How to Calculate Concrete for Footings
Strip, Pad & Perimeter

The footing concrete formula

Concrete footings are measured in cubic yards, the same as any other concrete pour. The formula differs from a slab calculation because footing dimensions are typically given in inches for width and depth, both must be converted to feet before calculating volume.

The 1.10 multiplier is the ACI-recommended 10% overage for below-grade pours. Footing forms are installed against uneven soil: the actual pour volume always exceeds the theoretical calculation. Running short mid-pour forces you to stop and creates a cold joint, which is a permanent plane of weakness in the footing. Always include the overage.

A practical example: a 40-foot continuous perimeter footing at 12 inches wide and 18 inches deep. CY = 40 × (12÷12) × (18÷12) ÷ 27 × 1.10 = 40 × 1.0 × 1.5 ÷ 27 × 1.10 = 2.44 CY. That is approximately 110 bags of 80lb concrete mix: the break-even point between bagged and ready-mix, depending on your local short-load fees. Use the concrete footing calculator to model any combination of dimensions.

Strip footings vs pad footings

The two fundamental footing types differ in geometry and application but use the same concrete. Strip footings (also called continuous or wall footings) run uninterrupted beneath load-bearing walls and spread the wall load across a long run of soil. Pad footings (also called isolated or spread footings) are individual rectangles or squares under single columns, posts, or heavily loaded points.

For residential construction, the vast majority of footings are either continuous strip footings under perimeter and interior load-bearing walls, or isolated pad footings under deck posts and columns. The IRC provides prescriptive sizing for both types in residential construction. Any footing supporting more than a standard residential load (steel columns, heavy masonry chimneys, commercial equipment) requires an engineered design.

Frost depth and footing depth requirements

The depth requirement for any footing is driven by the local frost line. Soil containing moisture freezes from the surface downward each winter, and the expansion force of freezing soil (frost heave) can exert thousands of pounds per square foot of upward pressure. A footing above the frost line will move up and down with the seasons, cracking every attached structure. The IRC mandates that all footings extend below the frost penetration depth.

These are regional ranges, your local building department has the adopted frost depth for your specific jurisdiction, which may be higher than the IRC map value. Some jurisdictions in frost-prone areas add 6 to 12 inches of additional depth as a safety margin. Always confirm with your building department before ordering footing forms. The footing calculator above adds the depth you enter directly: it does not automatically add frost depth margin.

IRC minimum footing dimensions

IRC 2024 Table R403.1 sets minimum footing widths and thicknesses based on the number of stories supported and the assumed soil bearing capacity of 1,500 psf. These are the minimum dimensions for prescriptive residential construction , if your soil has lower bearing capacity, your footings must be wider and a geotechnical report is recommended.

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. A 6-inch concrete foundation wall therefore requires a minimum 10-inch wide footing (6 + 2 + 2 = 10, though the code minimum is 12 inches anyway). For CMU (concrete block) walls, the footing must project beyond the outer block face by at least the wall thickness divided by 3 per ACI 530.

When working with pad footings under deck posts, the IRC does not provide prescriptive minimum sizes. Common residential practice for a standard 6×6 deck post in 1,500 psf soil is a 20-inch square by 10-inch deep footing, but this should be verified against local amendments. Use the concrete column calculator if you are combining a cylindrical tube form with a separate spread footing below it.

Pouring footings: practical guidance

Concrete footings for residential construction are typically poured directly into excavated trenches without forms for strip footings, or into tube forms for isolated pad footings. Both methods require attention to placement, consolidation, and curing.

For trench-poured strip footings, the trench walls act as the form. The soil must be undisturbed at the bottom, never pour over disturbed or filled soil without compaction verification. Set rebar chairs before pouring to maintain the 3-inch minimum bottom clearance required by ACI 318-25. Pour from one end of the trench and work continuously: stopping mid-pour creates a cold joint at the pour-stop location that becomes a plane of weakness.

For cold-weather pours below 40°F, ACI 306R-16 requires that the concrete be maintained above 50°F for at least 7 days after placement. This means heating the subgrade before pouring, using heated water in the mix, and insulating the placed concrete immediately. Never pour on frozen ground: the frozen subgrade will thaw and settle, dropping the footing. Costs for cold-weather concrete protection typically add $10 to $30 per cubic yard depending on conditions and duration.

After stripping forms (or after the surface has reached initial set for trench pours), protect the concrete from drying too quickly. ACI 308R recommends wet curing for at least 7 days for footings that will carry load within a few months. In practice, backfilling the trench within 24 to 48 hours provides adequate moisture retention for most residential footings in temperate climates.