordinary clays and silts. Stronger soils (sandy gravel at 3,000 psf) allow narrower footings; weak soils require wider.

Minimum footing thickness under the IRC is 6 inches, and the projection on each side of the wall should not exceed the footing thickness for plain concrete. A typical residential rule of thumb — footing width equal to twice the wall width — usually satisfies these minimums for a single-story home on average soil, but always size to the code table and your local soil report, not the rule of thumb alone.

Frost Depth: How Deep Footings Must Go (IRC R403.1.4)

IRC R403.1.4 requires the bottom of exterior footings to sit below the local frost line so the soil beneath them never freezes and heaves. Frost heave is the single most common cause of cracked foundations on shallow footings. Frost depth is set by your local building department, not a national number — these are typical ranges:

Where frost depth is deep, builders often use a frost-protected shallow foundation or a deeper stem wall on a shallow footing instead of pouring the full depth in solid concrete — see the stem-wall foundation guide. Footing depth (thickness of the pour) is separate from frost depth (how far down the bottom sits): most residential footings are poured 6–12 inches thick regardless of how deep the trench is excavated.

Strip Footing Volume per Linear Foot

Because strip footings run the full perimeter, volume scales with total linear feet. Use this table to estimate quickly: find your cross-section, then multiply the per-foot figure by your perimeter length. Add 10% for waste and over-excavation.

A standard 20-foot strip footing at 24 in × 12 in needs 1.48 cubic yards — about 74 bags of 80-lb mix before waste. A 100-foot basement perimeter at the same cross-section needs 7.4 cubic yards, firmly in ready-mix territory.

Spread Footing Volume by Pad Size

Spread footings are priced per pad. Multiply the volume below by the number of identical pads in your project (deck posts, fence-line columns, a row of steel posts).

*A 24×24×12 in pad is 0.148 cu yd; at 12-inch depth a deck pad alone runs about 7 bags. For deck-specific sizing including post and pier combinations, use the deck footing calculator and the pier footing calculator.

Rebar and Concrete Strength for Footings

Most building departments require continuous rebar in footings. A typical residential strip footing uses two #4 (½-inch) bars run lengthwise in the lower third of the section, with 3 inches of cover from the bottom and sides:

• Continuous bars: two #4 bars, lapped a minimum of 24 inches (or 40 bar diameters) at splices
• Corners and tees: add L-shaped bars so reinforcement is continuous around every corner
• Vertical dowels: #3 or #4 dowels tied into the footing to connect the foundation wall or stem above
• Spread footings: a two-way grid of #4 bars near the bottom for pads carrying heavy point loads

For concrete strength, footings are typically specified at 2,500–3,000 PSI. The IRC sets 2,500 PSI as the minimum for footings and foundation walls in most exposure conditions; 3,000–3,500 PSI is required where the footing is exposed to freezing and thawing in contact with de-icing or weathering. See what PSI concrete you need and when to use rebar in concrete for full reinforcement detailing.

Estimating Your Pour: Bagged vs. Ready-Mix

Footings consume more concrete per foot than people expect, and the volumes add up fast around a full perimeter. As a rule of thumb, use ready-mix for any footing over 2 cubic yards and bagged mix only for isolated pads, short piers, or a deck under 1 yard. A 40-foot perimeter at 16 × 12 in is roughly 2 cubic yards — about 90 bags of 80-lb mix — which is the practical break-even where ordering a truck beats hand-mixing.

Footing cost tracks volume plus excavation, rebar, and forms. To estimate the dollar figure, run your yardage through the concrete cost calculator, and for the full foundation pour above the footing use the concrete foundation calculator or the concrete wall calculator. Enter your footing dimensions in the tool above to get exact volume, bag count, and ready-mix yardage for your specific job.