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Concrete Footings Calculator

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Definition

What is Concrete Footings Calculator?

A concrete footings calculator determines the volume of concrete required for foundation footings — the structural base that transfers building loads to the ground. Footings spread the weight of walls, columns, and the entire structure over a wider area of soil to prevent settling and structural failure.

There are 3 main types of concrete footings:

  • Strip footings (continuous footings) — long, narrow footings that run under load-bearing walls. Typically 12–24 inches wide and 8–12 inches deep.
  • Pad footings (isolated footings) — square or rectangular pads under individual columns or posts. Typically 24×24 to 48×48 inches.
  • Stepped footings — footings on sloped ground that step up or down to follow the terrain while maintaining level bearing surfaces.

Building codes require footings to extend below the frost line and rest on undisturbed or compacted soil. Minimum footing width is typically 2× the wall width.

Formula

Concrete Footings Calculator Formula

Footing Volume = Length × Width × Depth × Quantity

For strip (continuous) footings: measure total linear length, width, and depth.

For pad (isolated) footings: measure one pad, then multiply by the number of pads.

Step-by-step for a 40 ft perimeter strip footing, 16 inches wide × 8 inches deep:

  1. Convert to feet: 16" = 1.333 ft width, 8" = 0.667 ft depth
  2. Volume: 40 × 1.333 × 0.667 = 35.6 ft³
  3. Convert to yd³: 35.6 ÷ 27 = 1.32 yd³
  4. Add 10% buffer: 1.32 × 1.10 = 1.45 yd³
  5. Bags needed: 35.6 ÷ 0.6 = 60 bags of 80 lb concrete
Examples

Worked Examples

Perimeter Strip Footing (40 ft)

40 ft long × 16" wide × 8" deep = 35.6 ft³ = 1.32 yd³. Needs 60 bags of 80 lb concrete.

8 Pad Footings (24×24×12")

Per pad: 2 × 2 × 1 = 4 ft³. Total: 32 ft³ = 1.19 yd³. Needs 54 bags of 80 lb concrete.

Garage Foundation (80 ft strip)

80 ft × 20" wide × 10" deep = 111 ft³ = 4.11 yd³. Order 4.5 yd³ ready-mix.

Deck Pier Pads (6 pads, 30×30×12")

Per pad: 2.5 × 2.5 × 1 = 6.25 ft³. Total: 37.5 ft³ = 1.39 yd³. Needs 63 bags.

FAQ

Frequently Asked Questions

Answers to common questions about the concrete footings calculator.

Footings must extend below the local frost line — ranging from 12 inches in southern US states to 48+ inches in northern states and Canada. Check your local building code for the exact frost depth requirement. The bottom of the footing must rest on undisturbed native soil or compacted gravel.

A footing should be at least 2× the width of the wall it supports. For an 8-inch block wall, the footing should be 16 inches wide minimum. For a 2×6 framed wall (5.5 inches), the footing should be at least 12 inches wide. Building codes specify minimum widths based on soil bearing capacity.

Yes — most building codes require at least 2 horizontal #4 rebar bars running the length of the footing. For strip footings, place rebar 3 inches from the bottom and 3 inches from each side. Pad footings require a grid of rebar (#4 on 12-inch centers both directions). Rebar prevents cracking from soil movement.

A footing is the widened base that distributes load to the soil. A foundation is the wall built on top of the footing. The footing sits at the bottom of the excavation. The foundation wall extends from the footing up to grade level. Together, they form the substructure of a building.

Yes — this is called a monolithic pour or turned-down slab edge. It combines the footing and slab into a single pour, saving time and creating a cold-joint-free structure. Monolithic pours are common for garage slabs, small buildings, and residential additions.

Residential footings typically use 2,500–3,000 psi concrete. Commercial footings may require 3,500–4,000 psi. The concrete strength (psi) refers to compressive strength at 28 days. Higher psi mixes are needed for heavy loads or poor soil conditions.